POPULAR
SCIENTIFIC RECREATIONS
IN
NATURAL PHILOSOPHY, ASTRONOMY, GEOLOGY, CHEMISTRY,
ETC., ETC., ETC.

Translated and Enlarged from “Les Récréations Scientifiques”
OF
GASTON TISSANDIER.
(Editor of “La Nature.”)

PROFUSELY ILLUSTRATED.

London:
WARD, LOCK, AND CO., WARWICK HOUSE,
SALISBURY SQUARE, E.C.
NEW YORK: 10, BOND STREET.

[Pg iv]

PREFACE.

A learned mathematician of the seventeenth century, Ozanam by name, a member of the Academy of Sciences and author of several distinguished works, did not think it derogatory to his dignity to write, under the title of “Mathematical and Physical Recreations,” a book designed for the amusement of youth, in which science lends itself to every pastime, even jugglery and tricks of legerdemain.

“Jeux d’esprit” says Ozanam, “are for all seasons and all ages; they instruct the young, they amuse the old, they are welcomed by the rich, and are not above the reach of the poor.”

The object of the book now presented to the reader is also to instruct while it amuses, but we have not thought proper to make use, as Ozanam did, of any physical feats, so called amusing. Such do not constitute experiments, and are but ingenious deceptions, intended to disguise the true mode of operation, and we have not desired to make use of or popularise such methods. We wish, on the contrary, that every game we describe, every pastime or amusement of which we give the exposition, should be rigorously based on the scientific method, and looked upon as a genuine exercise in physics, chemistry, mechanics, or natural science. It does not appear to us desirable to teach deception, even in play.

Science in the open air, in the fields, in the sunshine, is our first study; we point out how, in the country, it is possible, pleasantly and unceasingly, to occupy one’s leisure in observing nature, in capturing insects or aquatic animals, or in noting atmospheric phenomena.

We next teach a complete course of physics without any apparatus, and point out the methods for studying the different phenomena[Pg v] of heat, light, optics, and electricity, by means of a simple water-bottle, tumbler, stick of sealing-wax, and other ordinary objects, such as everyone has at hand. A series of chemical experiments, performed by means of some phials and inexpensive appliances, completes that part of the book relating to the physical sciences.

Another kind of recreation, both intelligent and useful, consists in collecting the ingenious inventions which are constantly being supplied to our requirements by the applied sciences, and learning how to use them. We have collected a number of mechanical inventions and appliances, with which most ingenious and skilful people will wish to supply themselves, from Edison’s electric pen, or the chromograph, which will produce a large number of copies of a letter, drawing, etc., to the more complicated, but not less valuable contrivances, for making science useful in the house.

Having described some scientific toys for the young, we have endeavoured to point out those interesting to persons of riper years, and have grouped together curious systems of locomotion, and ingenious mechanical appliances, such as small steam-boats, ice-boats, swimming apparatus, etc., under proper heads.

In addition to the foregoing subjects, we have included some of the experimental details of Chemical Science, with illustrations. We have added a chapter upon Aërial Navigation and Ballooning, with anecdotes of some of our celebrated aëronauts. We have also enlarged upon Light, Sound, Heat, Physical Geography, Mineralogy, Geology, Electrical Appliances, the Electric Light, and most of the latest adaptations of electricity.

It will be seen, therefore, that the present work is not only intended for the young; everyone, it is hoped, will find in it something interesting and also profitable, which, if not desired for self-instruction, may at any rate be turned to account as a means of teaching others that science, which is universal, can, when rightly apprehended, preside even over our pleasures and amusements.

The Editor.

[Pg vi]

CONTENTS.

[Pg 1]

SCIENTIFIC RECREATIONS.

CHAPTER I.—INTRODUCTORY.

SCIENCE AND RECREATION—THE BOOK OF NATURE—THE SENSES—NATURAL HISTORY—NATURAL PHILOSOPHY—MATTER—OBJECTS—PROPERTIES OF MATTER.

It may at the first glance appear paradoxical to combine Science and Recreation, but we hope to show that true scientific recreation is anything but the dry bones of learning. To those who study science with us, we will point out first how easy and pleasant it is to watch the sky and the plants and Nature generally in the open air. Then we will carry our readers along with us, and by means of illustrations and diagrams instruct them pleasantly in the reasons for things. “How?” and “Why?” will be questions fully answered. Not only will the usual scientific courses be touched upon, but we will show how Science is applied to Domestic Economy. We will have Chemistry put before us without needing a laboratory, and we will experiment in Physics without elaborate apparatus. We will have, in short, a complete Encyclopædia of Science free from dryness and technicalities—an amusing volume suited to old and young who wish to find out what is going on around them in their daily life in earth and sea and sky.

Bernard Palissy used to say that he wished “no other book than the earth and the sky,” and that “it was given to all to read this wonderful book.”

It is indeed by the study of the material world that discoveries are accomplished. Let an attentive observer watch a ray of light passing from the air into water, and he will see it deviate from the straight line by refraction; let him seek the origin of a sound, and he will discover that it results[Pg 2] from a shock or a vibration. This is physical science in its infancy. It is said that Newton was led to discover the laws of universal gravitation by beholding an apple fall to the ground, and that Montgolfier first dreamt of air-balloons while watching fogs floating in the atmosphere. The idea of the inner chamber of the eye may, in like manner, be developed in the mind of any observer, who, seated beneath the shade of a tree, looks fixedly at the round form of the sun through the openings in the leaves.

Luminous Cross seen at Havre, May 7th, 1877. Sketched from Nature.

Every one, of course, may not possess the ambition to make such[Pg 3] discoveries, but there is no one who cannot compel himself to learn to enjoy the pleasure that can be derived from the observation of Nature.

It must not be imagined that in order to cultivate science it is absolutely necessary to have laboratories and scientific work-rooms. The book of which Palissy spoke is ever present; its pages are always open, wherever we turn our eyes or direct our steps. So we may hope to introduce all our friends to a pleasant and lasting acquaintance with Dame Nature.

“But what is Nature?” We are fond of admiring Nature, and the effects of certain causes in the world, and we want to know why things are so. Very well—so you shall; and as to the question “What is Nature?” we will endeavour to answer you at once.

Nature is the united totality of all that the various Senses can perceive. In fact, all that cannot be made by man is termed “Nature”; i.e., God’s creation.

From the earliest ages man has sought to read the open leaves of the Book of Nature, and even now, with all our attainments, we cannot grasp all, or nearly all. One discovery only leads up to another. Cause and Effect are followed up step by step till we lose ourselves in the search. Every effect must have a cause. One thing depends upon another in the world, and it does not need Divine revelation to tell us that. Nothing happens by “mere chance.” “Chance!” said a Professor to us at the University, “Chance!—Remember, there is no such thing in the world as chance.”

Between our minds or consciousness and Nature are our Senses. We feel, we see, we hear, we taste, we smell,—so it is only through the Senses that we can come to any knowledge of the outer world. These attributes, or Senses, act directly upon a certain “primary faculty” called Consciousness, and thus we are enabled to understand what is going on around us. The more this great existing faculty is educated and trained, the more useful it will become. So if we accustom our minds to observation of Nature, we shall find out certain causes and effects, and discover Objects. Now an Object is a thing perceptible both to feeling and sight, and an Object occupies space. Therefore there are objects Artificial as well as Natural. The former are created by man from one or more Natural products. Natural Objects are those such as trees, rocks, plants, and animals. We may also class the heavenly bodies, etc., as Objects, though we cannot touch them, but we can feel their effects, and see them. The Phenomena of Nature include those results which are perceptible by only one sense, as thunder; light and sound may also be classed as Phenomena.

Take a familiar instance. A stone is a Natural Object. We take it up, open our fingers, and it falls. The motion of that object is a Phenomenon. We know it falls because we see it fall, and it possesses what we term weight; but we cannot tell why it possesses weight.

[Professor Huxley says: “Stones do not fall to the ground in consequence of a law of nature,” for a law is not a cause. “A law of nature[Pg 4] merely tells us what we may expect natural objects will do under certain circumstances.”]

A cause of a Phenomenon being independent of human will is called a Force, and the stone falls by the force of Gravitation, or that natural law which compels every material object to approach every other material object.

A single Force may produce a great number of Phenomena.

Nature being revealed to us by Objects, and by means of Phenomena, we have got already two Branches of Science extending from such Roots; viz., Natural History, the Science of Objects; and Natural Philosophy, the Science of Phenomena.

Both of these Branches have been subdivided thus:

These two great divisions comprehend, in their extended senses, all that is known respecting the material world.

We have spoken of Objects. Objects occupy Space. What is Space? Space is magnitude which can be conceived as extending in three directions—Length, Breadth, and Depth. Matter occupies portions of Space, which is infinite. Matter, when finite, is termed a body or object. The general properties of Matter are Magnitude, Form, Impenetrability, Inertia, Divisibility, Porosity, Elasticity, Compressibility, Expansibility.

Matter is present in Nature in three conditions. We find it as a Solid, a Liquid, and a Gas. We shall explain the various properties of Solids, Liquids, and Gases in their proper places (in Physics). To test the actual existence of Matter in one or other of these forms our Senses help us. We can touch a Solid, or taste it and see it. But touch is the test. We have said that Matter possesses certain properties. We will examine these briefly. The two which belong to all material bodies are Impenetrability and Magnitude. You cannot, strictly speaking, penetrate Matter. You can find the form of an object by touch or sight, but you cannot penetrate it. You will think you can drive a nail or a screw into a board, but you cannot; you only displace the fibres of the wood by the screw. Take water as a very common instance. Water is Matter, for it occupies a certain space. Water is impenetrable, for if you put your hand or foot into a basin full of it, it will overflow, thus proving that you displace, and do not penetrate it. It is almost impossible to compress water.

Divisibility is another quality of Matter; and when we attempt to show how far Matter can be divided, the brain refuses to grasp the infinity. A pin’s head is a small object, but it is gigantic compared to some[Pg 5] animals, of which millions would occupy a space no larger than the head of a pin. These tiny animals must contain organs and veins, etc., and those veins are full of blood globules. Professor Tyndall informs us that a drop of blood contains three millions of red globules. So these infinitesimally small animals must have millions of globules in their blood also. Thus we see to what an extent, far beyond our Senses’ power to grasp, Matter can be divided.

But there is something even more astonishing than this. It is stated that there are more animals in the milt of a single codfish than there are men in the world; and that one grain of sand is larger than four millions of these animals! each of which must be possessed of life germs of an equal amount, which would grow up as it grew to maturity. This carries us back again, and

Or take other interesting facts. One hundred threads of the silkworm must be placed side by side to make up the thickness of a line (—) about 1/25th of an inch; and metals can be drawn out to such exceeding fineness that twelve hundred of the fine wires will occupy only the space of one hundred silkworms’ threads, or one millimetre.

Porosity is another attribute of Matter, for in all Matter there are pores, or spaces, between the particles. Sometimes such openings are plainly visible; in very “solid” bodies they are, to a great extent, indistinguishable. But we know that the spaces exist, because we can compress the particles together.

Inertia is also a general property of Matter, and the meaning of the term is “inactivity,” or passiveness—a want of power in an object to move, or when moving, to stop of itself. It will come to rest apparently by itself, but the resistance of the air and the friction of the ground, or the attraction of the earth, will really occasion the stoppage of the object. We will speak more fully of Inertia presently. Elasticity and Expansibility are evident in fluids and gases.

We have thus introduced our readers to some of the most evident facts connected with Matter. The various Forces and Phenomena of attraction will be fully considered farther on; at present we are about to show our readers how they may first profitably study Science in the open air for themselves, and we will give them our experience of the Book of Nature.

[Pg 6]

CHAPTER II.

SCIENCE IN THE OPEN AIR—APHIDES—EVAPORATION BY LEAVES—AN AQUARIUM—THE CATALEPTIC FOWL—NEEDLE POINTS AND THORNS—MICROSCOPIC AQUARIUM—CAPE GRISNEZ—CRYSTALS—ICE ON THE GAS LAMPS.

Fig. 1.—Ants engaged in extracting aphides from a rose-tree (highly magnified)

Some years ago we were staying in Normandy, not far from the town of C——, enjoying, in the midst of most cordial hospitality, the peacefulness of country life; and my kind hosts, with me, took great pleasure in having what we called “a course of science in the open air.” The recollections of that time are some of the pleasantest in the whole course of my life, because all our leisure was intelligently occupied. Each of us set himself to provide the subject of some curious observation or instructive experiment; one made[Pg 7] a collection of insects, another studied botany. In the daytime we might have been seen examining, under a magnifying glass, the branch of a rose-tree, from which the ants were endeavouring to extract the aphides1 (fig. 1). At night we admired through the telescope the stars and planets that were visible; or if the sky was not clear, we examined under a strong magnifier grains of pollen from flowers, or the infusoria in a drop of stagnant water. Frequently some very insignificant object became the occasion for some scientific discussion, which terminated with an experimental verification.

Fig. 2.—Experiment showing evaporation
of water by leaves.

I recollect that one day one of us remarked that after a week of dry weather a stream of water had nearly dried up, although sheltered by thick trees, which necessarily impeded the calorific action of the sun; and he expressed surprise at the rapid evaporation. An agriculturist among the company, however, drew his attention to the fact that the roots of the trees were buried in the course of the stream, and that, far from preventing the evaporation of the water, the leaves had contributed to accelerate it. As the first speaker was not convinced, the agriculturist, on our return to the house, prepared an experiment represented in fig. 2. He placed the branch of a tree covered with foliage in a U-shaped tube, the two branches of unequal diameter, and filled with water. He placed the vegetable stem in the water, and secured it to the tube by means of a cork covered with a piece of india-rubber, and tied tightly to make it hermetically closed.

At the commencement of the experiment the water was level with A in the larger branch of the tube, and level with B in the smaller, rising by capillarity to a higher point in the more slender of the two. The evaporation of the water caused by the leaves was so active that in a very short time we beheld the water sink to the points C and C′.

[Pg 8]

Fig. 3.—Aquarium formed by means of a melon glass.

Thus did the excellent method of seeking the cause of phenomena by experiments often lead us to interesting results. We had among us many children and young people who had reached the age of ardent curiosity. We took pleasure in pointing out to them the means of studying natural science; and we were not long before feeling convinced that our lessons out in the fields had much greater success than those given between the four walls of a class-room. Insects were collected, and preserved by being carefully placed in a small bottle, into which was let fall a drop of sulphuret of carbon;2 the insect was immediately asphyxiated, and we[Pg 9] thus avoided the cruelty of passing a pin through a living body. Having chased butterflies and insects, we next desired to study the aquatic creatures which swarmed in the pools of the neighbourhood. For this purpose I constructed a fishing-net fitted to an iron ring, and firmly secured to a wooden handle. When this was plunged under the water and drawn quickly out again, it came back full of slime. In the midst of this muddy substance one generally succeeded in finding the hydrophilus, tadpoles, coleoptera, many curious kinds of caddis-worms, tritons, and sometimes frogs, completely astounded by the rapidity of their capture. All these creatures were transported in a bottle to the house, and I then constructed, at small expense, a glass aquarium, by means of the bell of a melon-glass turned upside down, thus forming a transparent receptacle of considerable size. Four wooden stakes were then fixed in the ground, and a plank with a circular hole nailed on the top, in which the glass bell was placed. I next scattered some large pebbles and shells at the bottom of the vase to form a stony bed, poured in some water, placed a few reeds and water plants among the pebbles, and then[Pg 10] threw a handful of water lentils on the surface; thus a comfortable home was contrived for all the captured animals.3 The aquarium, when placed under the shade of a fine tree in a rustic spot abounding with field flowers, became a favourite rendezvous, and we often took pleasure in watching the antics of the little inmates (fig. 3). Sometimes we beheld very sanguinary scenes; the voracious hydrophilus would seize a poor defenceless tadpole, and rend him in pieces for a meal without any compunction. The more robust tritons defended themselves better, but sometimes they also succumbed in the struggle.

Fig. 4.—Cage for preserving living insects.

Fig. 5.—Small aquarium, with frogs’ ladder.

The success of the aquarium was so complete that one of us resolved to continue this museum in miniature, and one day provided himself with an insects palace, which nearly made us forget the tadpoles and tritons. It was a charming little cage, having the form of a house, covered with a roof;[Pg 11] wires placed at equal distances forming the sides. In it was a large cricket beside a leaf of lettuce, which served as his food (fig. 4). The little creature moved up and down his prison, which was suspended from the branch of a tree, and when one approached him very closely gave vent to his lively chirps.

Fig. 6.—Frog lying in wait for a fly.

The menagerie was soon further augmented by a hitherto unthought-of object; namely, a frogs’ ladder. It was made with much skill. A large bottle served for the base of the structure. The ladder which was fixed in it was composed of the twigs of very small branches, recently cut from a tree, and undivested of their bark, which gave to the little edifice a more picturesque and rustic appearance. The pieces of wood, cleverly fixed into two posts, conducted the green frogs (tree-frogs) on to a platform, whence they ascended the steps of a genuine ladder. There they could disport themselves at pleasure, or climb up further to a branch of birch-tree placed upright in the centre of the bottle (fig. 5). A net with fine meshes prevented the little animals from escaping. We gave the tree-frogs flies for their food, and sometimes they caught them with remarkable dexterity. I have often seen a frog when at liberty watching a fly, on which it pounces as a cat does on a bird (fig. 6). The observations that we made on the[Pg 12] animals of our menagerie led us to undertake others of a very different nature; I recollect particularly a case of catalepsy produced in a cock. I will describe this remarkable experiment, certainly one of the most curious we ever performed.

Fig. 7.—Experiment of the cataleptic cock.

We place a cock on a table of dark colour, rest its beak on the surface, where it is firmly held, and with a piece of chalk slowly draw a white line in continuation from the beak, as shown in our engraving. If the crest is thick, it is necessary to draw it back, so that the animal may follow with his eyes the tracing of the line. When the line has reached a length of about two feet the cock has become cataleptic. He is absolutely motionless,[Pg 13] his eyes are fixed, and he will remain from thirty to sixty seconds in the same posture in which he had at first only been held by force. His head remains resting on the table in the position shown in fig. 7. This experiment, which we have successfully performed on different animals, can also be accomplished by drawing a straight line with a piece of chalk on a slate. M. Azam declares that the same result is also produced by drawing a black line on a table of white wood. According to M. Balbiani, German students had formerly a great predilection for this experiment, which they always performed with marked success. Hens do not, when operated on, fall into a cataleptic condition so easily as cocks; but they may often be rendered motionless by holding their heads fixed in the same position for several minutes. The facts we have just cited come properly under the little studied phenomena, designated by M. Braid in 1843 by the title of Hypnotism. MM. Littré and Ch. Robin have given a description of the hypnotic condition in their Dictionnaire de Médecine.

Fig. 8.—Ordinary pin and needle, seen through a microscope (magnified 500 diameters).

Fig. 9.—Thorn of a rose, and wasp’s sting through a microscope (magnified 500 diameters).

If any shining object, such as a lancet, or a disc of silver-paper gummed to a plate, is placed at about the distance of a foot from the eyes of a person, slightly above the head, and the patient regards this object fixedly, and without interruption for twenty or thirty minutes, he will become gradually motionless, and in a great number of cases will fall into a condition of torpor and genuine sleep. Dr. Braid affirms that under such circumstances he has been able to perform surgical operations, without the patient having any consciousness of pain. Later also, M. Azam has proved the complete insensibility to pricking on the part of individuals whom he has rendered cataleptic by the fixing of a brilliant object. The[Pg 14] experiment of the cataleptic cock was first described under the name of Experimentum Mirabile, by P. Kircher, in his Ars Magna, published at Rome in 1646. It evidently belongs to the class of experiments which were performed at the Salpêtrière asylum at Paris, by M. Charcot, on patients suffering from special disorders. It must now be evident to our readers that our scientific occupations were sufficiently varied, and that we easily found around us many objects of study. When the weather was wet and cloudy we remained indoors, and devoted ourselves to microscopical examinations. Everything that came under our hands, insects, vegetables, etc., were worthy of observation. One day, while engaged over a microscopical preparation, I was making use of one of those steel points generally employed in such purposes, when happening to pass it accidentally beneath the microscope, I was astonished to see how rough and uneven it appeared when highly magnified. The idea then occurred to me to have recourse to something still more pointed, and I was thus led to make comparisons between the different objects represented in figs. 8 and 9. It will here be seen how very coarse is the product of our industry when compared with the product of Nature. No. 1 of fig. 8 represents the point of a pin that has already been used, magnified 500 diameters. The point is evidently slightly blunted and flattened. The malleable metal has yielded a little under the pressure necessary to make it pass through a material. No. 2 is a little more pointed; it is a needle. This, too, will be seen to be defective when regarded by the aid of the microscope. On the other hand, what fineness and delicacy do the rose thorn and wasp’s sting[Pg 15] present when examined under the same magnifier! (See the two points in fig. 9.)

An examination of this exact drawing has led me to make a calculation which leads to rather curious results: at a half millimetre from the point, the diameters of the four objects represented are in thousandths of a millimetre respectively, 3·4; 2·2; 1·1; 0·38. The corresponding sections in millionths of a square millimetre are: 907·92; 380·13; 95·03; 11·34; or, in round numbers, 908; 380; 95; 11.

If one bears in mind, which is much below the truth, that the pressure exercised on the point must be proportional to the section, and admitting that a pressure of 11 centigrams suffices to thrust in the sting of a wasp half a millimetre, it will require more than 9 grams of pressure to thrust in a needle to the same extent. In fact, this latter figure is much too small, for we have not taken into account the advantage resulting from the elongated shape of the rose thorn, which renders it more favourable for penetration than a needle through a drop of tallow.

It would be easy to extend observations of this kind to a number of other objects, and the remarks I have just made on natural and artificial points will apply incontestably to textures for example. There is no doubt that the thread of a spider’s web would far surpass the thread of the finest lace, and that art will always find itself completely distanced by nature.

We amused ourselves frequently by examining the infusoria which are so easily procured by taking from some stagnant water the mucilage adhering to the vegetation on the banks, or attached to the lower part of water lentils. In this way we easily captured infusoria, which, when placed under a strong magnifier, presented the most remarkable spectacle that one can imagine. They are animalcules, having the form of transparent tulips attached to a long stem. They form bunches which expand and lengthen; then, suddenly, they are seen to contract with such considerable rapidity that the eye can scarcely follow the movement, and all the stems and flower-bells are folded up into the form of a ball. Then, in another moment, the stems lengthen, and the tulip-bells open once more. One can easily encourage the production of infusoria by constructing a small microscopic aquarium, in which one arranges the centre in a manner favourable to the development of the lowest organisms. It suffices to put a few leaves (a piece of parsley answers the purpose perfectly)4 in a small vase containing water (fig. 10), over which a glass cover is placed, and it is then exposed to the rays of the sun. In two or three days’ time, a drop of this water seen under the microscope will exhibit infusoria. After a certain time, too, the different species will begin to show themselves. Microscopical observations can be made on a number of different objects. Expose to the air some flour moistened by water, and before long a mouldiness will form on it; it is the penicillium glaucum, and when examined under a magnifier of 200 to 300 diameters, cells are dis[Pg 16]tinguishable, branching out from an organism remarkable for its simplicity. We often amused ourselves by examining, almost at hazard, everything that came within our reach, and sometimes we were led to make very instructive investigations. When the sky was clear, and the weather favourable to walking, we encouraged our young people to run about in the fields and chase butterflies. The capture of butterflies is accomplished, as every one knows, by means of a gauze net, with which we provided the children, and the operation of chasing afforded them some very salutary exercise. It sometimes happens that butterflies abound in such numbers, that it is comparatively easy to capture them. During the month of June 1879, a large part of Western Europe was thronged with swarms of Vanessa algina butterflies, in such numbers that their appearance was regarded as an important event, and attracted the lively attention of all entomologists (fig. 11). This passage of butterflies provided the occasion for many interesting studies on the part of naturalists.

Fig. 10.—Arrangement of a microscopic aquarium for examining infusoria.

Fig. 11.—Flight of butterflies seen near Berne, June 15th, 1879.

Fig. 12.—Group of rock crystal.

We cannot point out too strongly to our readers that the essential condi[Pg 17]tion for the student of natural science, is the possession of that sacred fire which imparts the energy and perseverance necessary for acquiring and enlarging collections. It is also necessary that the investigator should furnish himself with certain indispensable tools. For collecting plants, the botanist should be armed with a pickaxe set in a thoroughly strong handle, a trowel, of which there is a variety of shapes, and a knife with a sharp blade. A botanical case must also be included, for carrying the plants. The geologist, or mineralogist, needs no more elaborate instruments; a hammer, a chisel, and a pickaxe with a sharp point for breaking the rocks, and a bag for carrying[Pg 18] the specimens, will complete his outfit. We amused ourselves by having these instruments made by the blacksmith, sometimes even by manufacturing them ourselves; they were simple, but solid, and admirably adapted to the requirements of research. Often we directed our walks to the seashore, where we liked to collect shells on the sandy beach, or fossils among the cliffs and rocks. I recollect, in a walk I had taken some years previously along the foot of the cliffs of Cape Blanc-Nez, near Calais, having found an impression of an ammonite of remarkable size, which has often excited the admiration of amateurs; this ammonite measured no less than twelve inches in diameter. The rocks of Cape Grisnez, not far from Boulogne, also afford the geologian the opportunity of a number of curious investigations. In the Ardennes and the Alps I have frequently procured some fine mineral specimens; in the first locality crystallized pyrites, in the second, fine fragments of rock crystal (fig. 12). I did not fail to recount these successful expeditions to the young people who accompanied me, and their ardour was thereby inflamed by the hope that they also should find something valuable. It often happened when the sun was powerful, and the air extremely calm, that my young companions and I remarked some very beautiful effects of mirage on the beach, due to the heating of the lower strata of the atmosphere. The trees and houses appeared to be raised above a sheet of silver, in which their reflections were visible as in a sheet of tranquil water. It can hardly be believed how frequently the atmosphere affords interesting spectacles[Pg 19] which pass unperceived before the eyes of those who know not how to observe. I recollect having once beheld at Jersey a magnificent phenomenon of this nature, on the 24th June, 1877, at eight o’clock in the evening: it was a column of light which rose above the sinking sun like a sheaf of fire. I was walking on the St. Helier pier, where there were also many promenaders, but there were not more than two or three who regarded with me this mighty spectacle. Columns and crosses of light are much more frequent than is commonly supposed, but they often pass unperceived before indifferent spectators. We will describe an example of this phenomenon observed at Havre on the 7th May, 1877. The sun formed the centre of the cross, which was of a yellow, golden colour. This cross had four branches. The upper branch was much more brilliant than the others; its height was about 15°. The lower branch was smaller, as seen in the sketch on page 2, taken from nature by Monsieur Albert Tissandier. The two horizontal branches were at times scarcely visible, and merged in a streak of reddish-yellow colour, which covered a large part of the horizon. A mass of cloud, which the setting sun tinged with a deep violet colour, formed the foreground of the picture. The atmosphere over the sea was very foggy. The phenomenon did not last more than a quarter of an hour, but the conclusion of the spectacle was signalized by an interesting circumstance. The two horizontal branches, and the lower branch of the luminous cross, completely disappeared, whilst the upper branch remained alone for some minutes longer. It had now the appearance of a vertical column rising from the sun, like that which Cassini studied on the 21st May, 1672, and that which M. Renon5 and M. A. Guillemin observed on the 12th July, 1876.6 Vertical columns, which, it is well known, are extremely rare phenomena, may therefore indicate the existence of a luminous cross, which certain atmospheric conditions have rendered but partially visible.

How often one sees along the roads little whirlwinds of dust raised by the wind accomplishing a rotatory movement, thus producing the imitation of a waterspout! How often halos encompass with a circle of fire the sun or the stars! How often we see the rainbow develop its iridescent beauties in the midst of a body of air traversed by bright raindrops! And there is not one of these great natural manifestations which may not give rise to instructive observations, and become the object of study and research. Thus, in walks and travels alike, the study of Science may always be exercised; and this method of study and instruction in the open air contributes both to health of body and of mind. As we consider the spectacles which Nature spreads before us,—from the insect crawling on the blade of grass, to the celestial bodies moving in the dome of the heavens,—we feel a vivifying and salutary influence awaken in the mind. The habit[Pg 20] of observation, too, may be everywhere exercised—even in towns, where Nature still asserts herself; as, for example, in displays of meteorological phenomena. We will give an example of such.

Fig. 13.—Icicles on gas lamp.

The extraordinary abundance of snow which fell in Paris for more than ten consecutive hours, commencing on the afternoon of Wednesday, January 22nd, 1880, will always be looked upon as memorable among the meteorological events of the city of Paris. It was stated that in the centre of Paris, the thickness of the snow that had fallen at different times exceeded fourteen inches. The snow had been preceded by a fall of small transparent

icicles, of rather more than a millimetre in diameter, some having crystalline facets. They formed on the surface of the ground a very slippery glazed frost. On the evening of the 22nd January, flakes of snow began to hover in the atmosphere like voluminous masses of wool. The greater part of the gas-lamps were ornamented by frozen stalactites, which continually attracted the attention of passers-by. The formation of these stalactites, of which we give a specimen (fig. 13), is easy of explanation. The snow falling on the glass of the lamp became heated by the flame of gas, melted, and trickled down, freezing anew into the shape of a stalactite below the lamp, at a temperature of 0° centigrade. Not only can meteorology be studied in towns, but certain other branches of natural science—entomology, for[Pg 21] example. We will quote what a young student in science, M. A. Dubois, says on this very subject: “Coleoptera,” he declares, “are to be met with everywhere, and I think it may be useful to notice this fact, supporting it by examples. I desire to prove that there are in the midst of our large towns spots that remain unexplored, where some fine captures are to be made. Let us visit, at certain times, the approaches to the quays, even at low tide, and we shall be surprised to find there species which we have searched for far and near.” This opinion is confirmed by the enumeration of several interesting captures.

Was not the great Bacon right when he said, “For the keen observer, nothing in Nature is mute”?

The cliffs of Cape Grisnez.

[Pg 22]

CHAPTER III.

PHYSICS—THE MEANING OF PHYSICS—FORCES OF NATURE—GRAVITY—COHESION—CHEMICAL ATTRACTION—CENTRE OF GRAVITY—EXPERIMENTS—AUTOMATON TUMBLERS.

Having now introduced our readers to Science which they can find for themselves in the open air, and the pursuit of which will both instruct and amuse, we will proceed to investigate the Branch of Science called Physics.

Physics may be briefly described as the Branch of Natural Science which treats of such phenomena as are unaccompanied by any important changes in the objects wherein such phenomena are observed.

For instance, the sounding of a bell or the falling of a stone are physical phenomena, for the objects which cause the sound, or the fall, undergo no change. Heat is set free when coal burns. This disengagement of heat is a physical phenomenon; but the change during combustion which coal undergoes is a chemical phenomenon. So the objects may be the same, but the circumstances in which they are placed, and the forces which act upon them, may change their appearance or position.

This brings us at once to the Forces of Nature, which are three in number; viz., Gravity, Cohesion, and Affinity, or Chemical Attraction. The phenomena connected with the last-named forms the Science of Chemistry. We give these three Forces these names. But first we must see what is Force, for this is very important. Force is a CAUSE—the cause of Motion or of Rest. This may appear paradoxical, but a little reflection will prove it. It requires force to set any object in motion, and this body would never stop unless some other force or forces prevented its movement beyond a certain point. Force is therefore the cause of a change of “state” in matter.

We have said there are three forces in nature. The first is Gravity, or the attraction of particles at a distance from each other. We may say that Gravity, or Gravitation, is the mutual attraction between different portions of matter acting at all distances,—the force of attraction being, of course, in proportion to the mass of the bodies respectively. The greatest body is the Earth, so far as our purposes are concerned. So the attraction of the Earth is Gravity, or what we call Weight.

We can easily prove this. We know if we jump from a chair we shall come to the floor; and if there were nothing between us and the actual ground sufficient to sustain the force of the attracting power of the earth, we should[Pg 23] fall to the earth’s surface. In a teacup the spoon will attract air bubbles, and large air bubbles will attract small ones, till we find a small mass of bubbles formed in the centre of the cup of tea. Divide this bubble, and the component parts will rush to the sides of the cup. This form of attraction is illustrated by the accompanying diagrams.

Fig. 14.

Suppose two balls of equal magnitude, A and B (fig. 14). These being of equal magnitude, attract each other with equal force, and will meet, if not opposed, at a point (M) half-way between the two. But they do not meet, because the attraction of the earth is greater than the attraction they relatively and collectively exercise towards each other. But if the size of the balls be different, the attraction of the greater will be more evident, as shown below, where the points of meeting are indicated respectively (figs. 15 and 16). These experiments will illustrate the phenomena of falling bodies. Gravity is the cause of this, because every object on the surface of the earth is very much smaller than the earth itself, and therefore all bodies fall towards the centre of the earth. A certain time is thus occupied, and we can find the velocity or rapidity of a falling body very easily. On the earth a body, if let fall, will pass through a space sixteen feet in the first second; and as the attraction of the earth still continues and is exercised upon a body already in rapid motion, this rate of progress must be proportionately increased. Just as when steam is kept up in an engine running down hill, the velocity of the train will rapidly increase as it descends the gradient.

Figs. 15 and 16.

A body falling, then, descends sixteen feet in the first second, and for every succeeding second it assumes a greater velocity. The distance the body travels has been calculated, and the space it passes through has been found to increase in proportion to the square of the time it takes to fall. For instance, suppose you drop a stone from the top of a cliff to the beach, and it occupies two seconds in falling, if you multiply 2 × 2, and the result by sixteen, you will find how high the cliff is: in this (supposed) case it is (omitting decimals) sixty-four feet high. The depth of a well can also be ascertained in the same way, leaving out the effect of air resistance.

[Pg 24]

But if we go up into the air, the force of gravity will be diminished. The attraction will be less, because we are more distant from the centre of the earth. This decrease is scarcely, if at all, perceptible, even on very high mountains, because their size is not great in comparison with the mass of the earth’s surface. The rule for this is that gravity decreases in proportion to the square of the distance. So that if at a certain distance from the earth’s surface the force of attraction be 1, if the distance be doubled the attraction will be only one quarter as much as before—not one-half.

Gravity has exactly the same influence upon all bodies, and the force of the attraction is in proportion to the mass. All bodies of equal mass will fall in the same time in a given distance. Two coins (or a coin and a feather in vacuo) will fall together. But in the air the feather will remain far behind the coin, because nearly all the atoms of the former are resisted by the air, while in the coin only some particles are exposed to the resistance, the density of the latter preventing the air from reaching more than a few atoms, comparatively speaking. The theory of weight and gravitation, and experiments relating to the falling of bodies, may be easily demonstrated with ordinary objects that we have at hand. I take a halfpenny and a piece of paper, which I cut in the shape of the coin, and holding them side by side, I drop them simultaneously; the halfpenny reaches the ground some time before the paper, a result quite in accordance with the laws of gravitation, as one must bear in mind the presence of air, and the different resistance it offers to two bodies differing in density. I next place the paper disc on the upper surface of the piece of money, and then drop them simultaneously. The two objects now reach the ground at the same time, the paper, in contact with the halfpenny, being preserved from the action of the air. This experiment is so well known that we need not further discuss it; but it must be plainly evident that it is capable of development in experiments on phenomena relating to falling bodies.7 When a body influenced by the action of a force acts, in its turn, upon another, the latter reacts in an opposite manner upon the first, and with the same intensity.

The Attraction of Cohesion is the attraction of particles of bodies to[Pg 25] each other at very small distances apart. Cohesion has received various names in order to express its various degrees. For instance, we say a body is tough or brittle, or soft or hard, according to the degrees of cohesion the particles exercise. We know if we break a glass we destroy the cohesion; the particles cannot be reunited. Most Liquid particles can be united, but not all. Oil will not mix with water.

The force of cohesion depends upon heat. Heat expands everything, and the cohesion diminishes as temperature increases.

There are some objects or substances upon the earth the particles of which adhere much more closely than others, and can only, with very great difficulty, be separated. These are termed Solids. There are other substances whose particles can easily be divided, or their position altered. These are called Fluids. A third class seem to have little or no cohesion at all. These are termed Gases.

Adhesion is also a form of attraction, and is cohesion existing on the surfaces of two bodies. When a fluid adheres to a solid we say the solid is wet. We turn this natural adhesion to our own purposes in many ways,—we whitewash our walls, and paint our houses; we paste our papers together, etc.

On the other hand, many fluids will hot adhere. Oil and water have already been instanced. Mercury will not stick to a glass tube, nor will the oiled glass tube retain any water. We can show the attraction and repulsion in the following manner:—Let one glass tube be dipped into water and another into mercury, you will see that the water will ascend slightly at the side, owing to the attraction of the glass, while the mercury will be higher in the centre, for it possesses no attraction for the glass (fig. 17). If small, or what are termed capillary (or hair) tubes, be used (fig. 18), the water will rise up in the one tube, while in the other the mercury will remain lower than the mercury outside the tube. (See Capillarity.)

Figs. 17. and 18.

Chemical Attraction is the force by which two different bodies unite to form a new and different body from either. This force will be fully considered in Chemistry, in a future part.

It is needless for us to dwell upon the uses of these Forces of Nature. Gravity and Cohesion being left out of our world, we can imagine the result. The earth and sun and planets would wander aimlessly about; we should float away into space, and everything would fall to pieces, while our bodies would dissolve into their component parts.

The Balance and Centre of Gravity.—We have spoken at some length about Gravity, and now we must say something respecting that point called[Pg 26] the Centre of Gravity, and the Balance, and upon the latter we have a few remarks to make first, for a well-adjusted balance is a most useful thing, and we will show you how to make one, and then proceed to our illustrations of the Centre of Gravity, and explain it.

Fig. 19.—Torsion balance, which can easily be constructed, capable of weighing a milligram one-tenth of full size.

All those who cultivate experimental science are aware that it is useful to unite with theoretical ideas that manual dexterity which is acquired by the student accustoming himself to practical operations. One cannot too strongly urge both chemists and physicists to exercise themselves in the construction of the appliances they require, and also to modify those already existing, which may be adapted to their wants. In a large number of cases it is possible to manufacture, at small expense, delicate instruments, capable of rendering the same service as the most elaborate apparatus. Important scientific labours have often been undertaken by men whose laboratories were most simple, who, by means of skill and perseverance, knew how to do great things with small resources. A delicate balance, for instance, indispensable alike to chemist and physicist, can be manufactured at little cost in different ways. A thin platinum wire and a piece of wood is all that is needed to make a balance capable of weighing a milligram; and to make a very sensitive hydrostatic balance, little is required besides a glass balloon. Fig. 19 represents a small torsion balance of extreme simplicity. A thin platinum wire is stretched horizontally through two staples, from the wooden supports, A B, which are fixed in a deal board. A very thin, delicate lever, C D, cut in wood, or made with a wisp of straw, is fixed in the centre of the platinum wire by means of a small clip, which secures it firmly. This lever is placed in such a manner that it is raised perceptibly out of the horizontal line. At D is fixed a paper scale, on which is put the weight of a centigram. The lever is lowered to a certain point, slightly twisting the platinum wire. Near the end of the lever a piece of wood, F, is fixed, on which is marked the extreme point of its movements. Ten equi-distant divisions are marked between these two points, which represent the distance traversed by the lever under the weight of the milligram. If a smaller weight than a centigram is placed on the paper scale the lever falls, and balances itself[Pg 27] after a few oscillations. If it falls four divisions, it is evident that the substance weighs four milligrams. Taking a rather thicker platinum wire, to which a shorter lever must be adapted, one can weigh the decigram, and so on. It would be an easy matter, also, to make, on the same model, balances for weighing considerable weights. The platinum wire should be replaced by iron wires of larger diameter, firmly stretched, and the lever should be made of a piece of very resisting wood. One can also, by adaptation, find the exact value of the most trifling weights. By lengthening a very fine platinum wire several yards, and adapting a long, slender lever, it will not be impossible to ascertain the tenth of a milligram. In this latter case the balance can be set when it is wanted.

Fig. 20.—Nicholson’s Areometer, contrived to serve as a balance.

Fig. 20 represents Nicholson’s Areometer, which any one may construct for himself, and which, as it is here represented, constitutes another kind of balance. A glass balloon, filled with air, is hermetically closed with a cork, through which is passed a cylinder of wood, surmounted by a wooden disc, D. The apparatus is terminated at its lower end by a small tray, C, on which one can put pieces of lead in variable quantities. It is then plunged into a glass filled with water. The pieces of lead on the tray, C, are added by degrees, until the stem of the areometer rises almost entirely above the level of the water; it is next passed through a ring, which keeps it in position, and which is fastened to the upper part of the glass by means of four iron wires in the shape of a cross. The stem is divided in such a way that the space comprised in each division represents the volume of a cubic centimetre. Thus arranged, the apparatus constitutes a balance. The object to be weighed is placed on the disc, D, and the areometer sinks in the water, oscillates, and then remains in equilibrium. If the stem sinks five divisions, it is evident that the weight of the object corresponds to that of five cubic centimetres of displaced water, or five grams.

It is obvious, therefore, from the preceding examples, that it is not impossible to construct a weighing apparatus with ordinary and very inexpensive objects. We can, in the same way, show that it is possible to perform instructive experiments with no appliances at all, or, at any rate, with common things, such as everyone has at hand. The lamented Balard, whose loss science has had recently to deplore, excelled in chemical experiments without a laboratory; fragments of broken glass or earthenware[Pg 28] were used by him for improvising retorts, bottles and vases for forming precipitates, and carrying on many important operations.

Scheele also operated in like manner; he knew how to make great discoveries with the humblest appliances and most slender resources. One cannot too earnestly endeavour to imitate such leaders, both in teaching others and instructing oneself.

The laws relating to the weight of bodies, the centre of gravity, and stable or unstable equilibrium, may be easily taught and demonstrated by means of a number of very familiar objects. By putting into the hands of a child a box of soldiers cut in elder-wood, the end of each fixed into half a bullet, we provide him with the means of making some easy experiments on the centre of gravity. According to some authorities on equilibrium, it is not impossible, with a little patience and delicacy of manipulation, to keep an egg balanced on one of its ends. This experiment should be performed on a perfectly horizontal surface, a marble chimney-piece, for example. If one can succeed in keeping the egg up, it is, according to the most elementary principles of physics, because the vertical line of the centre of gravity passes through the point of contact between the end of the egg and the surface on which it rests.

Fig. 21.—Experiment on “centre of gravity.”

Fig. 21 reproduces a curious experiment in equilibrium, which is performed with great facility. Two forks are stuck into a cork, and the cork is placed on the brim of the neck of a bottle. The forks and the cork form a whole, of which the centre of gravity is fixed over the point of support. We can bend the bottle, empty it even, if it contains fluid, without the little construction over its mouth being in the least disturbed[Pg 29] from its balance. The vertical line of the centre of gravity passes through the point of support, and the forks oscillate with the cork, which serves as their support, thus forming a movable structure, but much more stable than one is inclined to suppose. This curious experiment is often performed by conjurors, who inform their audience, that they will undertake to empty the bottle without disturbing the cork. If a woodcock has been served for dinner, or any other bird with a long beak, take off the head at the extreme end of the neck; then split a cork so that you can insert into it the neck of the bird, which must be tightly clipped to keep it in place; two forks are then fixed into the cork, exactly as in the preceding example, and into the bottom of the cork a pin is inserted. This little contrivance is next placed on a piece of money, which has been put on the opening of the neck of the bottle, and when it is fairly balanced, we give it a rotatory movement, by pushing one of the forks as rapidly as we please, but as much as possible without any jerk (fig. 22). We then see the two forks, and the cork surmounted by the woodcock’s head, turning on the slender pivot of a pin. Nothing can be more comical than to witness the long beak of the bird turning round and round, successively facing all the company assembled round the table, sometimes with a little oscillation, which gives it an almost lifelike appearance. This rotatory movement will last some time, and wagers are often laid as to which of the company the beak will point at when it[Pg 30] stops. In laboratories, wooden cylinders are often to be seen which will ascend an inclined plane without any impulsion. This appears very surprising at first, but astonishment ceases when we perceive that the centre of gravity is close to the end of the cylinder, because of a piece of lead, which has been fixed in it.

Fig. 22.—Another experiment on the same subject.

Fig. 23.—Automatic puppets.

Fig. 23 gives a very exact representation of a plaything which was sold extensively on the Boulevards at Paris at the beginning of the New Year. This little contrivance, which has been known for some time, is one of the most charming applications of the principles relating to the centre of gravity. With a little skill, any one may construct it for himself. It consists of two little puppets, which turn round axles adapted to two parallel tubes containing mercury. When we place the little contrivance in the position of fig. 24, the mercury being at a, the two dolls remain motionless, but if we lower the doll S, so that it stands on the second step (No. 2) of the flight, as indicated in fig. 25, the mercury descends to b at the other end of the tube; the centre of gravity is suddenly displaced; the doll R then accomplishes a rotatory movement, as shown by the arrow in fig. 25, and finally alights on step No. 3. The same movement is also effected by the doll S, and so on, as many times as there are steps. The dolls may be replaced by a hollow cylinder of cartridge paper closed at both ends, and containing a marble; the cylinder, when placed vertically on an inclined[Pg 31] plane, descends in the same way as the puppets. The laws of equilibrium and displacement of the centre of gravity, are rigorously observed by jugglers, who achieve many wonderful feats, generally facilitated by the rotatory motion given to the bodies on which they operate, which brings into play the centrifugal force. The juggler who balances on his forehead a slender rod, on the end of which a plate turns round, would never succeed in the experiment if the plate did not turn on its axis with great rapidity. But by quick rotation the centre of gravity is kept near the point of support. We need hardly remark, too, that it is the motion of a top that tends to keep it in a vertical position.

Fig. 24.—First position of the puppets.

Fig. 25.—Second position of the puppets.

Many experiments in mechanical physics may occur to one’s mind. To conclude the enumeration of those we have collected on the subject, I will describe the method of lifting a glass bottle full of water by means of a simple wisp of straw. The straw is bent before being passed into the bottle of water, so that, when it is lifted, the centre of gravity is displaced,[Pg 32] and brought directly under the point of suspension. Fig. 26 shows the method of operation very plainly. It is well to have at hand several pieces of straw perfectly intact, and free from cracks, in case the experiment does not succeed with the first attempt.

Having now seen how this point we call the centre of gravity acts, we may briefly explain it.

Fig. 26.—Lifting a bottle with a single straw.

The centre of gravity of a body is that point in which the sum of the forces of gravity, acting upon all the particles, may be said to be united. We know the attraction of the earth causes bodies to have a property we call Weight. This property of weight presses upon every particle of the body, and acts upon them as parallel forces. For if a stone be broken all the portions will equal the weight of the stone; and if some of them be suspended, it will be seen that they hang parallel to each other, so we may call these weights parallel forces united in the whole stone, and equal to a single resultant. Now, to find the centre of gravity, we must suspend the body, and it will hang in a certain direction. Draw a line from the point of suspension, and suspend the body again: a line drawn from that point of suspension will pass through the same place as the former line did, and so on. That point is the centre of gravity of that suspended body. If the form of it be regular, like a ball or cylinder, the centre of gravity is the same as the mathematically central point. In such forms as pyramids it will be found near the largest[Pg 33] mass; viz., at the bases, about one-fourth of the distance between the apex and the centre of gravity of the base.

When the centre of gravity of any body is supported, that body cannot fall. So the well-known leaning towers are perfectly safe, because their lines of direction fall within the bases. The centre of gravity is in the centre of the leaning figure. The line of direction drawn vertically from that point falls within the base; but if the tower were built up higher, so that the centre of gravity were higher, then the structure would fall, because the line of direction would fall without the base.

We see that animals (and men) are continually altering the position of the centre of gravity; for if a man bears a load he will lean forward, and if he takes up a can of water in one hand he will extend the other to preserve his balance or equilibrium.

Fig. 27.—Balancing a weight on a nail and key.

The experiment shown in the accompanying illustration is apparently very difficult, but it will be found easy enough in practice if the hand be steady. Take a key, and by means of a crooked nail, or “holdfast,” attach it to a bar of wood by a string tied tightly round the bar, as in the picture. To the other extremity of the bar attach a weight, and then drive a large-headed nail into the table. It will be found that the key will balance, and even move upon the head of the nail, without falling. The weight is under the table, and the centre of gravity is exactly beneath the point of suspension.

[Pg 34]

Another simple experiment may prove amusing. Into a piece of wood insert the points of two knives, and at the centre of the end of the bar insert a needle between the knife handles. The wood and the knives may then be balanced on another needle fixed in a cork at A.

Fig. 28.—Another experiment.

We may conclude this chapter by summing up in a few words what the Centre of Gravity is. We can define it as “that point in a body upon which the body, acted on solely by the force of gravity, will balance itself in all positions.” Such a point exists in every body, and equally in a number of bodies fastened tightly together. The Centre of Gravity has by some writers been denominated the Centre of Parallel Forces, or the Centre of Magnitude, but the Centre of Gravity is the most usual and best understood term.

[Pg 35]

CHAPTER IV.

SOME PROPERTIES OF SOLID BODIES—INERTIA—MOTION—FRICTION—THE PENDULUM—EQUILIBRIUM.

Those who have followed us through the preceding pages have now, we hope, some ideas upon Gravity and the Forces of Nature. In speaking of Forces we said “Force was a cause of Motion.” Let us now consider Inertia, and Motion with its accompanying opponent, Friction.

Fig. 29.—Shock communicated by elasticity.

Inertia is the passiveness of Matter. This perfect indifference to either rest or motion makes the great distinction between living and lifeless matter. Inertia, or Vis Inertia, is this passiveness. Now, to overcome this indifference we must use force, and when we have applied force to matter we set it in motion; that is, we move it. When we move it we find a certain resistance which is always proportionate to the force applied. In mechanics this is termed Action, and Reaction, which are always equal forces acting in opposite directions. This is Newton’s law, and may be explained by a “weight” on a table, which presses against the table with the same force with which[Pg 36] the table presses against the “weight”; or when you strike a ball, it strikes the hand with the same force.

We can communicate motion by elasticity. For instance, if we place a number of coins upon a table touching each other and in a straight line, and strike the last coin of the line by pushing another sharply against it, the piece at the opposite extremity will slip out of its place from the effect of the shock transmitted by the coin at the other end (fig. 29).

Fig. 30.—Experiment to illustrate inertia.

When two forces act upon a body at the same time, it takes a direction intermediate. This is known as the resultant. The enormous forces exercised[Pg 37] by the heavenly bodies will be treated of later. We will first consider Inertia.

There are several experiments relating to the subject of Inertia which may be performed. I once witnessed one quite accidentally when taking a walk.

Fig. 31.—Another experiment on the same subject.

I was one day passing the Observatory at Paris, when I noticed a number of people collected round a professor, who after executing several juggling tricks, proceeded to perform the curious experiment I am about to describe. He took a broomstick and placed it horizontally, passing the ends through two paper rings. He then asked two children to hold the paper rings by means of two razors, so that the rings rested on the blade. This done, the operator took a stout stick, and, with all his strength, struck the broomstick in the centre; it was broken into shivers, but the paper rings were not torn in the least, or even cut by the razors! One of my friends, M. M——, a painter, showed me how to perform this experiment as represented in the illustration (fig. 30). A needle is fixed at each end of the broomstick, and these needles are made to rest on two glasses, placed on chairs; the needles alone must be in contact with the glasses. If the broomstick is then struck violently with another stout stick, the former will be broken, but the glasses will remain intact. The experiment answers all the better the more energetic the action. It is explained by the resistance of[Pg 38] inertia in the broomstick. The shock suddenly given, the impulse has not time to pass on from the particles directly affected to the adjacent particles; the former separate before the movement can be transmitted to the glasses serving as supports.8

Fig. 32.—Extracting a “man” from a pile of draughts without overturning the pile.

The experiment represented in fig. 31 is of the same nature. A wooden ball is suspended from the ceiling by a rather slender thread, and a similar thread is attached to the lower end of the ball. If the lower thread is pulled forcibly it will break, as shown in the illustration; the movement communicated to it has not time to pass into the ball; if, on the contrary, it is pulled very gradually and without any shock, the upper thread instead will break, because in this case it supports the weight of the ball. Motion is not imparted simultaneously to all parts of a body, but only to the particles first exposed to a blow, for instance. One might multiply examples of this. If a bullet be shot from a gun, it will make a round hole in a piece of wood or glass, whilst if thrown by the hand,—that is to say, with much less force,[Pg 39]— it will shiver the wood or the pane of glass to pieces. When the celerity of the motive force is very great, the particles directly affected are disturbed so quickly that they separate from the adjacent particles before there is time for the movement to be communicated to the latter.

It is possible, for the same reason, to extract from a pile of money a piece placed in the middle of the pile without overturning the others. It suffices to move them forcibly and quickly with a flat wooden ruler. The experiment succeeds very well also if performed with draughtsmen piled up on the draught-board (fig. 32).

Fig. 33.—Calling out a sixpence from the glass.

Fig. 33 represents another experiment which belongs to the laws of resisting force. A sixpence is placed on a table covered with a cloth or napkin. It is covered with a glass, turned over so that its brim rests on two penny pieces. The problem to be solved is how to extract the sixpence from underneath the glass without touching it, or slipping anything beneath it. To do this it is necessary to scratch the cloth with the nail of the forefinger; the elasticity of the material communicates the movement to the sixpence, which slowly moves in the direction of the finger, until it finally comes out completely from beneath the glass.

We may give another experiment concerning Inertia. Take a strip of paper, and upon it place a coin, on a marble chimney-piece, as in the illustration. If, holding the paper in the left hand, you strike it rapidly and[Pg 40] forcibly, you will be enabled to draw away the paper without causing the coin (say a five-shilling-piece) to fall down (fig. 34).

It is not impossible to draw away a napkin laid as a tablecloth for one person’s dinner, without disturbing the various articles laid upon it. A quick motion is all that is necessary, keeping the napkin tightly extended by the hands at the same time. This latter experiment, however, is not recommended to boys home for the holidays, as they may unwillingly practise a feat analogous to that executed by Humpty-Dumpty, and find equal difficulty to match the pieces.

Fig. 34.—Drawing a slip of paper from beneath a coin.

We will now examine the term Motion. A body is said to be in motion when it changes its position in relation to surrounding objects. To perceive motion the surrounding objects must be relatively at rest, for if they all hurried along at the same rate no motion would be perceptible. This is evident, for when we stand still trees and houses appear stationary, as do we ourselves, but we know we all are rushing round with the earth, though our relative positions are unchanged. Hence there is no absolute rest.

What are the causes of motion?—Gravity is one. The influence of heat, which is itself caused by the motion of atoms, the effects of electricity, etc., and finally, the power of force in men or animals—any of these causes will produce motion. But a body at rest cannot put itself in motion, nor can a body in motion stop itself, or change its condition of motion.

[Pg 41]

But you may say a body will stop itself. Your ball on the ground, or even upon ice, will eventually come to a stop. We fire a bullet, and it will stop in time. We reply it does not stop of itself, The resistance of the Air and Friction tend to bring the body in motion to a state of rest. In the case of a bullet gravity brings it down.

There is no need to insist upon the resistance offered by the air even when it is not rushing violently past to fill up a vacuum beyond us, and called a breeze, or high wind. But we may say something of Friction.

Friction is derived from the Latin frico, to rub, and expresses the resistance to motion which arises from uneven surfaces. It is a passive resistance, and depends upon the force which keeps the bodies together. Thus a train running upon a smooth iron rail would never be able to proceed but for friction, which gives the necessary purchase or grip to the wheel and rail in contact.

No surface is perfectly smooth, for we must push a body upon the smoothest surface we possess. Friction tends to resist motion always, and is the cause of a great loss of power in mechanics, though it is employed to stop motion by certain appliances, such as “brakes” and “drags,” for sliding friction is greater than rolling friction. But without friction most structures would fall to pieces, and all forward motion would cease. So though it is an inconvenient force to overcome, we could not do without it.

If a body is set in motion, we see that the tendency of it is to go on for ever. Such, indeed, is the case with the stars; but so long as we are within the influence of the earth’s attraction, we cannot expect such a result. We know now what motion is; we must also, to understand it perfectly, consider its direction and its velocity.

The line which indicates the way from the starting point to the end is the direction of the object in motion, and the rate it moves at its velocity. The latter is calculated at so many miles an hour, as a train; or so many feet in a second if the object be a shot, or other very rapidly-moving body. In equal velocity the same distance is traversed in the same time; and so if a train run a mile in a minute, we know it will travel sixty miles in an hour, and is therefore during that minute going at the rate of sixty miles an hour. We have already spoken of the velocity of a stone falling from a cliff as sixteen feet in a second, and a stone thrown into the air to rise sixteen feet will be a second in going up, and a second in descending. But the velocity will be accelerated in the descent after the first second of time, and retarded in the upward cast by gravity. So we have two terms—accelerated and retarded velocity—used to express an increased or decreased force of attraction.

Perpetual motion has often been sought, but never discovered, nor will it ever be till the elixir of life has been found. It is quite impossible to construct any machine that will work without friction; if any work be done energy will be expended and transformed into other energy, so the total must be diminished by so much as was employed to transform the remainder.[Pg 42] No body can give unlimited work, therefore the perpetual motion theory is untenable and impossible.

Fig. 35.—The pendulum.

The pendulum is considered the nearest approach to perpetual motion. This is so well known that no description is needed, but we may say a few words concerning it. By the diagram, we see that if we lift the ball to b, and let it fall, it will descend to l, and pass it to a opposite, nearly as far from l as b is from it. So the oscillations will continue, each beat being less and less, till rest is reached by the action of gravity (page 23). Were it not for friction and the pressure of the air, the oscillations would continue for ever; as it is, it declines by shorter swings till it remains in equilibrium.

The seconds’ pendulum oscillates sixty times an hour, and must be of a certain length in certain places. In London it is 39·1393 inches, and furnishes a certain standard of length, and by an Act of Parliament the yard is divided into 36 parts, and 39·1393 such parts make the seconds’ pendulum in the latitude of London (in vacuo) in a temperature of 62°.

Fig. 36.—Centrifugal Force.

But the same pendulum will not perform the same number of oscillations in one minute in all parts of the globe. At the equator they will be[Pg 43] less, and at the pole more. Thus it was discovered that, as the movements of the pendulum are dependent upon the force of gravity, and as this force decreases the farther we get from the centre of the earth, the equator must be farther from the earth’s centre than the poles, and therefore the poles must be depressed. The decline of the pendulum at the equator is also, in a measure, due to Centrifugal Force.

Centrifugal Force, which means “flying from the centre,” is the force which causes an object to describe a circle with uniform velocity, and fly away from the centre; the force that counteracts it is called the centripetal force. A very simple experiment will illustrate it.

Fig. 37.—Another illustration of centrifugal force.

To represent its action, we shall have recourse to an ordinary glass tumbler placed on a round piece of cardboard, held firmly in place by cords. Some water is poured in the glass, and we then show that it can be swung to and fro and round without the water being spilt, even when the glass is upside down (fig. 36).

Another experiment on the same subject is as shown in the above illustration, by which a napkin ring can be kept in revolution around the forefinger, and by a continued force the ring may be even held suspended at the tip of the finger, apparently in the air, without support (fig. 37).

[Pg 44]

CHAPTER V.

GASES AND LIQUIDS—PRESSURE OF THE AIR—EXPERIMENTS.

We have more than once referred to the pressure of the air which exerts a great influence upon bodies in motion, but a few experiments will make this more obvious, and clearly demonstrate the fact. We have also told you some of the properties of Solids, such as Weight, Inertia, Friction, and Resistance, or Strength. Solids also, as we have seen, occupy space, and cannot be readily compressed, nor bent to other shapes. Now the subject of the Pressure of the Air leads us to the other forms of Matter; namely, Gases and Liquids, which will be found very interesting to study.

Fig. 38.—Blowing an egg from one glass to another.

The force of air can very soon be shown as acting with considerable[Pg 45] pressure upon an egg in a glass. By blowing in a claret glass containing a hard-boiled egg, it is possible to cause the egg to jump out of the glass; and with practice and strength of lungs it is not impossible to make it pass from one glass to another, as per illustration (fig. 38).

The force of heated air ascending can also be ascertained by cutting up a card into a spiral, and holding it above the flame of a lamp (fig. 39). The spiral, if lightly poised, will turn round rapidly.

Now let us turn to a few experiments with the air, which is composed in two gases, Oxygen and Nitrogen, of which we shall hear more when we come to Chemistry.

Fig. 39.—Movement of heated air.

Fig. 40.—Pressure of the air.

It is not intended here to prosecute researches, but rather to sketch a programme for instruction, based on amusing experiments in Physics, performed without apparatus. The greater part of these experiments are probably well known, and we desire to say that we merely claim to have collected and arranged them for our descriptions. We must also add that we have performed and verified these experiments; the reader, therefore, can attempt them with every certainty of success. We will suppose that we are addressing a young auditory, and commence our course of Physics with some facts relating to the pressure of air. A wine glass, a plate, and water, will serve for our first experiments. Pour some water on the plate, light a piece of paper resting on a cork, and cover the flame with the glass[Pg 46] which I turn upside down (fig. 40). What follows?—The water rises in the glass. Why?—Because the burning of the paper having absorbed a part of the oxygen, and the volume of confined gas being diminished, the pressure of the outer air has driven back the fluid. I next fill a goblet with water up to the brim, and cover it with a sheet of paper which touches both the edge of the glass and the surface of the water. I turn the glass upside down, and the sheet of paper prevents the water running out, because it is held in place by atmospheric pressure (fig. 41). It sometimes happens that this experiment does not succeed till after a few attempts on the part of the operator; thus it is prudent to turn the glass over a basin, so that, in case of failure, the water is not spilt. Having obtained a vase and a bottle, both quite full of water, take the bottle, holding it round the neck so that the thumb can be used as a stopper, then turn it upside down, and pass the neck into the water in the vase. Remove your thumb, or stopper, keeping the bottle in a vertical position, and you will see that the water it contains does not escape, but remains in suspension. It is atmospheric pressure which produces this phenomenon. If, instead of water, we put milk in the bottle, or some other fluid denser than water, we shall see that the milk also remains suspended in the bottle, only there is a movement of the fluid in the neck of the bottle, and on careful examination we perceive very plainly that the milk descends to the bottom of the vase, and the water rises into the bottle. Here, again, it is atmo[Pg 47]spheric pressure which maintains the fluid in the bottle, but the milk descends, because fluids are superposed according to their order of density, and the densest liquid falls to the bottom.

This can be verified by means of the phial of the four elements, which is a plain, long, and narrow bottle, containing equal volumes of metallic mercury, salt water, alcohol, and oil. These four liquids will lie one on the top of the other without ever mixing, even if shaken.

Another experiment as to the pressure of the air may be made (fig. 42). Take a penny and press it against some oaken bookcase or press, rub the coin against the wood for a few seconds, then press it, and withdraw the fingers. The coin will continue to adhere to the wood. The reason of this is, because by the rubbing and the pressure you have dispersed the film of air which was between the penny and the wood, and under those conditions the pressure of the atmospheric air was sufficient to keep the penny in its place.

Fig. 41.—Pressure of the air.

Or, again, let us now add a water-bottle and a hard-boiled egg to our appliances; we will make use of the air-pump, and easily perform another experiment. I light a piece of paper, and let it burn, plunging it into a water-bottle full of air. When the paper has been burning a few seconds I close the opening of the water-bottle by means of a hard-boiled egg, which I have previously divested of its shell, so that it forms a hermetic stopper. The burning of the paper has now caused a vacuum of air in the bottle, and[Pg 48] the egg is gradually thrust in by the atmospheric pressure outside. Fig. 43 exhibits it slowly lengthening and stretching out as it passes through the aperture; then it is suddenly thrust completely into the bottle with a little explosive sound, like that produced by striking a paper bag expanded with air. This is atmospheric pressure demonstrated in the clearest manner, and at little cost.

Fig. 42.—Coin adhering by pressure of air.

If it is desired to pursue a little further the experiments relating to atmospheric pressure, it will be easy enough to add to the before-mentioned appliances a closed glass-tube and some mercury, and one will then have the necessary elements for performing Torricelli’s and Pascal’s experiments, and explaining the theory of the barometer (page 52).

An amusing toy, well-known to schoolboys, called the “sucker,” may also be made the object of many dissertations on the vacuum and the pressure of air. It is composed of a round piece of soft leather, to the centre of which is attached a small cord. This leather is placed on the ground and pressed under foot, and when the cord is pulled it forms a cupping-glass, and is only separated with difficulty from the pavement.

Atmospheric air, in common with other gases, has a tendency to fill any space into which it may enter. The mutual attraction of particles of air is nil; on the contrary, they appear to have a tendency to fly away from each other; this property is called “repulsion.” Air also possesses an expansive[Pg 49] property—a tendency to press against all the sides of any vessel in which it may be enclosed. Of course the larger the vessel containing a given quantity of air, the less actual pressure it will exert on the sides of the vessel. The elasticity of air therefore decreases with increasing expansion, but it gains in elasticity or force when compressed.

There is a law in Physics which expresses the relation between expansion and elasticity of gases, which may be said to be as follows:—

The elasticity (of a gas) is in inverse ratio to the space it occupies, and therefore by compressing air into a small space we can obtain a great force, as in the air-gun and the pop-gun of our youthful days.

Fig. 43.—Hard boiled egg, divested of its shell, passing through the neck of a glass bottle, under the influence of atmospheric pressure.

In the cut below we can illustrate the principle of the pop-gun. The chamber full of air is closed by a cork and by an air-tight piston (S) at p and p. When the piston is pushed into the chamber the air is compressed between it and the stopper, which at length flies out forcibly with a loud report

Fig. 44.—The principle of the pop-gun.

We have said that the tendency of air particles is to fly away from each other, and were it not for the earth’s attraction the air might be dispersed. The height of the atmosphere has been variously estimated from a height of[Pg 50] 45 miles to 212 miles in an attenuated form; but perhaps 100 miles high would be a fair estimate of the height to which our atmosphere extends.

Fig. 45.—Weighing the air.

The pressure of such an enormous body of gas is very great. It has been estimated that this pressure on the average human body amounts to fourteen tons, but being balanced by elastic fluids in the body, the inconvenience is not felt. The Weight of Air can easily be ascertained, though till the middle of the seventeenth century the air was believed to be without weight. The accompanying illustration will prove the weight of air. Take an ordinary balance; and suspend to one side a glass globe fitted with a stop-cock. From this globe extract the air by means of the air-pump, and weigh it. When the exact weight is ascertained turn the stop-cock, the air will rush in, and the globe will then pull down the balance, thus proving that air possesses weight. The experiments of Torricelli and Otto von Guerike, however, demonstrated that the air has weight and great pressure. Torricelli practically invented the barometer, but Otto von Guerike, by the cups known as Magdeburg Hemispheres, proved the pressure of the outward[Pg 51] air. This apparatus is well known, and consists of two hollow copper hemispheres which fit very closely. By means of the air-pump which he invented in 1650, Otto von Guerike exhausted the air from the closed hemispheres. So long as air remained in them, there was no great difficulty in separating them; but when it had been finally exhausted, the pressure of the surrounding atmosphere was so great that the hollow spheres could not be dragged asunder even by horses harnessed to rings which had been inserted in the globes.

Fig. 46.—Magdeburg Hemispheres.

The Air-Pump is a very useful machine, and we will now briefly explain its action. The inventor was, as remarked above, Otto von Guerike, of Magdeburg. The pump consists of a cylinder and piston and rod, with two valves opening upwards—one valve being in the bottom of the cylinder, the other in the piston. This pump is attached by a tube to a plate with a hole in it, one extremity of the tube being fixed in the centre of the plate, and the other at the valve at the bottom of the cylinder. A glass shade, called the receiver, is placed on the top of the plate, and of course this shade will be full of air (fig. 47).

Fig. 47.—The air-pump.

When the receiver is in position, we begin to work the pump. We have said there are two valves. So when the piston is drawn up, the cylinder would be quite empty did not the valve at the bottom, opening upwards, admit some air from the glass shade through the tube to enter the cylinder. Now the lower part of the cylinder is full of air drawn from the glass shade. When we press the piston down again, we press against the air in it, which, being compressed, tries to escape. It cannot go back, because the valve at the bottom of the cylinder won’t open, so it escapes by the valve in the piston, and goes away. Thus a certain amount of air is got rid of at each stroke of the piston. Two cylinders and pistons can be used, and so by means of cog-wheels, etc., the air may be rapidly exhausted from the receiver. Many experiments are made with the assistance of the air-pump and receiver, though the air is never entirely exhausted from the glass.

The “Sprengel” air-pump is used to create an almost perfect vacuum, by putting a vessel to be exhausted in connection with the vacuum at the top of a tube of mercury thirty inches high. Some air will bubble out, and the mercury will fall. By filling up again and repeating the process, the air vessel will in time be completely exhausted. This is done by Mr. Sprengel’s pump, and a practically perfect vacuum is obtained, like the Torricellian vacuum.

The “Torricellian vacuum” is the empty space above the column of mercury in the barometer which we will proceed to describe. Air has a certain weight or pressure which is sufficient to raise a column of mercury thirty[Pg 52] inches. We will prove this by illustration. Take a bent tube and fill it with mercury; the liquid will stand equally high in both arms, in consequence of the ratio of equilibrium in fluids, of which we shall read more when we come to consider Water. So the two columns of mercury are in equilibrium. (See A.) Now stop the arm a with a cork, and take out half the mercury. It will remain in one arm only. Remove the cork, and the fluid will fall in both arms, and remain in equilibrio. If a long bent glass tube be used, the arms being thirty-six inches high, the mercury will fall to a point c, which measures 29·9 inches from the bottom. If the tube be a square inch in bore, we have 29·9 cubic inches of mercury, weighing 14⅘ lbs., balancing a column of air one square inch thick and as high as the atmosphere. So the mercury and the column of air must weigh the same. Thus every square inch on the earth supports a weight of (nearly) 15 lbs (figs. 48 and 50).

Fig. 48.—Air Pressure.

The barometer invented by Pascal, working on the investigations of Torricelli, is a very simple and useful instrument. Fill a tube with mercury from which all moisture has been expelled, and turn it over in a dish[Pg 53] of mercury; the mercury will rise to a certain height (30 inches), and no higher in vacuo. When the pressure of the air increases the mercury rises a little, and falls when the pressure is removed. Air charged with aqueous vapour is lighter than dry air, so a fall in the mercury indicates a certain amount of water-vapour in the air, which may condense and become rain. The action of mercury is therefore used as a weather-glass, by which an index-point shows the movements of the fluid, by means of a wheel over which a thread passes, sustaining a float and a counterpoise. When the mercury rises the float goes up, and the weight falls, and turns the wheel by means of the thread. The wheel having a pointer on the dial tells us how the mercury moves. This weather-glass is the usual syphon barometer with the float on the surface and a weight (fig. 50).

Fig. 49.—The Barometer.

Fig. 50.—Syphon barometer.

The Syphon Barometer is a bent tube like the one already shown, with one limb much shorter than the other.

The Aneroid Barometer, so called because it is “without moisture,” is now in common use. In these instruments the atmospheric pressure is held in equilibrium by an elastic metal spring or tube. A metal box, or tube, is freed from air, and then hermetically sealed. This box has a flexible side, the elasticity of which, and the pressure of the air on it, keep each other in equilibrium. Upon this elastic side the short arm of a lever is pressed, while the longer arm works an index-point, as in the circular barometer. When pressure increases the elastic box “gives”; when pressure diminishes it returns to its former place, and the index moves in the opposite direction. It is necessary to compare and “set” the aneroid with the mercurial barometer to ensure correctness. A curved tube is sometimes used, which coils and uncoils like a spring, according to the pressure on it.

Fig. 51.—The Water Barometer.

There are other barometers, such as the Water Barometer, which can be fixed against the side of a house, and if the water be coloured, it will prove a useful indicator. As the name indicates, water[Pg 54] is used instead of mercury, but as the latter is thirteen-and-half times heavier than water, a much longer tube is necessary; viz., one about thirty-five feet in length. The construction is easy enough. A leaden pipe can be fixed against the house; on the top is a funnel furnished with a stop-cock, and placed in a vase of water. The lower part of the tube is bent, and a glass cylinder attached, with another stop-cock—the glass being about three feet long, and graduated. Fill the tube with water, shut the upper stop-cock, and open the lower one. The vacuum will be formed in the top of the tube, and the barometer will act on a larger scale than the mercury.

The Glycerine Barometer, invented by Mr. Jordan, and in use at the Times office, registers as more than one inch movements which on the mercurial thermometer are only one-tenth of an inch, and so are very distinctly visible. The specific gravity of pure glycerine is less than one-tenth that of mercury, so the mean height of the glycerine column is twenty-seven feet at sea level. The glycerine has, however, a tendency to absorb moisture from the air, but Mr. Jordan, by putting some petroleum oil upon the glycerine, neutralized that tendency, and the atmospheric pressure remains the same. A full description of this instrument was given in the Times of 25th October, 1880.

Fig. 52.—The principle of the diving-bell.

The uses of the barometer are various. It is employed to calculate the heights of mountains; for if a barometer at sea level stand at 30", it will[Pg 55] be lower on a mountain top, because the amount of air at an elevation of ten thousand feet is less than at the level of the sea, and consequently exercises less pressure, and the mercury descends. [The pressure is on the bulb of mercury at the bottom, not on the top, remember.]

The pressure of the air at the tops of mountains sometimes decreases very much, and it is not sufficiently dense for perfect respiration, as many people find. Some climbers suffer from bleeding at the nose, etc., at great altitudes. This is occasioned by the action of the heart, which pumps with great force, and the outward pressure upon the little veins being so much less than usual, they give way.

Fig. 53.—Diver under water.

Many important instruments depend upon atmospheric pressure. The most important of these is the pump, which will carry us to the consideration of water and Fluids generally. The fire-engine is another example, but we will now proceed to explain the diving-bell already referred to.

[Pg 56]

Fig. 52 represents the experiment of the diving-bell, which is so simple, and is explained below. It belongs to the same category of experiments as those relating to the pressure of air and compression of gas. Two or three flies have been introduced into the glass, and they prove by their buzzing about that they are quite at their ease in the rather confined space.

The Diving-Bell in a crude form appears to have been used as early as 1538. It was used by two Greeks in the presence of the Emperor Charles V., and numerous spectators. In the year 1720 Doctor Halley improved the diving-bell, which was a wooden box or chamber open at the bottom. Air casks were used to keep the inmate supplied with air. The modern diving-bell was used by Smeaton in 1788, and was made of cast iron. It sinks by its own weight. The pressure of the air inside is sufficient to keep the water out. Air being easily compressed, it is always pumped in to keep the hollow iron “bell” full, and to supply the workmen. There are inventions now in use by which the diver carries a supply of air with him on his back, and by turning a tap can supply himself for a long time at a distance from the place of descent, and thus is able to dispense with the air-tube from the boat at the surface. This apparatus was exhibited at the Crystal Palace some years ago.

Fig. 54.—The Hand Fire-Engine

The Pump.

We have seen in the case of the Water Barometer that the pressure of the air will sustain a column of water about thirty feet high. So the distance between the lower valve and the reservoir or cistern must not be more than thirty-two feet, practically the distance is about twenty-five feet in pumps.

[Pg 57]

We can see by the illustration that the working is much the same as in the air-pump. The suction pipe B is closed by the valve C, the cylinder D and spout E are above, the piston rod F lifts the air-tight piston in which is a valve H. When the piston is raised the valve C opens and admits the water into the cylinder. When the piston is depressed the valve C is closed, the water already in forces H open, and passing through the piston, reaches the cylinder and the spout (fig. 55).

The hand fire-engine depends upon the action of compressed air, which is so compressed by pumping water into the air chamber a. The tube is closed at g, and the pumps e e drive water into the air chamber. At length the tap is opened, and the air drives the water out as it is continually supplied (fig. 54).

Compressed air was also used for driving the boring machines in the Mount Cenis tunnel. In this case also the air was compressed by water, and then let loose, like steam, to drive a machine furnished with boring instruments.

Fig. 55..—The Pump.

A pretty little toy may be made, and at the same time exemplify an interesting fact in Physics. It is called the ludion, and it “lies in a nut shell” in every sense. When the kernel has been extracted from the shell, fasten the portions together with sealing wax, so that no water can enter. At one end O, as in the illustration, leave a small hole about as large as a pin’s head; fasten two threads to the sealing wax, and to the threads a wooden doll. Let a weight be attached to his waist. When the figure is in equilibrium, and will float, put it into a jar of water, and tie a piece of bladder over the top. If this covering be pressed with the finger, the doll will descend and remount when the finger is removed. By quick successive pressure the figure may be made to execute a pas seul. The reason of the movement is because the slight cushion of air in the upper part of the vase is compressed, and the little water thus caused to enter the nut shell makes it heavier, and it descends with the figure (fig. 56).

We have now seen that air is a gas, that it exercises pressure, that it possesses weight. We know it can be applied to many useful purposes, and that the air machines and inventions—such as the air-pump and the “Pneumatic Despatch”—are in daily use in our laboratories, our steam engines, our condensed milk manufactories, and in many other industries, and for our social benefit. Compressed air is a powerful motor for boring machinery in tunnels where steam cannot be used, even if water could be supplied, for smoke or fire would suffocate the workers. To air we owe our life and our happiness on earth.

Pneumatics, then, deals with the mechanical properties of elastic fluids[Pg 58] represented by air. A gas is an elastic fluid, and differs very considerably, from water; for a gas will fill a large or small space with equal convenience, like the genii which came out of the bottle and obligingly retired into it again to please the fisherman. We have seen that the pressure of the air is 14⅘ per square inch at a temperature of 32°. It is not so easy to determine the pressure of air at various times as that of water. We can always tell the pressure of a column of water when we find the height of the column, as it is the weight of so many cubic inches of the liquid. But the pressure of the atmosphere per square inch at any point is equal to the weight of a vertical column of air one inch square, reaching from that point to the limit of the atmosphere above it. Still the density is not the same at all points, so we have to calculate. The average pressure at sea level is 14·7 per square inch, and sustains a column of mercury 1 square inch in thickness, 29·92, or say 30 inches high. These are the data upon which the barometer is based, as we have seen.

Fig: 56.—The “Ludion.”

In our article upon “Chemistry” we will speak more fully of the atmosphere and of its constituents, etc.

[Pg 59]

CHAPTER VI.

ABOUT WATER—HYDROSTATICS AND HYDRAULICS—LAW OF ARCHIMEDES—THE BRAMAH PRESS—THE SYPHON.

At present we will pass from Air to Water, from Pneumatics to Hydrostatics and Hydraulics. We must remember that Hydrostatics and Hydraulics are very different. The former treats of the weight and pressure of liquids when they are at rest, the latter treats of them in motion. We will now speak of the properties of Liquids, of which Water may be taken as the most familiar example.

We have already seen that Matter exists in the form of Solids, Liquids, and Gases, and of course Water is one form of Matter. It occupies a certain space, is slightly compressible; it possesses weight, and exercises force when in motion. It is a fluid, but also a liquid. There are fluids not liquid, such as air or steam, to take equally familiar examples. These are elastic fluids and compressible, while water is inelastic, and termed incompressible.

The chemical composition of water will be considered hereafter, but at present we may state that water is composed of oxygen and hydrogen, and proportions of eight of the former to one of the latter by weight; in volume the hydrogen is as two to one.

From these facts, as regards water, we learn that volume and weight are very different things,—that equal volumes of various things may have different weights, and that volume (or bulk) by no means indicates weight Equal volumes of feathers and sand will weigh very differently.

[The old “catch” question of the “difference in weight between a pound of lead and a pound of feathers” here comes to the mind. The answer generally given is that “feathers make the heavier ‘pound’ because they are weighed by avoirdupois, and lead by troy weight.” This is an error. They are both weighed in the same way, and pound for pound are the same weight, though different in volume.]

Fluids in equilibrium have all their particles at the same distance from the centre of the earth, and although within small distances liquids appear perfectly level (in a direct line), they must, as the sea does, conform to the shape of the earth, though in small levels the space is too limited to admit of any deviation from the plane at right angle to the direction of gravity.

Liquids always fall to a perfectly level surface, and water will seek to find its original level, whether it be in one side of a bent tube, in a watering pot and its spout, or as a fountain. The surface of the water will be on the same level in the arms of a bent tube, and the fountain will rise to a height[Pg 60] corresponding with the elevation of the parent spring whence it issues. The waterworks companies first pump the water to a high reservoir, and then it rises equally high in our high-level cisterns.

As an example of the force of water, a pretty little experiment may be easily tried, and, as many of our readers have seen in a shop in the Strand in London, it always is attractive. A good-sized glass shade should be procured and placed over a water tap and basin, as per the illustration herewith. Within the glass put a number of balls of cork or other light material. Let a stop-cock, with a small aperture, be fixed upon the tube leading into the glass. Another tube to carry away the water should, of course, be provided, but it may be used over again. When the tap is properly fixed, if the pressure of the water be sufficient, it will rush out with some force, and catching the balls as they fall to the bottom of the glass shade bear them up as a juggler would throw oranges from hand to hand. If coloured balls be used the effect may be enhanced, and much variety imparted to the experiment, which is very easy to make.

Fig. 57.—Water jet and balls.

Water exercises an enormous pressure, but the pressure does not depend upon the amount of water in the vessel. It depends upon the vessel’s height, and the dimensions of the base. This has been proved by filling vessels whose bases and heights are equal, but whose shapes are different, each holding a different quantity of water. The pressure at the bottom of each[Pg 61] vessel is the same, and depends upon the depth of the water. If we subject a portion of the liquid surface to certain force, this pressure will be dispersed equally in all directions, and from an acquaintance with this fact the Hydraulic Press was brought into notice. If a vessel with a horizontal bottom be filled with water to a depth of one foot, every square foot will sustain a pressure of 62·37 lbs., and each square inch of 0·433 lbs.

Figs. 58, 59, 60, 61.—Pressure of Water.

We will now explain the principle of this Water Press. In the small diagram, the letters A B represent the bottom of a cylinder which has a piston fitted in it (P). Into the opposite side a pipe is let in, which leads from a force-pump D, which is fitted with a valve E, opening upwards. When the piston in D is pulled up water enters through the valve; when the piston is forced down the valve shuts, and the water rushes into the chamber A B. The pressure pushes up the large piston with a force multiplied as many times as the area of the small piston is contained in the large one. So if the large one be ten times as great as the small one, and the latter be forced down with a 10 lb. pressure, the pressure on the large one will be 100 lbs., and so on.

Fig. 62.—Water Press.

The accompanying illustration shows the form of the Hydraulic or Bramah Press. A B C D is a strong frame, F the force-pump worked by means of a lever fixed at G, and H is the counterprise. E is the stop-cock to admit the water (fig. 63).

Fig. 63.—Bramah Press.

The principles of hydrostatics will be easily explained. The Lectures of M. Aimé Schuster, Professor and Librarian at Metz, have taught us in a very simple manner the principle of Archimedes, in which it is laid down that “a body immersed in a liquid loses a portion of its weight equal to the weight of the liquid displaced by it.” We take[Pg 62] a body of as irregular form as we please; a stone, for example. A thread is attached to the stone, and it is then placed in a glass of water full up to the brim. The water overflows; a volume of the liquid equal to that of the stone runs over. The glass thus partially emptied is then dried, and placed on the scale of a balance, beneath which we suspend the stone; equilibrium is established by placing some pieces of lead in the other scale. We then take a vase full of water, into which we plunge the stone suspended from the scale, supporting the vase by means of bricks. The equilibrium is now broken; to re-establish it, it is necessary to fill up with water the glass placed on the scale; that is to say, we put back in the glass the weight of a volume of water precisely equal to that of the stone.

Fig. 64.—Demonstration of the upward pressure of liquids.

If it is desired to investigate the principles relating to connected vessels, springs of water, artesian wells, etc., two funnels, connected by means of an india-rubber tube of certain length, will serve for the demonstration; and by placing the first funnel at a higher level, and pouring in water abundantly, we shall see that it overflows from the second.

A disc of cardboard and a lamp-glass will be all that is required to show the upward pressure of liquids. I apply to the opening of the lamp-glass a round piece of cardboard, which I hold in place by means of a string; the tube thus closed I plunge into a vessel filled with water. The piece of cardboard is held by the pressure of the water upwards. To separate it[Pg 63] from the opening it suffices to pour some water into the tube up to the level of the water outside (fig. 64). The outer pressure exercised on the disc, as well as the pressure beneath, is now equal to the weight of a body of water having for its base the surface of the opening of the tube, its depth being the distance from the cardboard to the level of the water.

Syringes, pumps, etc., are the effects of atmospheric pressure. Balloons rise in the air by means of the pressure of gas; a balloon being a body plunged in gas, is consequently submitted to the same laws as a body plunged in water.

Boats float because of the pressure of liquid, and water spurts from a fountain for the same reason. I recollect having read a very useful application of the principles of fluid pressure.

Fig 65.—Experiment on the convexity of a meniscus.

A horse was laden with two tubs for carrying a supply of water, and in the bottom of the tubs a valve was fixed. When the horse entered the stream the tubs were partly immersed; the water then exercised its upward pressure, the valve opened, and the tubs slowly filled. When they were nearly full the horse turned round and came out of the water; the pressure had ceased.

Thus the action of the water first opened the valve, and then closed it.

The particular phenomena observable in the water level in narrow spaces, as of a fine glass tube, or the level of two adjoining waves, capillary[Pg 64] phenomena, etc., do not need any special appliance for demonstration, and it is the same with the convexity or concavity of meniscuses.

Fig. 65 represents a pretty experiment in connection with these phenomena. I take a glass, which I fill up to the brim, taking care that the meniscus be concave, and near it I place a pile of pennies. I then ask my young friends how many pennies can be thrown into the glass without the water overflowing. Everyone who is not familiar with the experiment will answer that it will only be possible to put in one or two, whereas it is possible to put in a considerable number, even ten or twelve. As the pennies are carefully and slowly dropped in, the surface of the liquid will be seen to become more and more convex, and one is surprised to what an extent this convexity increases before the water overflows.

Fig. 66.—The Syphon.

The common syphon may be mentioned here. It consists of a bent tube with limbs of unequal length. We give an illustration of the syphon (fig. 66). The shorter leg being put into the mixture, the air is exhausted from the tube at o, the aperture at g being closed with the finger. When the finger is removed the liquid will run out. If the water were equally high in both legs the pressure of the atmosphere would hold the fluid in equilibrium, but one leg being longer, the column of water in it preponderates, and as it falls, the pressure on the water in the vessel keeps up the supply.

Apropos of the syphon, we may mention a very simple application of the principle. Cut off a strip of cloth, and arrange it so that one end shall remain in a glass of water while the other hangs down, as in the illustration. In a short time the water from the upper glass will have passed through the cloth-fibres to the lower one (fig. 67).

This attribute of porous substances is called capillarity, and shows itself by capillary attraction in very fine pores or tubes. The same phenomenon is exhibited in blotting paper, sugar, wood, sand, and lamp-wicks, all of which give familiar instances of capillarity. The cook makes use of this property by using thin paper to absorb grease from the surface of soups.

Capillarity (referred to on page 25) is the term used to define capillary force, and is derived from the word capillus, a hair; and so very small bore tubes are called capillary tubes. We know that when we plunge a glass tube into water the liquid will rise up in it, and the narrower the tube the higher the water will go; moreover, the water inside will be higher than at the outside. This is in accordance with a well-known law of adhesion, which induces concave or convex surfaces9 in the liquids in the tubes, according as the tube is wetted with the liquid or not. For instance, water, as we[Pg 65] have said, will be higher in the tube, and concave in form; but mercury will be depressed below the outside level, and convex, because mercury will not adhere to glass. When the force of cohesion to the sides of the tube is more than twice as great as the adhesion of the particles of the liquid, it will rise up the sides, and if the forces be reversed, the rounded appearance will follow. This accounts for the convex appearance, or “meniscus,” in the column of mercury in a barometer.

Amongst the complicated experiments to demonstrate molecular attraction, the following is very simple and very pretty:—Take two small balls of cork, and having placed them in a basin half-filled with water, let them come close to each other. When they have approached within a certain distance they will rush together. If you fix one of them on the blade of your penknife, it will attract the other as a magnet, so that you can lead it round the basin (fig. 68). But if the balls of cork are covered with grease they will repel each other, which fact is accounted for by the form of the menisques, which are convex or concave, according as they are moistened, or preserved from action of the water by the grease.

Fig. 67.—An improvised syphon.

This attribute is of great use in the animal and vegetable kingdoms. The rising of the sap is one instance of the latter.

Experience in hydrostatics can be easily applied to amusing little experiments. For instance, as regards the syphon, we may make an image[Pg 66] of Tantalus as per illustration (fig. 69). A wooden figure may be cut in a stooping posture, and placed in the centre of a wide vase, as if about to drink. If water be poured slowly into the vase it will never rise to the mouth of the figure, and the unhappy Tantalus will remain in expectancy. This result is obtained by the aid of a syphon hidden in the figure, the shorter limb of which is in the chest. The longer limb descends through a hole in the table, and carries off the water. These vases are called vases of Tantalus.

The principle of the syphon may also be adapted to our domestic filters. Charcoal, as we know, makes an excellent filter, and if we have a block of charcoal in one of those filters,—now so common,—we can fix a tube into it, and clear any water we may require. It sometimes (in the country) happens that drinking-water may become turgid, and in such a case the syphon filter will be found useful.

Fig. 68.—Molecular attraction.

The old “deception” jugs have often puzzled people. We give an illustration of one, and also a sketch of the “deceptive” portion (figs. 70 and 71). This deception is very well managed, and will create much amusement if a jug can be procured; they were fashionable in the eighteenth century, and previously. A cursory inspection of these curious utensils will lead one to vote them utterly useless. They are, however, very quaint, and if not exactly useful are ornamental. They are so constructed, that if an inexperienced[Pg 67] person wish to pour out the wine or water contained in them, the liquid will run out through the holes cut in the jug.

To use them with safety it is necessary to put the spout A in one’s mouth, and close the opening B with the finger, and then by drawing in the breath, cause the water to mount to the lips by the tube which runs around the jug. The specimens herein delineated have been copied from some now existent in the museum of the Sèvres china manufactory.

The Buoyancy of Water is a very interesting subject, and a great deal may be written respecting it. The swimmer will tell us that it is easier to float in salt water than in fresh. He knows by experience how difficult it is to sink in the sea; and yet hundreds of people are drowned in the water, which, if they permitted it to exercise its power of buoyancy, would help to save life.

Fig. 69.—Vase of Tantalus.

The sea-water holds a considerable quantity of salt in solution, and this adds to its resistance, or floating power. It is heavier than fresh water, and the Dead Sea is so salt that a man cannot possibly sink in it. This means that the man’s body, bulk for bulk, is much lighter than the water of the Dead Sea. A man will sink in fresh, or ordinary salt water if the air in his lungs be exhausted, because without the air he is much heavier than water, bulk for bulk. So if anything is weighed in water, it apparently loses in weight exactly equal to its own bulk of water.

[Pg 68]

Water is the means by which the Specific Gravity of liquids or solids is found, and by it we can determine the relative densities of matter in proportion. Air is the standard for gases and vapours. Let us examine this, and see what is meant by Specific Gravity.

We have already mentioned the difference existing between two equal volumes of different substances, and their weight, which proves that they may contain a different number of atoms in the same space. We also know, from the principle of Archimedes, that if a body be immersed in a fluid, a portion of its weight will be sustained by the fluid equal to the weight of the fluid displaced.

Fig. 70.—Deception jugs of old pattern.

[This theorem is easily proved by filling a bucket with water, and moving it about in water, when it will be easy to lift; and likewise the human body may be easily sustained in water by a finger under the chin.]

Fig. 71.—Section of jug.

The manner in which Archimedes discovered the displacement of liquids is well known, but is always interesting. King Hiero, of Syracuse, ordered a crown of gold to be made, and when it had been completed and delivered[Pg 69] to His Majesty, he had his doubts about the honesty of the goldsmith, and called to Archimedes to tell him whether or not the crown was of gold, pure and simple. Archimedes was puzzled, and went home deep in thought. Still considering the problem he went to the bath, and in his abstraction filled it to the brim. Stepping in he spilt a considerable quantity of water, and at once the idea struck him that any body put into water would displace its own weight of the liquid. He did not wait to dress, but ran half-naked to the palace, crying out, “Eureka, Eureka! I have found it, I have found it! “What had he found?—He had solved the problem.

He got a lump of gold the same weight as the crown, and immersed it in water. He found it weighed nineteen times as much as its own bulk of water. But when he weighed the kings crown he found it displaced more water than the pure gold had done, and consequently it had been adulterated by a lighter metal. He assumed that the alloy was silver, and by immersing lumps of silver and gold of equal weight with the crown, and weighing the water that overflowed from each dip, he was able to tell the king how far he had been cheated by the goldsmith.

Fig. 72.—Weighing metal in water.

It is by this method now that we can ascertain the specific gravity of bodies. One cubic inch of water weighs about half an ounce (or to be exact, 252½ grains). Take a piece of lead and weigh it in air; it weighs, say, eleven ounces. Then weigh it in a vase of water, and it will be only ten ounces in weight. So lead is eleven times heavier than water, or eleven ounces of lead occupy the same space as one ounce of water.

Fig. 73.—Hydrometer.

[The heavier a fluid is, or the greater its density, the greater will be the weight it will support. Therefore we can ascertain the purity or otherwise of certain liquids by using hydrometers, etc., which will float higher or lower in different liquids, and being gauged at the standard of purity, we can ascertain (for instance) how much water is in the milk when supplied from the dairy.]

But to return to Specific Gravity, which means the “Comparative[Pg 70] density of any substance relatively to water,” or as Professor Huxley says, “The weight of a volume of any liquid or solid in proportion to the weight of the same volume of water, at a known temperature and pressure.”

Water, therefore, is taken as the unit; so anything whose equal volume under the same circumstances is twice as heavy as the water, is declared to have its specific gravity 2; if three-and-a-half times it is 3·5, and so on. We append a few examples; so we see that things which possess a higher specific gravity than water sink, which comes to the same thing as saying they are heavier than water, and vice versâ.

To find the specific gravity of any solid body proceed as above, in the experiment of the lead. By weighing the substance in and out of water we find the weight of the water displaced; that is, the first weight less the second. Divide the weight in air by the remainder, and we shall find the specific gravity of the substance.

Fig. 74.—Over-shot wheel of mill.

The following is a table of specific gravities of some very different substances, taking water as the unit.

But we have by no means exhausted the uses of water. Hydrodynamics, which is the alternative term for hydraulics, includes the consideration of many forms of water-wheels, most of which, as mill-wheels, are[Pg 71] under-shot, or over-shot accordingly as the water passes horizontally over the floats, or acts beneath them. These wheels are used in relation to the fall of water. If there is plenty of water and a slight fall, the under-shot wheel is used. If there is a good fall less water will suffice, as the weight and momentum of the falling liquid upon the paddles will turn the wheel. Here is the Persian water-wheel, used for irrigation (fig. 75). The Archimedian Screw, called after its inventor, was one of the earliest modes of raising water. It consists of a cylinder somewhat inclined, and a tube bent like a screw within it. By turning the handle of the screw the water is drawn up and flows out from the top.

Fig. 75.—Irrigation wheel in Egypt.

The Water Ram is a machine used for raising water to a great height by means of the momentum of falling water.

The Hydraulic Lift is familiar to us all, as it acts in our hotels, and we need only mention these appliances here; full descriptions will be found in Cyclopædias.

We have by no means exhausted the subject of Water in this chapter. Far from it. But when we come to Chemistry and Physical Geography we shall have more to tell, and our remarks as to the application of science to Domestic Economy, in accordance with our plan, will also lead us up to some of the uses of water. So for the present we will take our leave of water in a liquid form, and meet it again under the application of Heat, which subject will take us to Ice and Steam,—two very different conditions of water.

[Pg 72]

CHAPTER VII.

HEAT—WHAT IT IS—THEORY OF HEAT—THE THERMOMETER—EXPANSION BY HEAT—EBULLITION AND DISTILLATION—LATENT HEAT—SPECIFIC HEAT.

What is Heat?—We will consider this question, and endeavour to explain it before we speak of its effects on water and other matter.

Heat is now believed to be the effects of the rapid motion of all the particles of a body. It is quite certain that a heated body is no heavier than the same body before it was made “hot,” so the heat could not have gone into it, nor does the “heat” leave it when it has become what we call “cold,” which is a relative term. Heat is therefore believed to be a vibratory motion, or the effects of very rapid motion of matter.

The Science of Heat, as we may term it, is only in its infancy, or certainly has scarcely come of age. Formerly heat was considered a chemical agent, and was termed caloric, but now heat is found to be motion, which affects our nerves of feeling and sight; and, as Professor Stewart tells us, “a heated body gives a series of blows to the medium around it; and although these blows do not affect the ear, they affect the eye, and give us a sense of light.”

Although it is only within a comparatively few years that heat has been really looked upon as other than matter, many ancient philosophers regarded it as merely a quality of matter. They thought it the active principle of the universe. Epicurus declared that heat was an effluxion of minute spherical particles possessing rapid motion, and Lucretius maintained that the sun’s light and heat are the result of motion of primary particles. Fire was worshipped as the active agent of the universe, and Prometheus was fabled to have stolen fire from heaven to vivify mankind. The views of the ancients were more or less adopted in the Middle Ages; but John Locke recognized the theory of heat being a motion of matter. He says: “What in our sensation is heat, in the object is nothing but motion.”

Gradually two theories arose concerning heat;—one, the Material theory—the theory of Caloric or Phlogiston; the other, the Kinetic theory. Before the beginning of the present century the former theory was generally accepted, and the development of heat was accounted for by asserting that the friction or percussion altered the capacity for heat of the substances acted upon. The heat was squeezed out by the hammer, and the amount[Pg 73] of heat in the world was regarded as a certain quantity, which passed from one body to another, and that some substances contained, or could “store away,” more of the material called heat than other substances. Heat was the material of fire—the principle of it, or materia ignis; and by these theories Heat, or Caloric, was gradually adopted as a separate material agent—an invisible and subtle matter producing certain phenomena when liberated.

So the two theories concerning heat arose at the end of the last century. One, as we have said, is known as the Material, the other as the Kinetic theory. The latter is the theory of motion, so called from the Greek kinesis (motion), or sometimes known as the Dynamic theory of heat, from dunamis (force); or again as Thermo-dynamics.

But any possibility of producing a new supply of heat was denied by the materialists. They knew that some bodies possessed a greater capacity for heat than others; but Count Rumford, at Munich, in 1797, astonished an audience by making water boil without any fire! He had observed the great extent to which a cannon became heated while being bored in the gun factory, and influenced by those who maintained the material theory of heat, paid great attention to the evolution of heat. He accordingly endeavoured to produce heat by friction, and by means of horse power he caused a steel borer to work upon a cylinder of metal. The shavings were permitted to drop into a pan of water at 60° Fahrenheit. In an hour after the commencement of the operation the temperature of the water had risen to 107°: in another half-hour the heat of it was up to 142°: and in two hours had measured 170°. Upon this he says: “It is hardly necessary to add that anything which any insulated body or system of bodies can continue to furnish without limitation cannot possibly be a material substance, and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in these experiments except by motion.”

A few years later Sir Humphrey Davy made his conclusive experiments, and the Material theory of heat received its death-blow.

Sir Humphrey Davy—referring to the fact that water at a freezing temperature has “more heat in it” (as it was believed) than ice at the same temperature—said: “If I, by friction, liquify ice, a substance will be produced which contains a far greater absolute amount of heat than ice. In this case it cannot reasonably be affirmed that I merely render sensible heat which had been previously insensible in the frozen mass. Liquification will conclusively prove the generation of heat.

This reasoning could not be doubted. Sir Humphrey Davy made the experiment. He rubbed together two pieces of ice in the air, and in a vacuum surrounded by a freezing mixture. The ice became liquified, and so the generation of heat by “mechanical means” was proved. Its immateriality was demonstrated, but the Material theory was not even then abandoned by its adherents.

So things continued, until in 1842-3, Doctor Julius Meyer, of Heilbronn,[Pg 74] and Doctor Joule, of Manchester, separately, and by different means, arrived at the conclusion that a certain definite amount of mechanical work corresponds to a certain definite amount of Heat, and vice versâ. Thus was a great support afforded to the Dynamic theory. This fact Doctor Joule communicated to the Philosophical Magazine in 1843, and the conclusions he came to were—

1. “That the quantity of heat produced by the friction of bodies, whether solid or liquid, is always in proportion to the force expended;

2. “That the quantity of heat capable of increasing the temperature of a pound of water (weighed in vacuo and taken at between 55° and 60° Fahr.) by 1° Fahr., requires for its evolution the expenditure of a mechanical force represented by the fall of 772 lbs. through the space of one foot.”

Fig. 76.—Melting a piece of tin on a card.

This is the “mechanical equivalent of heat.” The first paper written by Mr. Joule demonstrated that the temperature of water rises when forced through narrow tubes; and to heat it one degree, the force of 770 foot pounds was necessary, which means that the 1 lb. of water falling 770 feet, got hotter by one degree when it reached the earth. He subsequently arrived at the more exact conclusions quoted above.

So heat is now known to be a series of vibrations, or vibratory motions, as sound vibrations, which we cannot hear nor see, but the effects of which are known to us as light and heat.

[Pg 75]

In considering heat we must put aside the idea of warmth and cold, for they are only different degrees of heat, not the absence of it.

The study of heat can be briefly undertaken without any complicated apparatus. If we desire a proof of the great conducting power of metals, let us place a fine piece of muslin tightly stretched over a lump of polished metal. On the muslin we put a burning ember, and excite combustion by blowing on it; the muslin is not burned in the least, the heat being entirely absorbed by the metal, which draws it through the material into itself. Fig. 76 represents a similar experiment: it consists of melting some tin on a playing card, held over the flame of a spirit lamp. The metal becomes completely melted without the card being burnt. It is through a similar effect that metals appear cold to us when we take them in our hands; by their conductibility they remove the heat from our hands, and give us the peculiar impression which we do not experience when in contact with substances that are bad conductors, such as wood, woollen materials, etc.

Fig. 77.—Boiling water in a paper case.

Fig. 77 shows the method of boiling water in paper. We make a small paper box, such as those made by school-boys, and suspend it by four threads to a piece of wood held horizontally at a suitable height. We fill this improvised vessel with water, and place it over the flame of a spirit lamp. The paper is not burnt, because the water absorbs all the heat into itself. After a few minutes the water begins to boil, sending forth clouds[Pg 76] of steam, but the paper remains intact. It is well to perform this operation over a plate, in case of accident, as the water may be spilt. We may also make use of an egg-shell as a little vessel in which to heat the water, by resting it on a wire ring over the flame of the spirit lamp.

Fig. 78.—Experiment on the regelation of ice.

Fig. 78 shows the arrangement of a very remarkable experiment, but little known, on the refreezing of ice. A block of ice is placed on the edge of two iron chairs, and is encircled by a piece of wire, to which is suspended the weight of say five pounds. The wire penetrates slowly, and in about an hour’s time has passed completely through the lump of ice, and the weight, with the piece of wire, falls to the ground. What happens then to the block of ice?—You imagine, doubtless, that it is cut in two. No such thing; it is intact, and in a single lump as it was previous to the experiment. In proportion as the wire was sunk through the mass, the slit has been closed again by refreezing. Ice or snow during the winter may serve for a number of experiments relating to heat. If we wish to demonstrate the influence of colours on radiation, we take two pieces of cloth of the same size,—one white, and the other black,—and place them both on the snow, if possible, when there is a gleam of sunlight. In a short time it will be found that the snow underneath the black cloth has melted to a much greater extent than that beneath the white cloth, because black absorbs heat more than white, which, on the contrary, has a tendency to reflect it. We perceive very plainly the[Pg 77] difference in temperature by touching the two cloths. The white cloth feels cold in comparison with the black cloth.

It is hardly necessary to point out experiments on the expansion of bodies. They can be performed in a number of different ways; by placing water in a narrow-necked bottle, and warming it over the fire, we can ascertain the expansion of liquids under the influence of heat. We may in this way construct a complete thermometer.

We may now consider the Sources of Heat, or causes of its development, which are various, and in many cases apparent. The first great source is the Sun, and it has been calculated that the heat received by the earth in one year is sufficient to melt an envelope of ice surrounding it one hundred and five feet thick. Of course the heat at the surface of the sun is enormously greater than this, about one-half being absorbed in the atmosphere before it reaches us at all. In fact, it is impossible to give you an idea of the enormous heat given out by the sun to the earth (which is a very small fraction indeed of the whole), stars, and planets, all of which give out heat. We know that heat is stored in the earth, and that it is in a very active condition we can perceive from the hot springs, lava, and flame which are continually erupting from the earth in various places. These sources of heat are beyond our control.

But apart from the extra- and intra-terrestrial sources of heat there are mechanical causes for its generation upon our globe, such as friction, percussion, or compression. The savage or the woodman can procure heat and fire by rubbing a pointed stick in a grooved log. The wooden “breaks” of a locomotive are often set on fire by friction of the wheels, so they require grease, and the wheels on the rails will develop heat and sparks. Our matches, and many other common instances of the generation of heat (and fire) by friction, will occur to every reader. Water may be heated by shaking it in a bottle, taking care to wrap something round it to keep the warmth of the hand from the glass. By percussion, such as hammering a nail or piece of iron, the solid bar may be made “red-hot”; and when cannon are bored at Woolwich the shavings of steel are too hot to hold even if soap-and-water has been playing upon the boring-machine.

The production of heat by chemical action is termed combustion, and this is the means by which all artificial heat for our daily wants is supplied. We can also produce heat by electricity. A familiar and not always pleasant instance of this is seen in the flash of lightning which will fuse metals, and experiment may do the same upon a smaller scale. These are, in brief, the Sources of Heat, and we may speak of its effects.

We may take it for granted that no matter from what source heat is derived, it exhibits the same phenomena in its relation to objects. One of the most usual of these phenomena is expansion. Let us take water, and see the effect of heat upon it.

We know that a certain weight of water under the same conditions[Pg 78] has always the same volume; and although the attributes of the liquid vary under different circumstances, under the same conditions its properties are exactly the same. Now, water expands very much when under the influence of heat, like all liquids; solids and gases also expand upon the application of heat.

We can easily establish these statements. A metallic ring when heated is larger than when cool. A small quantity of air in a bladder when heated will fill the bladder, and water will boil over the vessel, or expand into steam, and perhaps burst the boiler. So expansion is the tendency of what we term heat.

We make use of this quality of heat in the thermometer, by which we can measure the temperature not only of liquids or solids, but of the atmosphere. The reading of the thermometer varies in different countries, for the degrees are differently marked, but the construction of the instrument is the same. It is called thermometer from two Greek words signifying the measure of heat. It is a notable fact that Castelli, writing in 1638, says to Ferdinand Cæsarina: “I remembered an experiment which Signor Galileo had shown me more than thirty-five years ago. He took a glass bottle about the size of a hen’s egg, the neck of which was two palms long, and as narrow as a straw. Having well heated the bulb in his hands, he placed its mouth in a vessel containing water, and withdrawing the heat of his hand from the bulb, the water instantly rose in the neck more than a palm above the level of the water in the vessel.”

Here, then, we have an air-thermometer, but as it was affected by the pressure as well as the temperature of the atmosphere, it could not be relied upon as a “measurer of heat.” Until Torricelli propounded the principle of the barometer, this “weather-glass” of Galileo was used, for the philosopher divided the stem into divisions, and the air-thermometer served the purpose of our modern instruments.

The actual inventor of the thermometer is not known. It has been attributed to Galileo, to Drebbel, and to Robert Fludd. There is little doubt, however, that Galileo and Drebbel were both acquainted with it, but whether either claimed the honour of the invention, whether they discovered it independently, or together, we cannot say. Sanctorio, of Padua, and Drebbel have also been credited with the invention. We may add that the spirit thermometer was invented in 1655-1656. It was a rough form of our present thermometer, and roughly graduated. But it was hermetically closed to the air, and a great improvement on the old “weather-glass. Edmond Halley introduced mercury as the liquid for the instrument in 1680. Otto von Guerike first suggested the freezing point of water as the lowest limit, and Renaldini, in 1694, proposed that the boiling and freezing points of water should be the limit of the scale.

Let us now explain the construction and varied markings of the three kinds of thermometers in use. By noting the differences between the scales[Pg 79] every reader will be able to read the records from foreign countries noted upon the Centigrade and Réaumur instruments, which are all based upon the theory that heat expands liquids.

Fig. 79.—Thermometer.

[We used to hear the expression, “Heat expands, and cold contracts,” but we trust that all our readers have now learnt that there is no such thing as cold. It is only a negative term. We feel things cold because they extract some warmth from our fingers, not because the substances have no heat.]

Thermometers are made of very fine bore glass tubes. One end has a bowl, or bulb, the other is at first open. By heating the bowl the air in the tube is driven away by the open end, which is quickly dipped in a bowl of mercury. The mercury will then occupy a certain space in the tube; and if it be heated till the liquid boils, all the air will be driven out by the mercurial vapour. By once again dipping the tube in the quicksilver the glass will be filled. Then, before it cools, close the open end of the tube, and the thermometer is so far made. Having now caught our thermometer we must proceed to mark it, which is an easy process. By plunging the mercury into pounded melting ice we can get the freezing point, and boiling water will give us the boiling point. The intermediate scale can be then indicated.

If mercury and glass expanded equally there would be no rise in the latter. Extreme delicacy of the thermometer can be arrived at by using a very fine tube, particularly if it be also flat.

The freezing point in Fahrenheit’s scale is 32°; in the Centigrade it is 0°, and the boiling point 100°. This was the scale adopted by Celsius, a Swede, and is much used. Réaumur called the freezing point 0°, and the boiling point 80°. There is another scale, almost obsolete,—that of Delisle, who called boiling point zero, and freezing point 150°.

There is no difficulty in converting degrees on one scale into degrees on the other. Fahrenheit made his zero at the greatest cold he could get; viz., snow and salt. The freezing point of water is 32° above his zero. Therefore 212-32 gives 180° the difference between the freezing and boiling points of water. So 180° Fahr. corresponds to 100° Cent., and to 80° Réaumur, reckoning from freezing point.

The following tables will explain the differences:—

[Pg 80]

Alcohol is used in thermometers in very cold districts, as it does not freeze even at a temperature of -132° Fahr.

We have now explained the way in which we can measure heat by the expansion of mercury in a tube. We can also find out that solids and gases expand also. Engineers always make allowances for the effects of winter and summer weather when building bridges; in summer the bridge gets longer, and unless due provision were made it would become strained and weakened. So there are compensating girders, and the structure remains safe.

The effects of expansion by heat are very great and very destructive at times. Instances of boilers bursting will occur to every reader. It is very important to be able to ascertain the extent to which solid bodies will expand. Such calculations have been made, and are in daily use.

We can crack a tumbler by pouring hot water into it, or by placing it on the “hob.” A few minutes’ consideration will assure us that the lower particles of the glass expanded before the rest, and cracked our tumbler. A gradual heating, particularly if the glass be thin, will ensure safety. Thick glass will crack sooner than thin.

Again, many people at railway stations have asked us, “Why don’t they join the rails together on this line?” We reply that if every length of rail were tightly fixed against its neighbour, the whole railway would be displaced. The iron expands and joins up close in hot weather. In wet weather, also, the wooden pegs and the sleepers swell with moisture, and get tightened up. Everyone knows how much more smoothly a train travels in warm, wet weather. This is due to the expansion of the iron and the swelling of the sleepers and pegs in the “chairs.” A railway 400 miles long expands 338[Pg 81] yards in summer,—that is the difference in length between the laid railroad in summer and in winter.

This can be proved. Iron expands 0·001235 of its length for every 180° Fahr. Divided by 180 it gives us the expansion for 1°, which is 0·00000686, taking the difference of winter and summer at 70° Fahr. Multiply these together, and the result (0·00048620 of its length) by the number of yards in 400 miles, and we find our answer 338 yards. Expansion acts in solids and most liquids by the destruction of cohesion between the particles. Gases, however, having much less cohesion amid the particles, will expand far more under a given heat than either solids or liquids, and liquids expand more than solids for the same reason, and more rapidly at a high temperature than at a low one.

We have spoken of expansion. We may give an instance in which the subsequent contraction of heated metal is useful. Walls sometimes get out of the perpendicular, and require pulling together. No force which can be conveniently applied would accomplish this so well as the cooling force due to the potential energy of iron. Rods are passed through the walls and braced up by nuts. The rods are then heated, and as they cool they contract and pull the walls with them.

When glass is suddenly cooled, the inner skin, as it were, presses with great force against the cooled surface, but as it is quite tight no explosion can follow. But break the tail, or scratch it with a diamond, and the strain is taken off. The glass drop crumbles with the effect of the explosion, as in the cases of Prince Rupert’s drops, and the Bologna flasks; the continuity is broken, and pulverization results.

But a very curious exception to the general laws of expansion is noticed in the case of nearly freezing water. We know water expands by heat, at first gradually, and then to an enormous extent in steam. But when cooling water, instead of getting more and more contracted, only contracts down to 39·2° Fahr., it then begins to expand, and at the moment it freezes into ice it expands very much—about one-twelfth of its volume, but according to Professor Huxley it weighs exactly the same, and the steam produced from that given quantity of water will weigh just exactly what the water and the ice produced by it weigh individually. At 39·2° Fahr. water is at its maximum density, or in other words, a vessel of a certain size will hold more water when it is at 39° Fahr. than at any other time. Whether the water be heated or cooled at this temperature, it expands to the boiling or freezing point when it becomes steam or ice, as the case may be.

Water, when heated, is lighter than cold water. You can prove this in filling a bath from two taps of hot and cold water at the same time. The cold falls to the bottom, and if you do not stir up the water when mixed you will have a hot surface and a cold foundation. The heat increases the volume of water, it becomes lighter, and comes uppermost.

Steam and Water and Ice are all the same things under different conditions, although to the eye they are so different. They are alike inasmuch[Pg 82] as a given weight of water will weigh as much when converted into ice or developed into steam. The half ounce of water will weigh half an ounce as ice or as steam, but the volume or bulk will vary greatly, as will be understood when we state that one cubic inch of water will produce 1,700 cubic inches of steam, and 1-1/11 cubic inch of ice; but at the same time each will yield, when decomposed, just the same amount of oxygen and hydrogen.

Let us now consider the Effects of Heat upon Water. We have all seen the vapour that hangs above a locomotive engine. We call it “steam.” It is not pure steam, for steam is really invisible. The visible vapour is steam on its way to become water again. On a very hot dry day we cannot distinguish the vapour at all.

The first effect of heat upon water is to expand it; and as the heat is applied we know that the water continues to expand and bubble up; and at last, when the temperature is as high as 212°, we say water “boils”—that is, at that heat it begins to pass away in vapour, and you will find that the temperature of the steam is the same as the boiling water. While undergoing this transformation, the water increases in volume to 1,700 times its original bulk, although it will weigh the same as the water. So steam has 1,700 less specific gravity than water.

It is perhaps scarcely necessary to remind our readers that water, when heated, assumes tremendous force. Air likewise expands with great violence, and the vessels containing either steam or air frequently burst, with destructive effects. Solid bodies also expand when heated, and the most useful and accurate observations have been made, so that the temperatures at which solid bodies expand are now exactly known. Air also expands by heat.

While speaking of Expansion by Heat, we may remark that a rapid movement is imparted to the air by Heat. In any ordinary room the air below is cool, while if we mount a ladder to hang up a picture, for instance, we shall find the air quite hot near the ceiling. This is quite in keeping with the effects of heat upon water. The hot particles rise to the top in a vessel, and thus a motion is conveyed to the water. So in our rooms. The heated air rushes up the chimney and causes a draught, and this produces motion, as we have seen by fig. 39, in which the cardboard spiral was set in motion by heated air. A balloon will ascend, because it is filled with heated air or gas; and we all have seen the paper balloons which will ascend if a sponge containing spirit of wine be set on fire underneath them.

Winds are also only currents of air produced by unequal temperature in different places. The heated air ascends, and the colder fluid rushes in sometimes with great velocity to fill the space. “Land” and “sea” breezes are constant; the cool air blows in from the sea during the day, and as the land cools more rapidly at night, the breeze passes out again. When we touch upon Meteorology, we will have more to say respecting Air Currents and the various Atmospheric Phenomena.

We know that water can be made to boil by heat, but it is not perhaps generally known that it will apparently boil by cold, and the experiment may[Pg 83] thus be made:—A flask half-full of water is maintained at ebullition for some minutes. It is removed from the source of heat, corked, inverted, and placed in one of the rings of a retort stand. If cold water is poured on the upturned bottom of the flask, the fluid will start into violent ebullition. The upper portion of the flask is filled with steam, which maintains a certain pressure on the water. By cooling the upper portion of the flask some of this is condensed, and the pressure reduced. The temperature at which water boils varies with the pressure. When it is reduced, water boils at a lower heat. By pouring the cold water over the flask we condense the steam so that the water is hot enough to boil at the reduced pressure. To assert that water boils by the application of cold is a chemical sophism.

Ebullition and Evaporation may be now considered, and these are the two principal modes by which liquids assume the gaseous condition. The difference is, when water boils we term it ebullition (from the Latin ebullio, I boil); evaporation means vapour given out by water not boiling (from evaporo, I disperse in vapour).

There are two operations based upon the properties which bodies possess of assuming the form of vapour under the influence of heat, which are called Distillation and Sublimation. These we will consider presently.

Ebullition then means a bubbling up or boiling; and when water is heated in an open vessel two forces oppose its conversion into vapour; viz., its own cohesive force and atmospheric pressure. At length, at 212° Fahr., the particles of water have gained by heat a force greater than the opposing forces; bubbles of vapour rise up from the bottom and go off in vapour. This is ebullition, and at that point the tension of the vapour is equal to the pressure of the atmosphere, for if not, the bubbles would not form. All this time of boiling, notwithstanding any increase of heat, the thermometer will not rise above 212° (Fahr.), for all the heat is employed in turning the water to steam.

We have said the ebullition takes place at 212° Fahr. (or 100° C.), but that is only at a certain level. If we ascend 600 feet high we shall find that water will boil at a less temperature; and on the top of a mountain (say Mont Blanc) water will boil at 185° Fahr.; so at an elevation of three miles water boils at a temperature less by 27° Fahr. An increase of pressure similarly will raise the boiling point of water. The heights of mountains are often ascertained by noticing the boiling point of water on their summits, the general rule being a fall of one degree for every 530 feet elevation at medium altitudes. We append a few instances taken at random:—

[Pg 84]

[The difference for a degree depends upon the height, varying between 510 and 590 feet, according to the elevation reached. The approximate height of a mountain can be found by multiplying 530 by the number of degrees between the boiling point and 212°. In some very elevated regions travellers have even failed to boil potatoes.]

The boiling point of liquid may be altered by mixing some substance with it; and although such a substance as sawdust would not alter the boiling point of water, yet if the foreign matter be dissolved in the liquid it will alter the boiling point. Even the air dissolved in liquids alters their boiling point, and water freed from air will not boil till it is raised to a temperature much higher than 212° Fahr. Water will boil at a higher temperature in a glass vessel than in metal, because there is a greater attraction between water and glass.

We said above that an increase of pressure will raise the boiling point of water. Under the pressure of one atmosphere—that is, when there is a pressure of 15 lbs. on the square inch—water boils at 212°. But under a pressure of two atmospheres, the boiling point rises to 234°, and of four atmospheres, 294°. So we see by increasing the pressure the water may be almost indefinitely heated, and it will not boil. We can understand that in a very deep vessel the layer of water at the bottom has to sustain the pressure of the water in addition to the weight of the atmosphere above it. The pressure of thirty-four feet of water is equal to the atmospheric pressure of 15 lbs. on the square inch, and thus at such a distance water must be heated to 234° before it will boil. Professor Bunsen founded his Theory of the Geysers upon this fact, for he maintained that water falling into the earth lost much air, and required with the super-incumbent pressure a very high temperature to boil it. When it did boil it generated steam so suddenly that it exploded upwards, throwing up vapour and the water with it, as water poured into a very hot basin will do.

Evaporation may now be considered, and is distinguished from Ebullition by the production of vapour on the surface of liquids, the latter term signifying the formation of vapour in the body of the liquid. Evaporation takes place at all temperatures, and from every liquid surface exposed to the air. We know what we call a “drying wind.” The air in fresh layers continually passing over the wet ground, takes up the moisture; like the east wind, for instance, which has great capabilities of that nature. Damp air can only take up a certain quantity, and when it contains as much water as corresponds to the temperature it can take no more, and is “saturated with moisture”; then evaporation ceases. Heat is a great cause of evaporation, and the greater the surface the more rapid the process, and in a vacuum more readily than in atmospheric air. Evaporation is resorted to very commonly to produce coolness; for instance, the universal fan, by increasing evaporation from a heated skin, generates a feeling of coolness; and we know the vaporization of ether will freeze into insensibility. When a fluid evaporates we can tell that the heat passes away at the same time, for we[Pg 85] cool water in porous jars, which permit some of it to pass off in vapour, the remainder being cooled.

Sir John Leslie invented a method of freezing water by rapid evaporation on sulphuric acid under the receiver of an air-pump, and water has been frozen even on a hot plate by these means. By pouring sulphurous acid and water on this plate, the acid evaporates so quickly that it produces sufficient cold to freeze the water it quitted into solid ice.

We leave the phenomena of clouds and watery vapour in the atmosphere for consideration on another opportunity, under the head of Meteorology, Rain, etc.

Fig. 80.—Apparatus for freezing carafes of water.

An experiment is often performed by which water is frozen in a vacuum. By putting a saucer full of water under the receiver of an air-pump it will first boil, and then become a solid mass of ice. It is not difficult to understand the cause of this. The water boils as soon as the air is removed; but in order to pass from the liquid to the gaseous state without the assistance of exterior heat, it gives out heat to the surroundings, and in so doing becomes ice itself. This fact Mr. Carré has made use of in the apparatus shown above (fig. 80). A small pump creates a vacuum in the water bottles, and ice is formed in them.

This apparatus might easily be adopted in country houses, and in places where ice is difficult to procure in summer. The only inconvenience attending it is the employment of sulphuric acid, of which a considerable quantity is[Pg 86] used to absorb the vapour from the water, as already referred to. If proper precautions are taken, however, there will be no danger in using the apparatus.

The mode of proceeding is as follows:—The bottle full of water is joined to the air-pump by a tube, and after a few strokes the water is seen in ebullition. The vapour thus disengaged traverses an intermediate reservoir filled with sulphuric acid, which absorbs it, and immediately condenses it, producing intense cold. In the centre of the liquid remaining in the carafe some needles of ice will be seen, which grow rapidly, and after a few more strokes of the pump the water will be found transformed into a mass of ice. This is very easy of accomplishment, and in less than a minute the carafe full of water will be found frozen.

The problem for the truly economical formation of ice by artificial means is one of those which have occupied chemists for a long time, but hitherto, notwithstanding all their efforts, no satisfactory conclusion has been arrived at. Nearly every arrangement possesses some drawback to its complete success, which greatly increases the cost of the ice, and causes inconvenience in its production. The usual mode in large towns is to collect the ice, in houses constructed for the purpose, during the winter, and this simple method is also the best, so far as at present has been ascertained.

Fig. 81.—Retort and Receiver.

In connection with vaporization we may now mention two processes referred to just now (page 83); viz., sublimation and distillation. The former is the means whereby we change solid bodies into vapour and condense the vapour into proper vessels. The condensed substances when deposited are called sublimates, and when we go into Chemistry we shall hear more of them. The mode of proceeding is to place the substance in a glass tube, and apply heat to it. Vapour will be formed, and will condense at the cool end of the tube. The sublimate of sulphur is called “Flowers of Sulphur,” and that of perchloride of mercury “Corrosive Sublimate.”

Distillation is a more useful process, or, at any rate, one more frequently employed, and is used to separate a volatile body from substances not volatile. A distilling apparatus (distillo, to drop) converts a liquid to vapour by means of heat, and then condenses it by cold in a separate vessel.

The distilling apparatus consists of three parts,—the vessel in which the liquid is heated (the still, or retort), the condenser, and the receiver. The simple retort and receiver are shown in fig. 81. But when very volatile vapours are dealt with, the arrangement shown on next page is used (fig. 82).[Pg 87] Then the vapour passes into the tube encased in a larger one, the intervening space being filled with cold water from the tap above (c), the warm water dropping from g. The vapours are thus condensed, and drop into the bottle (or receiver) B.

Fig. 82.—Distilling apparatus.

The apparatus for distilling spirits is shown below. The “still” A is fitted into a furnace, and communicates with a worm O in a metal cylinder filled with water, kept constantly renewed through the tube TT′. This spirit passes through the spiral, and being condensed, goes out into the receiver C.

Fig. 83.—Spirit still.

There are even more simple apparatus for spirit distilling, but the diagram above will show the principle of all “stills.” In former days, in Ireland, whiskey was generally procured illegally by these means.

[Pg 88]

CHAPTER VIII.

SPECIFIC HEAT—FUSION—LATENT HEAT—CONDUCTION AND CONVECTION OF HEAT—CALORESCENCE.

We have considered the effects of heat upon water, and touched upon one or two kindred experiments. But we have some other subjects to discuss, two in particular; viz., Specific Heat, and Latent Heat.

The specific heat of any substance is “the number of units of heat required to raise one pound of such substance one degree.” We can explain this farther. When heat is communicated to a body it has two or three functions to perform. Some of it has to overcome the resistance of the air in expanding the body, more of it expands, and the remainder increases the temperature of the body. So some heat disappears as heat, and is turned into energy,—“molecular potential energy,”—as it is called, and the rest remains. Of course in objects the molecules vary very much in weight and in their mutual attraction, and the heat requisite to raise equal weights of different substances through the same number of degrees of temperature will vary. This is called capacity for heat, or specific heat. The capacity of different metals for heat can easily be shown. The specific heat of water is very high, because its capacity for heat is great. We can cool a hot iron in very little water, and it takes thirty times as much heat to raise a given weight of water a certain number of degrees, as it would to raise the same weight of mercury to the same temperature. Water has greater specific heat, generally speaking, than other bodies, and it is owing to this circumstance that the climate is so affected by ocean currents.

Nearly all substances can be melted by heat, if we go far enough, or frozen, if we could take the heat away. Solid can be made liquid, and these liquids can be made gases and fly off in vapour. Similarly, if we could only get heat away sufficiently from the atoms of a substance we could freeze it. We cannot freeze alcohol, nor make ice from air, nor can we liquify it, for we are unable to take away its heat sufficiently. But we can turn water into steam, and into ice; or ice into water, and then into steam. But there is one body we cannot melt by heat, that is carbon. In the hottest fire coal will not melt, it becomes soft. We call this melting fusion, and every body has its melting point, or fusing point, which is the same at all times if the air pressure be the same.

It is a curious fact that when a body is melting it rises to a certain temperature (its fusing point), and then gets no hotter, no matter whether or[Pg 89] not the fire be increased;—all the extra heat goes to melt the remainder of the substance. The heat only produces changes of state. So this heat above fusing point disappears apparently, and is called Latent Heat. This can easily be proved by melting ice. Ice melts at 32° Fahr., or 0° Cent., and at that temperature it will remain so long as any ice is left; but the water at 32°, into which the ice has melted, contains a great deal of latent heat, for it has melted the ice quickly, and yet the thermometer does not show it. It is just the same with boiling water.

When substances are fused they expand as a rule, but ice contracts; so does antimony. On the other hand, when water solidifies it does not contract as most things do. It expands, as many of us are aware, by finding our water pipes burst in the winter; and the geologist will tell us how the tiny trickling rills of water fall in between the cracks of rocks and there freeze. In freezing the drops expand and split the granite blocks. Type-metal expands also when it becomes solid, and leaves us a clear type; but copper contracts, and won’t do for moulding, so we have to stamp it when we want an impression on it.

There is no doubt that chemical combinations produce heat, as we can see every day in house-building operations, when water is poured upon lime; but there are also chemical combinations which produce cold. Fahrenheit produced his greatest cold by combining snow and salt, for in the act of combining, a great quantity of heat is swallowed up by reason of the heat becoming latent, as it will do when solid bodies become liquid. Such mixtures or combinations are used as Freezing Mixtures when it is necessary to produce intense cold artificially. Sulphate of Sodium and Hydrochloric acid will also produce great cold, and there are many other combinations equally or even more efficacious.

Heat is communicated to surrounding objects in three well-known ways—by conduction, by radiation, and by convection. Conduction of heat is easily understood, and is the propagation of heat through any body, and it varies very much according to the substance through which it passes. Some substances are better conductors of heat than others. Silver has a far greater conductivity than gold, and copper is a better heat-conductor than tin. Flannel is a non-conductor, or rather a bad conductor, for no substance can be termed actually a non-conductor. Flannel, we know, will keep ice from melting, and a sheep’s wool or a bird’s feathers are also bad conductors of heat; so Nature has provided these coverings to keep in the animal heat of the body. A good conductor of heat feels cold to the touch of our fingers, because it takes the heat from our hands. This can be tried by touching silver, lead, marble, wood, and wool. Each in turn will feel cold and less cold, because they respectively draw away, or conduct less and less heat from our bodies. So our clothes are made of bad-conducting substances. The bark of a tree is a bad conductor, and if you strip off this clothing the tree will die.

Solids conduct heat the better the more compact they are. Air being[Pg 90] a bad conductor it follows that the less tightly the molecules are packed the less conductibility there will be; and even a substance powdered will be a worse conductor than the same substance in solid form; and also more readily in the direction of the fibres than crossways.

Liquids do not possess great conductivity, but they, as well as gases, are influenced by convection, or the transport of heat from the bottom layers to the top (conveho, to carry up). We have already mentioned that the heated particles of water rise to the top because they expand, and so become lighter. This is convection of heat; and by it liquids and gases, though actually bad conductors, may become heated throughout to a uniform temperature. Of course the more easily expansible the body is the more rapidly will convection take place—so gases are more readily affected than liquids. Solids are not affected, because convection of heat depends upon molecular movement or mobility, and it is obvious that the particles of solid bodies are not mobile. Professor Balfour Stewart says with reference to this that “were there no gravity there would be no convection,” for the displacement of the light warm particles by the heavier cold ones is due to gravity. The instances of convection of heat in nature are numerous, and on a gigantic scale. The ocean currents, trade winds, lake freezing, etc., while the chimney draught already referred to, is another example; and in all these cases the particles of air or water are replaced by convection. In the case of the lake freezing the cold particles at the top sink, and the warmer ones ascend, until all the lake is at a temperature of 36·2°, or say 4° above freezing. At this temperature water assumes its maximum density, and then expands, as we have seen, instead of contracting. Ice is formed, and being thus lighter than water, floats; and so unites to cover in the water underneath, which is never frozen solid, because the cold of the atmosphere cannot reach it through the ice in time to solidify the whole mass.

Fig. 84.—Radiant heat.

Radiant heat is the motion of heat transmitted to the ether, and through it in the form of waves. The sun’s heat is radiant heat, and radiation may be defined as “The communication of the motion of heat from the articles of a heated substance to the ether.” The fire gives out radiant heat, and so does heated metal, and it is transmitted by an unseen medium. It is quite certain that the heat of a suspended red-hot poker is not communicated to the air, because it will cool equally in a vacuum. Sir Humphrey Davy proved that radiant heat could traverse a vacuum, for by putting tin reflectors in an exhausted receiver he found that a hot substance in the focus of one reflector caused an increase in the heat of the other. If we put a red-hot or a hot substance in one reflector, and tinder in the other, the latter will take fire. The velocity of heat rays is equal to that of light, 186,000 miles in a[Pg 91] second, and indeed, radiant heat is identical with light. Heat is reflected as is light, and is refracted in the same way as sound.

Some bodies allow the heat rays to pass through them, as air does, and as rock salt will do. White clothing is preferable in summer (and also in winter if we could only make people believe it). White garments radiate less heat in winter, and absorb less heat in summer. An old black kettle will boil water more quickly than a new bright one, but the latter will keep the water hotter for the longer time when not on the fire.

Heat, then, is movement of particles. Energy can be changed into heat, as the savage finds when he rubs the bits of wood to produce heat and fire. Friction causes heat, and chemical combination produces heat; and, if “visible energy can be turned into heat, heat can be turned back into visible energy.” For fire heats water, water expands into steam, and steam produces motion and energy in the steam-engine.

If we heat water in Wollaston’s bulb,—the opening of which is hermetically stopped by a piston,—the vapour will raise the piston. If we cool the bulb we condense the steam, and the piston falls. Here we have the principle of the steam-engine.

Steam is the vapour of water educed by heat, and we may give a few particulars concerning it. Its mechanical properties are the same as those of other gases, and pure steam is colourless and transparent—in fact, invisible. Its power when confined in boilers and subjected to pressure is enormous, for the volume of the steam is far greater than the water which gave rise to it. One cubic inch of water will produce 1,700 cubic inches of steam—in other words, a cubic inch of water produces a cubic foot of steam. When we obtain steam at 212°, we do so under the pressure of one atmosphere; but by increasing the pressure we can raise the boiling point, and thus water at the pressures of sixteen atmospheres will not steam till it reaches 398°. It is thus we obtain pressure for locomotives, and other engines, although a very small portion of the steam does work. Much the largest portion is expended in overcoming cohesion, and one way and another, taking into consideration defects in machinery, only about one-tenth of the heat is employed in doing the work. The force exercised by steam under atmospheric pressure is sufficient to raise a ton weight one foot.

To obtain very high temperatures we shall find the thermometer of no use, for mercury boils at 662°, so an instrument called a Pyrometer is used to ascertain the fusing point of metals. Mr. Wedgwood, the celebrated china manufacturer, invented an instrument made of small cylinders of clay moulded and backed, placed between two brass rods as gauges divided into inches and tenths. But this instrument has been long superseded by Professor Daniell’s Pyrometer, which consists of a small bar of platina in an earthenware tube. The difference of expansion between the platina and the tube is measured on a scale on which one degree is equal to seven degrees of Fahrenheit. Thus the melting temperatures of metals are ascertained.

[Pg 92]

The reflection and refraction of heat are ruled by the same laws as the reflection and refraction of light. A convex lens will bring the heat or light to a focus, and will act as a burning-glass if held in the sunlight. Gunpowder has been ignited by a lens of ice, and more than one house has been mysteriously set on fire at midday in summer by the sun’s rays shining through a glass globe of water containing gold fish, and falling upon some inflammable substance. Professor Tyndall performed a series of experiments of a very interesting nature, described in his book, “Heat considered as a Mode or Motion,” and showed the transmutation of invisible heat rays into visible rays, by passing a beam of electric light through an opaque solution, and concentrating it upon a lens. The dark heat rays were thus brought to a focus, all the light was cut off, and at the dark focus the heat was found to be intense enough to melt copper and explode gunpowder. This change of invisible heat into light is termed Calorescence.

It was Sir William Herschell who discovered that there were heat rays beyond the red end of the spectrum. When light is split up into its component rays, or decomposed, Sir William found that the heat increased as the thermometer passed from violet to indigo, and so on to blue, green, orange, and red, and the last were the hottest, while beyond the spectrum there was heat even greater. A Heat Spectrum was thus discovered, and by comparing, by means of the thermometer, the various degrees of heat within certain limits, Professor Tyndall found that the invisible Heat Spectrum is longer than the visible Light Spectrum.

[Pg 93]

CHAPTER IX.

LIGHT AND ITS SOURCES—WHAT IS LIGHT?—VELOCITY OF LIGHT—REFLECTION AND REFRACTION—RELATIVE VALUE OF LIGHTS.

The subject of Light and the science of Optics are so interesting to all of us that some short history of light is necessary before we can enter upon the scientific portion of the subject. The nature of the agent (as we may term light) upon which our sight depends has employed man’s mind from a very early period. The ancients were of opinion that the light proceeded from the eye to the object looked at. But they discovered some of the properties of light. Ptolemy of Alexandria, who was born A.D. 70, made some attempts to discover the law of Refraction; and we are informed that Archimedes set the Roman fleet on fire with burning-glasses at Syracuse. The Arabian treatise of Alhagen, in 1100 A.D., contains a description of the eye and its several parts; and the writer notices refraction and the effects of magnifying glasses (or spectacles). Galen, the physician, practically discovered the principle of the stereoscope, for he laid down the law that our view of a solid body is made up of two pictures seen by each eye separately.

Still the science of optics made little progress till the law determining the path of a ray of light was made known, and the laws of refraction discovered. Refraction means that a ray is deflected from its straight course by its passage from one transparent medium to another of different density. The old philosophers found out the theory of sound, and they applied themselves to light. Newton said light consisted of minute particles emanating from luminous bodies. Huyghens and Euler opposed Newton’s theory of the emission of light; and it was not till the celebrated Thomas Young, Professor at the Royal Institution, grappled with the question that the undulating or wave theory of light was found out. He based his investigations upon the theory of sound waves; and we know that heat, light, and sound are most wonderfully allied in their manner of motion by vibration. But he was ridiculed, and his work temporarily suppressed by Mr. Brougham.

Light, then, is a vibratory motion (like sound and heat), a motion of the atoms of our ether. But how is the motion transmitted? Sound has its medium, air; and in a vacuum sounds will be very indistinctly heard, if heard at all. But what is the medium of communication of light? It is decided that light is transmitted through a medium called ether, a very elastic substance surrounding us. The vibrations, Professor Tyndall and[Pg 94] other philosophers tell us, of the luminous atoms are communicated to this ether, or propagated through it in waves; these waves enter the pupil of the eye, and strike upon the retina. The motion is thus communicated by the optic nerve to the brain, and then arises the great primary faculty, Consciousness. We see light, the waves of which, or ether vibrations, are transversal; air waves or sound vibrations are longitudinal.

We have spoken of radiant heat. Light acts in the same way through the ether; and when we consider Sound we shall learn that a certain number of vibrations of a string give a certain sound, and the quicker the vibration the shriller the tone. So in light. The more quickly the waves of luminosity travel to our eye, and the faster they strike it, the greater the difference in the colour, or what we call colour. Light as we see it is composed of different colours, as visible in the rainbow. There are seven primary colours in the sunlight, which is white. These can be divided or “dispersed,” and the shortest rays of the spectrum are found to be violet, the longest red. It has been calculated that 39,000 red waves make an inch in length. Light travels at a rate of nearly 190,000 miles a second, so if we multiply the number of inches in that distance by the number of red waves, we shall have millions of millions of waves entering the eye in a single second of time. The other waves enter more rapidly still, and “the number of shocks corresponding to the impression of violet is seven hundred and eighty-nine millions of millions” per second! Or taking the velocity of light at 186,000 miles in a second, it would be six hundred and seventy-eight millions of millions (Tyndall). There may be other colours which we cannot see because the impressions come too rapidly upon the retina; but the violet impression has been thus accurately determined. See page 168.

We have seen that heat is a kind of motion of particles in a body—a vibratory motion which, instead of being apparent to the ear, is apparent to the eye in rays of light. Thus heat, sound, and light are all intimately connected in this way. We have also learnt that rays of light radiate and travel with tremendous speed to our eyes, but without any shock. There is no feeling connected with the entrance of light to the eye any more than there is any sensation of sound when entering the ear, except when the light is vividly and very suddenly revealed, or when a very piercing sound is heard. Then the nerves are excited, and a painful sensation is the result; but under ordinary circumstances we are not physically conscious of the entrance of light or sound.

Heat and light are considered to be one and the same thing in different degrees of intensity. The sources of light are various. The sun and fixed stars, heat, electricity, many animals, and some plants, as well as decaying animal matter, give out light. There are luminous and non-luminous bodies. The moon is non-luminous, as she derives her light from the sun, as does the earth, etc.

Light is distributed in rays. These rays are straight in all directions. The velocity of light is almost inconceivable. It travels at a rate of 186,500[Pg 95] miles a second. The latest computation with electric light has given a rate of 187,200 miles a second; but the blue rays in the light experimented on probably account for the difference, for blue rays travel quicker by one per cent. than red rays. Römer first found out the velocity of light, which comes to us from the sun—ninety millions of miles—in eight minutes. Fizeau calculated the velocity by means of a wheel, which was set moving with tremendous speed by making the light pass between the teeth of the wheel and back again.

When rays of light meet substances they are deflected, and the phenomena under these circumstances are somewhat similar to the phenomena of heat and sound. There are three particular conditions of rays of light: (1) they are absorbed; (2) they are reflected; (3) they are refracted.

Firstly. Let us see what we mean by light being absorbed; and this is not difficult to understand, for any “black” substance shows us at once that all the sunlight is taken in by the black object, and does not come out again. It does not take in the light and radiate it, as it might heat. The rose is red, because the rays of light pass through it, and certain of them are reflected from within. So colour may be stated to be the rays thrown out by the objects themselves—those they reject or reflect being the “colour” of the object.

Fig. 85.—Angle of reflection, etc.

Secondly. Bodies which reflect light very perfectly are known as mirrors, and they are termed plane, concave, or convex mirrors, according to form. A plane mirror reflects so that the reflected ray d i forms the same angle with the perpendicular as the incident ray r i; in other words, the angle of incidence is always equal to the angle of reflection, and these rays are perpendicular to the plane from which they are reflected. The rays diverge, so that they appear to come from a point as far behind the mirror as the luminous point is in front, and the images reflected have the same appearance, but reversed. There is another law, which is that “the angular velocity of a beam reflected from a mirror is twice that of the mirror.” The Kaleidoscope, with which we are all familiar, is based upon the fact of the multiplication of images by two mirrors inclining towards each other.

Fig. 86.—Concave mirror.

A concave mirror is seen in the accompanying diagram, and may be called the segment of a hollow sphere—V W. The point C is the geometrical centre, and O C the radius; F is the focus; the line passing through it is the optical axis; O being the optical centre. All perpendicular rays pass through C. All rays falling in a direction parallel with the optical axis are reflected and collected at F. Magnified images will be produced, and if the object be placed between the mirror and the focus, the image will appear at the back; while if the object be placed between the geo[Pg 96]metrical centre and the focus, the image will appear to be in front of the mirror.

We can understand these phenomena by the accompanying diagrams. Suppose a ray A n passes from one object, A B, at right angles, it will be reflected as n A C, the ray A C being reflected to F. These cannot meet in front of the mirror, but they will if produced meet at a, and the point A will be reflected there; similarly B will be reflected at b, and thus a magnified image will appear behind or at the back of the mirror’s surface. In the next diagram the second supposed case will produce the image in the air at a b, and if a sheet of paper be held so that the rays are intercepted, the image will be visible on the sheet. In this case the perpendicular ray, A n, is reflected in the same direction, and the ray, a c, parallel with the axis is reflected to the focus. These rays meet at a and corresponding rays at b, when the image will be reproduced; viz., in front of the mirror.

Fig. 87.—Reflection of mirrors (I).

Fig. 88.—Reflection of mirrors (II).

The concave mirror is used in the manufacture of telescopes, which, with other optical instruments, will be described in their proper places. We will now look at the Refraction of light.

Bodies which permit rays of light to pass through them are termed transparent. Some possess this property more than others, and so long as the light passes through the same medium the direction will remain the same. But if a ray fall upon a body of a different degree of density it cannot proceed in the same direction, and it will be broken or refracted, the angle it makes being termed the angle of refraction.

[Pg 97]

Fig. 89.—Refraction in water.

For instance, a straight stick when plunged into water appears to be broken at the point of immersion. This appearance is caused by the rays of light taking a different direction to our eyes. If in the diagram (fig. 89) our eye were at o, and the vessel were empty, we should not see m; but when water is poured into the vessel the object will appear higher up at n, and all objects under water appear higher than they really are.

Fig. 90.—A water-bottle employed as a convergent lens.

One may also place a piece of money at the bottom of a basin, and then stoop down gradually, until, the edge of the basin intervening, the coin is lost to view. If an operator then fills the basin with water, the piece of money appears as though the bottom had been raised. The glass lenses used by professors may be very well replaced by a round water-bottle full of water. A candle is lighted in the darkness, and on holding the bottle between the light and a wall which acts as a screen, we see the reflected light turned upside down by means of the convergent lens we have improvised (fig. 90). A balloon of glass constitutes an excellent microscope. It must be filled with perfectly clear, limpid water, and closed by means of a cork. A piece of wire is then rolled round its neck, and one end is raised, and turned up towards the focus; viz., to support the object we wish to examine, which is magnified several diameters. If a fly, for instance, is at the end of the wire, we find it is highly magnified when seen through the glass balloon (fig. 91). By examining the insect through the water in the balloon, we can distinguish every feature of its organism, thanks to this improvised magnifier. This[Pg 98] little apparatus may also serve to increase the intensity of a luminous focus of feeble power, such as a lighted candle. It is often employed in this manner by watchmakers. If a bottle full of water is placed on a table, and exposed to the rays of the sun, the head of a lucifer match being placed in the brightest centre of light caused by the refracted rays, the match will not fail to ignite. I have succeeded in this experiment even under an October sun, and still more readily in warm weather.

Fig. 91.—A simple microscope formed with a glass balloon full of water.

In the Conservatoire des Arts in Paris a visitor will always notice a number of people looking at the mirrors in the “optical” cabinets. These mirrors deform and distort objects in a very curious manner, and people find much amusement in gazing into them till they are “moved on” by the attendants. Such experiments create great interest, and a very excellent substitute for these may be found in a coffee-pot or even in a large spoon, and all the grotesque appearance will be seen in the polished surface. The least costly apparatus will sometimes produce the most marvellous effects. Look at a soap-bubble blown from the end of a straw. When the sphere has a very small diameter the pellicule is colourless and transparent; but as the air enters by degrees, pressing upon all parts of the concave surface equally, the bubble gets bigger as the thickness decreases, and then the colours appear,—feeble at first, but stronger and stronger as the thickness[Pg 99] diminishes. The study of soap-bubbles and of the effects of the light is very interesting. Newton made the soap-bubble the object of his studies and meditations, and it will ever hold its place amongst the curious phenomena of the Science of Optics. But before going into all the phases of Lights and Optics we will proceed to explain the structure of the eye, as it is through that organ that we are enabled to appreciate light and its marvellous effects.

Fig. 92.—Grotesque effects of curved surfaces.

It is often considered an embarrassing matter to fix precisely the value of two lights. Nothing, however, can be easier in reality, as we will show. In comparing different lights, it is necessary to bear in mind the amount of waste, the colour of the light, the luminous value of the source, and the steadiness of the flame. The luminous value of a lamp-burner is generally equalled by that of a wax candle, and we will take as an example one of those at six to the pound. Very precise appliances are used for this experiment when great exactness is required; but it is easy to calculate in a simple manner the differences in ordinary lights. Supposing we desire to test the value of light given by a lamp and a wax candle, they must both be placed on the table at an equal height, B and A, (fig. 93), in front of some opaque body, A, and then a large sheet of paper must be fixed as vertically as possible to form a screen. When B and A are lighted, two shadows, E and F, are produced, to which it is easy to give exactly the same[Pg 100] intensity, by advancing or withdrawing one of the two sources of light. The intensities of the two lights will then be inversely proportional to the squares of the measured distances, AB and AC. By a similar careful calculation it has been possible to draw up a table of the relative values of various ordinary lights. We have not included here the electric light, which has recently attracted so much attention, because this system of lighting can hardly be said to have yet penetrated the domain of domestic life; but when we consider electricity, as we hope to do in a future part, we[Pg 101] intend to study this question fully, for there is no doubt that electricity is becoming more and more adapted to our daily life.

Fig. 93.—An elementary Photometer.

The measurement of intensity of light is called Photometry, and the instruments used are Photometers. Bunsen’s instrument consists of a screen of writing-paper, saturated in places with spermaceti to make it transparent. A sperm candle is placed on one side, and the light to be compared on the other. The lights are provided with graduated bars, and these lights are then removed farther and farther from the screen till the spots of grease are invisible. The relative intensities are as the squares of the distance from the screen.

Fig. 94.—The soap-bubble.

We append a table showing the comparative cost of light given by Dr. Frankland at the Royal Institution some few years ago. The standard of comparison was 20 sperm candles burning for 10 hours at the rate of 120 grains an hour:—

There are many other interesting experiments connected with Light,—Spectrum Analysis, etc., etc.,—all of which we will defer for a time until we have examined the Eye and some effects produced upon it by Light, illustrated by numerous diagrams in the pages next following.

[Pg 102]

CHAPTER X.

VISION AND OPTICAL ILLUSIONS—THE EYE DESCRIBED—ACCOMMODATION OF THE EYE—CHROMATIC ABERRATION—SPINNING TOPS.

The eye is an optical instrument that may be compared with those constructed by physicists themselves; the media of which it is composed have surfaces like those which enter into the construction of optical instruments. It was Kepler who at the end of the eighteenth century discovered the passage of light into the eye. Soon after the discovery of the inner chamber he found that the eye realized the conditions that Porta had combined to obtain the reflection of external objects.

Fig. 95.—Structure of the eye.

We will now briefly state that the coats of this organ are constituted of a fibrous membrane, T (fig. 95), termed sclerotic, which is opaque, except in the anterior portion of the eye, where it forms the transparent cornea. The crystalline, C, enshrined behind the cornea, is the convergent lens of the inner chamber; it is covered with a transparent membrane, or capsule, and is bathed in two fluids, the aqueous humour, between the crystalline humour and the cornea, and the vitreous body, a gelatinous humour lodged between the crystalline and the back of the eye. The image of exterior objects which is produced by the passage of light through these refracting surfaces, is received by a nervous membrane, the retina, B, formed by an expansion of the optic nerve, N. We must also mention the choroid, a membrane lined with a dark pigment, which absorbs the light, and prevents interior reflections, and in front of the crystalline lens, a curtain with an opening, H, called the iris, which gives to the eyes their colour of blue, grey, or black. The opening in the centre of the iris is called the pupil.

The penetration of light through the surfaces of the eye is easily demonstrated. An object throws divergent rays on the cornea, a part penetrates into the eye and falls upon the retina, leaving a perfectly retained image of the object. Magendie has proved in the following manner the truth of this mathematical deduction. The eye of a rabbit is very similar to an[Pg 103] albino’s; that is to say, the choroid contains no black pigment, but a transparent matter, and when placed before a brilliant object, the image can be seen inverted on the retina. The experiment succeeds also with the eye of a sheep or a cow, if the sclerotic has been lessened. The optic centre of the eye is the point where the secondary axes cross; the optic axis passes through the geometrical axis of the organ, and directs itself spontaneously towards the point that attracts the eye.

Fig. 96.—Diagram of mode of vision.

We will now point out in what distinct vision consists. A screen placed behind a lens will only receive the image of a lighted object, A B, if placed in a position, R R (fig. 96). If placed nearer at R″ R″, or further off at R´ R´, the light from the object is thrown on the screen, and the image is confused. To prove the imperfection of sight which is shown by the application of these theoretic rules, MM. Boutan and d’Alméïda10 cite the following experiment:—If the head of a pin is placed from one to two inches from the eye, nothing will be perceived but a confused haziness of vague outline. The distance of distinct vision is that at which an object of small dimensions may be placed to be plainly perceived. This distance, which averages fifteen inches, varies with different individuals. It can be determined for different sights by means of an apparatus constructed by Lepot. A white thread, a, is stretched horizontally on a dark board (fig. 97). We look at it by placing our eye at one end behind a little screen pierced with an aperture, O; it then appears much reduced in length, but either nearer or farther off it seems to enlarge and swell, having the appearance of a white surface, becoming larger and larger in proportion as we move away from the point at which it is seen most distinctly. In this manner we can easily obtain a measure of the distance of distinct vision. One of the most remarkable properties of the eye consists in the faculty which this organ possesses of seeing different distances. If we consider it as a dark chamber, there is but one distance at which an object will be perfectly visible; nevertheless a metal wire, for example, can be seen as well at a distance of seven, as ten, fifteen, or twenty inches by good sights.

Fig. 97.—Experiment for sight.

This faculty of accommodation in the eye is thus demonstrated: we place two pins, one in front of the other, one eye only being open; we first look at the nearest pin, which appears confused if it is near the eye, but by an effort of will the image becomes clear. If, while preserving the clearness of the image, we then carry our attention to the second pin, we find that it, too, presents a confused appearance. If we make an effort to distinguish the[Pg 104] contour of the second pin, we at last succeed, and the first once more appears ill-defined. It is only since the experiments of M. Cramer and M. Helmholtz that the explanation of this phenomenon could have been given. M. Cramer has succeeded in determining on the living eye the curved ray of the cornea, and of the two surfaces of the crystalline lens. In so doing he followed Samson’s method, and observed the images thrown by a luminous object, whose rays strike the different refracting surfaces of the eye. A candle, L (fig. 98), is placed before the eye, O, and throws as in a convex mirror a straight image of the flame, A (fig. 99). The other portion of the light, which has penetrated the pupil, falls on the crystalline lens, and produces likewise a second straight image, B. Then the light refracted by the lens reaches the posterior surface; a portion is reflected on a concave mirror, and gives the inverted image, C, very small and brilliant. M. Cramer observed it through a microscope, and studied the variations in the size of images when the eye passed from the observation of adjacent to distant objects. He stated:—

Fig. 98.—M. Cramer’s experiment.

Fig. 99.—Images in the eye.

1. That the image, A, formed on the surface of the cornea, remains the same size in both cases; the form of the cornea therefore remains unaltered.

2. That the image, B, formed on the upper surface of the lens, diminishes in proportion as the eye is nearer the object; the surface therefore becoming more and more convex, as the focal distance diminishes—a result indicated by the theory that it is possible in the vision of near objects to receive the image on the retina.

3. That the third image, C, produced on the posterior surface of the lens, remains nearly invariable.

We may confirm Cramer’s statements by an easy experiment. We place ourselves in front of the eye of someone who looks in turn at two objects placed on the same black line at unequal distances from him, and are able to distinguish by the dimension of the images of the candle, which object it is that he is regarding. M. Helmholtz has carried M. Cramer’s methods to perfection, and has been able to formulate a complete theory of all the phenomena of accommodation. The laws of optics show that the rays emitted by a luminous point may unite at another point by the action of the refracting surfaces of the eye. Nevertheless, a white light being composed of rays of diverse refrangibility, particular effects, known under the name of chromatic aberration, are produced through the decomposition of light, which we will proceed to study, under M. Helmholtz’s auspices11.[Pg 105] We make a narrow opening in a screen, and fix behind this opening a violet glass, penetrable only by red and violet rays. We then place a light, the red rays of which reach the eye of the observer after having passed through the glass and the opening in the screen. If the eye is adapted to the red rays, the violet rays will form a circle of diffusion, and a red point encircled with a violet aureola is seen. The eye may also be brought to a state of refraction, so that the point of convergence of the violet rays is in front, and that of the red rays behind the retina, the diameters of the red and violet circles of diffusion being equal. It is then only that the luminous point appears monochromatic. When the eye is in this state of refraction, the simple rays, whose refrangibility is maintained between the red and the violet rays, unite on the retina.

There is another kind of aberration of luminous rays of one colour emitted through a hole, which generally only approach approximately to a mathematical focus, in consequence of the properties of refracting surfaces; it is called aberration of sphericity. The phenomena are as follows:—

Fig. 100.

1. We take for our object a very small luminous point (the hole made by a pin in some black paper, through which the light passes), and having also placed before the eye a convex glass, if we are not near-sighted, we fix it a little beyond the point of accommodation, so that it produces on the retina a little circle of diffusion. We then see, instead of the luminous point, a figure representing from four to eight irregular rays, which generally differ with both eyes, and also with different people. We have given the result of M. Helmholtz’s observations in fig. 100; a corresponds to the right eye, and b to the left. The outer edges of the luminous parts of an image, produced in this way by a white light, are bordered with blue; the edges towards the centre are of a reddish yellow. The writer adds that the figure appears to him to have greater length than breadth. If the light is feeble, only the most brilliant parts of the figure can be seen, and several images of the luminous point are visible, of which one is generally more brilliant than the others. If, on the other hand, the light is very intense,—if, for example, the direct light of the sun passes through a small opening,—the rays mingle with each other, and are surrounded by aureola of rays, composed of numberless extremely fine lines, of all colours, possessing a much larger diameter, and which we distinguish by the name of the aureola of capillary rays.

Fig. 101.

The radiating form of stars, and the distant light of street-lamps belong to the preceding phenomena. If the eye is accommodated to a greater distance than that of the luminous point,—and for this purpose, if the luminous point itself is distant, we place before the eye a slightly convex lens,—we see another radiating image appear,[Pg 106] which M. Helmholtz represents thus (fig. 101): at c as it is presented to the right eye, and at d as seen by the left.

If the pupil is covered on one side, the side opposite to the image of diffusion disappears; that is to say, that part of the retinal image situated on the same side as the covered half of the pupil. This figure, then, is formed by rays which have not yet crossed the axis of the eye. If we place the luminous point at a distance to which the eye can accommodate itself, we see, through a moderate light, a small, round, luminous spot, without any irregularities. If the light, on the contrary, is intense, the image is radiated in every position of accommodation, and we merely find that on approaching nearer, the figure which was elongated, answering to a distant accommodation, gradually diminishes, grows rounder, and gives place to the vertically elongated figure, which belongs to the accommodation of a nearer point. When we examine a slender, luminous line, we behold images developed, which are easily foreseen, if for every point of the line we suppose radiating images of diffusion, which encroach on each other. The clearest portions of these images of diffusion mingle together and form distinct lines, which show multiplied images of the luminous line. Most persons will see two of these images; some, with the eyes in certain positions, will see five or six.

Fig. 102.

To show clearly by experiment the connection existing between double images and radiated images from points, it is sufficient to make in a dark sheet of paper a small rectilinear slit, and at a little distance from one end, on a line with the slit, a small round hole, as shown at a in fig. 102. Looking at it from a distance we shall see that the double images of the line have exactly the same distance between them that the most brilliant parts of the starred figure of diffusion have from the point, and that the latter are in a line with the first, as will be seen at b (fig. 102), where in the image of diffusion of the luminous point, we only see the clearest parts of star a of the figure.

On lighted surfaces, to which the eye is not exactly accommodated, multiplied images are often remarked through the passage from light to darkness being made by two or three successive steps.

A series of facts which have been collected under the title of irradiation, and which show that brightly-lighted surfaces appear larger than they are in reality, and that the dark surfaces which surround them appear diminished to a corresponding degree, explains this by the circumstance that the luminous sensation is not proportional to the intensity of the objective light. These phenomena affect very various appearances, according to the form of respective figures; they are generally seen with the greatest ease and intensity when the eye is not exactly accommodated to the object examined, either by the eye being too near or too far off, or by using a concave or convex lens, which prevents the object being seen clearly. Irradiation is not completely wanting, even when the accommodation is exact, and we notice it clearly in very luminous objects, above all when they are small; small[Pg 107] circles of diffusion increase relatively the dimensions of small objects much more than of large ones, with regard to which, the dimensions of the small circles of diffusion which the eye furnishes, when properly accommodated, become insensible.

Fig. 103.—Experiment 1.

1. Luminous surfaces appear larger. We can never judge exactly of the dimensions of a slit or small hole through which a bright light escapes; it always appears to us larger than it really is, even with the most exact accommodation. Similarly, the fixed stars appear in the form of small luminous surfaces, even when we make use of a glass which allows of perfect accommodation. If a gridiron with narrow bars—the spaces intervening being exactly equal to the thickness of the bars—is held over a light surface, the spaces will always appear wider than the bars. With an inexact accommodation, these phenomena are still more remarkable. Fig. 103 exhibits a white square on a black foundation, and a black square on a white foundation. Although the two squares have exactly the same dimensions, the white appears larger than the black, unless with an intense light and an inexact accommodation.

Fig. 104.—Experiment 2.

2. Two adjacent luminous surfaces mingle together. If we hold a fine metallic wire between the eye and the sun, or the light of a powerful lamp, we shall cease to see it; the lighted surfaces on all sides of the wire in the visual range pass one into the other, and become mingled. In objects composed of black and white squares, like those of a draught-board (fig. 104), the angles of the white squares join by irradiation, and separate the black squares.

3. Straight lines appear interrupted. If a ruler is held between the eye and the light of a bright lamp or the sun, we perceive a very distinct hollow on the edge of the ruler in the part corresponding to the light. When one point of the retina is affected by a light which undergoes periodical and regular variations, the duration of the period being sufficiently short, there results a continuous impression, like that which would be produced if the light given during each period were distributed in an equal manner through[Pg 108]out the whole duration of the period. To verify the truth of this law, we will make use of some discs, such as that represented in fig. 105. The innermost circle is half white and half black; the middle circle has two quarters, or half its periphery, white, and the outer circle has four eighths’ white, the rest being black. If such a disc is turned round, its entire surface will appear grey; only it is necessary to turn it with sufficient force to produce a continuous effect. The white may also be distributed in other ways, and provided only that on all the circles of the disc the proportion of the angles covered with white is the same, they will always exhibit the same grey colour. Instead of black and white we may make use of different colours, and obtain the same resultant colour from all the circles, when the proportion of the angles occupied by each of the colours in the different circles is the same.

Fig. 105.—Disc which appears uniformly grey by reason of its rotation.

If we paint on a disc a coloured star, which is detached from a foundation of another colour (fig. 106), during the rapid rotation of the disc the centre affects the colour of the star; the outer circle assumes that of the background, and the intermediate parts of the disc present the continuous series of the resultant colours. These results are in accordance with the theory of the mixture of colours.

Fig. 106.—Disc with a star painted on the background of another colour.

Rotative discs, which are so much used in experiments in optical physiology, were employed for the first time by Müsschenbroeck; the most simple is the top. M. Helmholtz ordinarily uses a brass spinning-top, which fig. 107 represents at a third the natural size. It is set in motion by the hand, and its quickness may be increased or moderated at will; but it cannot be made to spin quicker than six rounds in a second; this motion will be kept up for three or four minutes. Thus, with a feeble movement of rotation, a uniform luminous impression can only be obtained by dividing the disc into four or six sections, on each of which we repeat the same arrangement of colours, light, and shade. If the number of repetitions of the design is less, we obtain, with a bright light, a more or less shot-coloured disc.

Fig. 107.—M. Helmholtz’s top for studying the impression of light on the retina.

It is easy to place designs on the disc, even when in motion, or to make any desired modification, by superposing on the first disc another disc with sectors, of which we can vary the position by slightly touching it, or even blowing on it, thus producing during the rotation of the disc very[Pg 109] varied modifications. If, for instance, we place on a disc covered with blue and red sectors of equal size, a black disc, of which the sectors are alternately filled in or empty, the disc, as it turns round, will appear blue if the black sectors of the upper disc exactly cover the [red] sectors of the lower disc; and it appears red, if, on the contrary, the blue sectors are covered with the black; while in the intermediate positions we obtain different mixtures of red and white, and during the rotation of the disc may vary the colour insensibly by a gentle touch. By dividing the different sectors with broken or curved lines, instead of straight ones, we can produce an arrangement of coloured rings of great variety and beauty. To give the top greater speed, we set it in motion by drawing a string twined round its stem. The simplest method, as shown in fig. 108, consists in the employment of a handle similar to that of the German top. It is a hollow cylinder of wood set into a handle with two circular holes; and at right angles with these is a groove for the passage of the string. The stem of the top is passed through the holes of the cylinder, one end of the string is fixed in the small hole in the stem, and is rolled round by turning the top in the hand. The part of the stem on which the string is twisted becomes sufficiently thick for the top to remain suspended to the handle; then holding it a little above the table, and giving the string a powerful pull, we set the top in motion, and as the string unrolls it falls on the table, where it will continue its rotation for some time. The top represented in fig. 109 is constructed so that the discs may be firmly pressed by the stem, which is necessary in experiments for demonstrating Newton’s theory of the mingling of colours. We make use for this purpose of a variety of discs, made of strong paper of different sizes, having an opening in the centre and a slit, as in fig. 110; each of the discs is covered uniformly with a single colour; and if two or more are superposed, with their slits placed one over the other, we obtain sectors, the size of which we may vary at will, so that we can modify in a continuous manner the proportions of the colours. The most perfect construction is that of Busold’s chromatic top (fig. 111), which should only be employed for very rapid rotations. The disc, which weighs 5 lbs., is made of an alloy of zinc and lead, about an inch and a quarter in diameter. The brass axis[Pg 110] terminates at its lower end with a blunt point of untempered steel; the cylindrical part of the axis is roughened to encourage the adherence of the string; the axis is placed between the clamps of a vice, and a plate is put underneath; we then pull the string firmly with the right hand, and when the top is in motion it is separated from the clamps. By pulling the string very powerfully it is possible to obtain a speed of sixty turns in a second, and the movement will be kept up for three quarters of an hour.

Fig. 108.—Spinning a top with coloured discs.

Fig. 109.—Top for experiments demonstrating Newton’s theory of the mingling of colours.

Fig. 110.—Disc.

Besides tops, we may make use of different kinds of discs, with an axis rotating between two clamps; they are moved either by a kind of clock-work, or by the unrolling of a string, like the tops. Generally, however, these contrivances have this inconvenience, that the discs cannot be changed without stopping the instrument, and partly taking it to pieces. On the other hand, we have the advantage of being able to turn them on a vertical plane, so that we can conveniently carry on our experiments before a numerous auditory, which is a more difficult matter with tops. Montigny contrived to obtain the mingling of colours by means of a turning prism, which he caused to throw its shadow on a white screen. The Thaumatrope is a small rectangle of cardboard, which is made to rotate on an axis passing through the centres of the longest sides. We shall describe it at greater length when we come to consider a new apparatus known under the name of the Praxinoscope.

Fig. 111.—Busold’s chromatic top.

More complicated contrivances have also been constructed on the same principle, by which one may perceive the rotating disc through slits which turn at the same time. We will now describe the construction of some discs invented by Plateau under the name of the Phenakistoscope. These discs are made of strong cardboard, from six to ten inches in diameter (fig. 112), on which a certain number of figures (eight to twelve) are placed[Pg 111] in circles at an equal distance from each other, presenting the successive phases of a periodical movement. This disc is placed on another opaque circle of rather larger diameter, which has on its margin as many openings as the first disc has figures. The two discs are placed one on the other, and are fixed in the centre by means of a screw at the anterior extremity of a small iron axis, the other end being fitted into a handle. To make use of this contrivance we place ourselves in front of the glass, towards which we turn the disc with the figures, placing the eye so as to see the figures through one of the holes of the large disc. Directly the apparatus begins to turn round, the figures seen in the glass appear to execute the particular movements which they represent in different positions. Let us designate by means of the figures 1, 2, 3, the different openings through which the eye successively looks, and point out by the same numbers the figures in the radiuses thus numbered. If the experimenter looks in the glass through opening 1, he will see first figure 1, which appears in the glass to pass before his eyes; then the rotation of the disc displaces opening 1, and the cardboard intervenes, until opening 2 appears; then figure 2 takes the place of figure 1, until it in turn disappears, and opening 3 presents figure 3 to view. If these figures were all similar, the spectator would have but a series of visual impressions, separate but alike, which by a sufficiently rapid rotation mingle together in one durable impression like a perfectly immovable object. If, on the contrary, the figures differ slightly from each other, the luminous sensations will also mingle in a single object, which will however appear to be modified in a continuous manner, conformably with the differences of successive images. With a difference of speed, we obtain a new series of phenomena. A most simple contrivance of this kind is a top[Pg 112] of C. B. Dancer, of Manchester (fig. 113). It will be seen that the axis carries another disc, pierced with openings of different shapes, to the edge of which a thread is attached. This second disc is carried along by the friction of the axis, but its rotation is less rapid because of the great resistance offered by the air to the piece of thread which participates in the movement. If the lower disc has several differently-coloured sectors, they produce a very motley appearance, which seems to move sometimes by leaps, and sometimes by continuous motion. We must distinguish between the phenomena of successive contrast and simultaneous contrast.

Fig. 112.—Rotating disc.

Fig. 113.—Mr. Dancer’s top.

Phenomena of successive contrast develop what are called accidental images. If we fix our eyes for a considerable time on a coloured object,[Pg 113] and then suddenly direct them towards a uniform white surface, we experience the sensation of the object as it is, but it appears coloured with a complementary tint; that is to say, it has the colour which, superposed on the genuine tint, we obtain from pure white. Thus a red object produces a consecutive green object. The experiment can be tried by gazing at the sun when it is setting, and then directing one’s eyes towards a white wall in the same direction.

Phenomena of simultaneous contrast arise from the influence exercised over each other by different shades and colours which we see simultaneously. That we may be certain that we have really obtained phenomena of this kind, the experiments must be arranged in such a manner that accidental images are not produced, and that the part of the retina affected by the sensation of colour does not receive, even momentarily, a passing image.

Fig. 114.—Disc, which exhibits, when in rotation, a series of concentric rings.

The phenomena of simultaneous contrast appear with the greatest clearness with slight differences of colour, and are therefore exactly the contrary of phenomena of successive contrast, which are favoured by strong oppositions of colour and light. We can, in general, characterise phenomena of simultaneous contrast as governed by this law, common to all perceptions of the senses: the differences clearly perceived appear greater than the differences equal to them, but perceived with greater difficulty, either because they only affect the observation in an uncertain manner, or that the memory fails to judge of them. A man of middle height appears small beside a tall man, because at the moment it is forcibly impressed on us that there are taller men than he, and we lose sight of the fact that there are smaller. The same man of medium height appears tall beside a man of small stature. We can easily make experiments on simultaneous contrast with a sheet of transparent paper. We fasten together a sheet of green and a sheet of rose-coloured paper, so as to obtain a sheet half red and half green. On the line of separation between the two colours we place a strip of grey paper, and cover the whole with a sheet of thin letter-paper of the same size. The grey strip will then appear red at the edge touching the green, and green at the edge touching the red; the centre presenting an intermediate shade. It presents a still more decided appearance if the grey strip is perpendicular with the[Pg 114] line of separation of the two colours; the piece of grey then stretching into the green will present as deep a red as the red foundation on the other side. If the line of grey colour exactly covers the line of separation between the two colours, the contrasting colour is more feeble; the edges of the grey paper then present complementary strips of colour. Similar effects may be obtained by superposing, in gradually diminishing layers, strips of thin paper, so as to form successive bands of different thicknesses. If it is then lit up from behind, the objective intensity is evidently constant through the extent of each layer; nevertheless every strip appears darker at the edge touching a more transparent layer, and lighter at the edge in contact with a thicker layer. The dull tints of China ink, superposed in layers, will produce a similar effect. The phenomena are produced by means of rotative discs of most beautiful and delicate gradations of colour. Let us give the sectors of the disc the form represented by fig. 114, and make them black and white; and when in rotation we shall see a series of concentric rings of a shade that becomes darker and darker towards the centre. The angular surface of the dark portions is constant in each of these rings. The intensity, therefore, of each ring is uniform during rapid rotation; it is only between one ring and another that the intensity varies. Each ring also appears lighter on its inner side when it borders on a darker ring, and darker on its outer side when in contact with a lighter ring. If the differences of intensity in the rings are very slight, one can scarcely judge sometimes if the inner rings are darker than the outer; the eye is only struck by the periodical alternations of light and shade presented by the edges of the rings. If, instead of white and black, we take two different colours, each ring will present two colours on its two edges, although the colour of the rest of the ring will be uniform. Each of the constituent colours presents itself with more intensity on that edge of the ring which borders on another ring containing a smaller quantity of the colour. Thus, if we mix blue and yellow, and the blue predominates in the exterior and the yellow in the[Pg 115] interior, every ring will appear yellow at its outer, and blue at its inner edge; and if the colours present together very slight differences, we may fall into the illusion which causes the differences really existing between the colours of the different rings to disappear, leaving instead, on a uniformly coloured background, the contrasting blue and yellow of the edges of the rings. It is very characteristic that in these cases we do not see the mixed colours, but seem to see the constituent colours separately, one beside the other, and one through the other.

All the experiments we have described afford great interest to the student; they can easily be performed by those of our readers who are particularly interested in these little-known subjects. Any one may construct the greater part of the appliances we have enumerated, and others can be obtained at an optician’s. The discs in particular are extensively manufactured, and with great success.

[Pg 116]

CHAPTER XI.

OPTICAL ILLUSIONS—ZOLLNER’S DESIGNS—THE THAUMATROPE—PHENOKISTOSCOPE—THE ZOOTROPE—THE PRAXINOSCOPE—THE DAZZLING TOP.

We shall now continue the subject by describing some illusions more curious still—those of ocular estimation. These illusions depend rather on the particular properties of the figures we examine, and the greater part of these phenomena may be placed in that category whose law we have just formulated: the differences clearly perceived appear greater than the differences equal to them, but perceived with greater difficulty. Thus a line —— when divided appears greater than when not divided; the direct perception of the parts makes us notice the number of the sub-divisions, the size of which is more perceptible than when the parts are not clearly marked off. Thus, in fig. 115, we imagine the length ab equals bc, although ab is in reality longer than bc. In an experiment consisting of dividing a line into two equal parts, the right eye tends to increase the half on the right, and the left eye to enlarge that on the left. To arrive at an exact estimate, we turn over the paper and find the exact centre.

Fig. 115.

Fig. 116.

Illusions of this kind become more striking when the distances to be compared run in different directions. If we look at A and B (fig. 116), which are perfect squares, A appears greater in length than width, whilst B, on the contrary, appears to have greater width than length. The case is the same with angles. On looking at fig. 117, angles 1, 2, 3, 4 are straight, and should appear so when examined. But 1 and 2 appear pointed, and 3 and 4 obtuse. The illusion is still greater if we look at the figure with the right eye. If, on the contrary, we turn it, so that 2 and 3 are at the bottom, 1 and 2 will appear greatly pointed to the left eye. The divided angles always appear relatively greater than they would appear without divisions.

[Pg 117]

The same illusion is presented in a number of examples in the course of daily life. An empty room appears smaller than a furnished room, and a wall covered with paperhangings appears larger than a bare wall. It is a well-known source of amusement to present someone in company with a hat, and request him to mark on the wall its supposed height from the ground. The height generally indicated will be a size and a half too large.

We will relate an experience described by Bravais: “When at sea,” he says, “at a certain distance from a coast which presents many inequalities, if we attempt to draw the coastline as it presents itself to the eye, we shall find on verification that the horizontal dimensions have been correctly sketched at a certain scale, while all the vertical angular objects have been represented on a scale twice as large. This illusion, which is sure to occur in estimates of this kind, can be demonstrated by numerous observations.”

M. Helmholtz has also indicated several optical illusions.

Fig. 117.

Fig. 118.

Fig. 119.

If we examine fig. 118, the continuation of the line a does not appear to be d,—which it is in reality,—but f, which is a little lower. This illusion is still more striking when we make the figure on a smaller scale (fig. 119), as at B, where the two fine lines are in continuation with each other, but do not appear to be so, and at C, where they appear so, but are not in reality. If we draw the figures as at A (fig. 118), leaving out the line d, and look at them from a gradually increasing distance, so that they appear to diminish, it will be found that the further off the figure is placed, the more it seems necessary to lower the line f to make it appear a continuation of a. These effects are produced by irradiation; they can also be produced by black lines on a white foundation. Near the point of the two acute angles, the circles of diffusion of the two black lines touch and mutually reinforce each other; consequently the retinal image of the narrow line presents its maximum of darkness nearest to the broad line, and appears to deviate on that side. In figures of this kind, however, executed on a larger scale, as in fig. 118, irradiation can scarcely be the only cause of[Pg 118] illusion. We will continue our exposition as a means of finding an explanation. In fig. 120, A and B present some examples pointed out by Hering; the straight, parallel lines, a b, and c d, appear to bend outwards at A, and inwards at B. But the most striking example is that represented by fig. 121, published by Zollner.

The vertical black strips of this figure are parallel with each other, but they appear convergent and divergent, and seem constantly turned out of a vertical position into a direction inverse to that of the oblique lines which divide them. The separate halves of the oblique lines are displaced respectively, like the narrow lines in fig. 119. If the figure is turned so that the broad vertical lines present an inclination of 45° to the horizon, the convergence appears even more remarkable, whilst we notice less the apparent deviation of the halves of the small lines, which are then horizontal and vertical. The direction of the vertical and horizontal lines is less modified than that of the oblique lines. We may look upon these latter illusions as fresh examples of the aforesaid rule, according to which acute angles clearly defined, but of small size, appear, as a rule, relatively larger when we compare with obtuse or right angles which are undivided; but if the apparent enlargement of an acute angle shows itself in such a manner that the two sides appear to diverge, the illusions given in figs. 118, 120, and 121, will be the result.

Fig. 120.—The horizontal lines, a, b, c, d, are strictly parallel; their appearance of deviation is caused by the oblique lines.

In fig. 118 the narrow lines appear to turn towards the point where they penetrate the thick line and disappear, to appear afterwards in continuation of each other. In fig. 120 the two halves of each of the two straight lines seem to deviate through the entire length in such a manner that the acute angles which they form with the oblique lines appear enlarged. The same effect is shown by the vertical lines of fig. 121.

M. Helmholtz is of opinion (figs. 120, 121) that the law of contrast is insufficient to entirely explain the phenomena, and believes that the effect is also caused by the movements of the eye. In fact, the illusions almost entirely[Pg 119] disappear, if we fix on a point of the object in order to develop an accidental image, and when we have obtained one very distinctly, which is quite possible with Zollner’s design (fig. 121), this image will present not the slightest trace of illusion. In fig. 118 the displacement of the gaze will exercise no very decided influence on the strengthening of the illusion; on the contrary, it disappears when we turn our eyes on the narrow line, ad. On the other hand, the fixing of the eyes causes the illusion to disappear with relative facility in fig. 120, and with more difficulty in fig. 121; it will, however, disappear equally in the latter design, if we fix it immovably, and instead of considering it as composed of black lines on a white background, we compel ourselves to picture it as white lines on a black foundation; then the illusion vanishes. But if we let our eyes wander over the illustration, the illusion will return in full force. We can indeed succeed in completely destroying the illusion produced by these designs by covering them with a sheet of opaque paper, on which we rest the point of a pin. Looking fixedly at the point, we suddenly draw away the paper, and can then judge if the gaze has been fixed and steady according to the clearness of the accidental image which is formed as a result of the experiment.

Fig. 121.—The vertical strips are parallel; they appear convergent or divergent under the influence of the oblique lines.

Fig. 122.—Observation of electric spark.

The light of an electric spark furnishes the surest and simplest means of counteracting the influence of movements of the eyes, as during the momentary duration of the spark the eye cannot execute any sensible movement. For this experiment the present writer has made use of a wooden box, A B C D (fig. 122), blackened on the inside. Two holes are made for the eyes on each side of the box, f and g. The observer looks through the openings, f, and in front of openings, g, the objects are placed; these are pierced through with a pin, which can be fixed by the eyes in the absence[Pg 120] of the electric spark, when the box is perfectly dark. The box is open, and rests on the table, B D, to allow of changing the object. The conducting wires of electricity are at h and i; in the centre of the box is a strip of cardboard, white on the side facing the spark, the light of which it shelters from the eye of the observer and throws back again on the object. With the electric light the illusion was completely perceptible with fig. 118, while it disappeared altogether in fig. 120; with fig. 121 it was not entirely absent, but when it showed itself, it was much more feeble and doubtful than usual, though the intensity of light was quite sufficient to allow of the form of the object being very distinctly examined. Thus two different phenomena have to be explained; first, the feeble illusion which is produced without the intervention of movements of the eye; and secondly, the strengthening of the illusion in consequence of these movements. The law of contrast sufficiently explains the first; that which one perceives most distinctly with indirect vision is the concordance of directions with dimensions of the same kind. We perceive more distinctly the difference of direction presented at their intersection by the two sides of an acute or obtuse angle, than the deviation that exists between one of the sides and the perpendicular which we imagine placed on the other side, but which is not marked. By being distributed on both sides, the apparent enlargement of the angles gives way to displacements, and changes of direction of the sides. It is difficult to correct the apparent displacement of the lines when they remain parallel to their true direction; for this reason, the illusion of the figure is relatively more inflexible. Changes of direction, on the contrary, are recognised more easily if we examine the figure attentively, when these changes have the effect of causing the concordance of the lines (which accord in reality) to disappear; it is probably because of the difference in aspect of the numerous oblique lines of figs. 120 and 121 that the concordance of these lines escapes the observer’s notice. As regards the influence exercised by the motion of[Pg 121] the eyes in the apparent direction of the lines, M. Helmholtz, after discussing the matter very thoroughly, proves the strengthening of the illusion in Zollner’s illustration to be caused by those motions. It is not now our intention to follow out the whole of this demonstration; it will be sufficient to point out to the reader a fruitful force of study, with but little known results.

The Romans were well acquainted with the influence of oblique lines. At Pompeii, fresco paintings are to be found, in which the lines are not parallel, so that they satisfy the eye influenced by adjacent lines. Engravers in copper-plate have also studied the influence of etchings on the parallelism of straight lines, and they calculate the effect that they will produce on the engraving. In some ornamentations in which these results have not been calculated, it sometimes happens that parallel lines do not appear parallel because of the influence of other oblique lines, and a disagreeable effect is produced. A similar result is to be seen at the railway station at Lyons, the roof of which is covered with inlaid work in point de Hongrie. The wide parallel lines of this ceiling appear to deviate, a result produced by a series of oblique lines formed by the planks of wood.

Fig. 123.—Two sides of a Thaumatrope disc.

Having given a long account of the result of M. Helmholtz’s labours, we will pass to the consideration of another kind of experiments, or rather appliances, based on the illusions of vision, and the persistence of impressions on the retina.

The Thaumatrope, to which we have already referred, is a plaything of very ancient origin, based on the principle we have mentioned. It consists of a cardboard disc, which we put in motion by pulling two cords. On one side of the disc a cage, a, is portrayed, on the other a bird, b (fig. 123). When the little contrivance is turned round, the two designs are seen at the same time, and form but one image—that of a bird in its cage (fig. 124). It is of course hardly necessary to add that the designs may be varied.

We have already referred to M. Plateau’s rotating disc (the Phenakistoscope). Through the narrow slits we perceive in succession representations of different positions of a certain action. The persistence of the luminous impressions on the retina gives to the eye the sensation of a continuous image, which seems animated by the same movements as those portrayed in the different phases (fig. 125).

Fig. 124.—Appearance of the Thaumatrope in rotation.

Fig. 125.—Plateau’s Phenokistoscope.

The Zootrope (fig. 126) is a perfected specimen of this apparatus. It is composed of a cylinder of cardboard, turning on a central axis. The cylinder is pierced with vertical slits at regular intervals, and through which the[Pg 122] spectator can see the designs upon a band of paper adapted to the interior of the apparatus in rotation. The designs are so executed that they represent the different times of a movement between two extremes; and in consequence of the impressions upon the retina the successive phases are mingled, so the spectator believes he sees, without transition, the entire movement. We give a few specimens of the pictures for the Zootrope (fig. 127). We have here an ape leaping over a hedge, a dancing “Punch,” a gendarme pursuing a thief, a person holding the devil by the tail, a robber coming out of a box, and a sportsman firing at a bird. The extremes of the movement are right and left; the intermediary figures make the transitions, and they are usually equal in number to the slits in the Zootrope. It is not difficult to construct such an instrument, and better drawings could be made than the specimens taken at random from a model. The earth might be represented turning in space, or a fire-engine pumping water could be given, and thus the Zootrope might be quite a vehicle of instruction as well as of amusement. This instrument is certainly one of the most curious in the range of optics, and never fails to excite interest. The ingenious contrivances which have up to the present time reproduced it, all consist in the employment of narrow slits, which besides reducing the light to a great extent, and consequently the light and clearness of the object, require the instrument to be set in rapid rotation, which greatly exaggerates the rapidity of the movements represented, and without which the intermissions of the spectacle could not unite in a continuous sensation.

Fig. 126.—The Zootrope.

Fig. 127.—Pictures used in the Zootrope.

We present here an apparatus based on a very different optical[Pg 123] arrangement. In the Praxinoscope12 (a name given by the inventor, Mr. Reynaud, to this new apparatus), the substitution of one object for another is accomplished without interruption in the vision, or solution of continuity, and consequently without a sensible reduction of light; in a word, the eye beholds continuously an image which, nevertheless, is incessantly changing before it. The result was obtained in this manner. Having sought unsuccessfully by mechanical means to substitute one object for another without interrupting the continuity of the spectacle, the inventor was seized with the idea of producing this substitution, not with the objects themselves, but with their virtual images. He then contrived the arrangement which we will now describe. A plane mirror, AB (fig. 129), is placed at a certain distance from an object, CD, and the virtual image will be seen at C′D′. If we then turn the plane mirror and object towards the point, O, letting BE and DF be their new positions, the image will be at C″D″. Its axis, O, will not be displaced. In the positions, AB and CD, first occupied by the plane mirror and the object, we now place another mirror and object. Let us imagine the eye placed at M. Half of the first object will be seen at OD″, and half of the second at OC′. If we continue the rotation of the instrument, we shall soon have mirror No. 2 at TT′, and object No. 2 at SS′. At the same moment the image of object No. 2 will be seen entirely at C‴D″. Mirror No. 2 and its object will soon after be at BE and DF. If we then imagine another mirror and its corresponding object at AB and CD, the same succession of phenomena will be reproduced. This experiment therefore shows that a series of objects placed on the perimeter of a polygon[Pg 124] will be seen successively at the centre, if the plane mirrors are placed on a concentric polygon, the “apothème” of which will be less by one-half, and which will be carried on by the same movement. In its practical form, M. Reynaud’s apparatus consists of a polygonal or simply circular box (fig. 128), (for the polygon may be replaced by a circle without the principle or result being changed), in the centre of which is placed a prism of exactly half a diameter less, the surface of which is covered with plane mirrors. A strip of cardboard bearing a number of designs of the same object, portrayed in different phases of action, is placed in the interior of the circular rim of the box, so that each position corresponds to a plate of the glass prism. A moderate movement of rotation given to the apparatus, which is raised on a central pivot, suffices to produce the substitution of the figures, and the animated object is reflected on the centre of the glass prism with remarkable brightness, clearness, and delicacy of movement. Constructed in this manner, the Praxinoscope forms an optical toy both interesting and amusing. In the evening, a lamp placed on a support ad hoc, in the centre of the apparatus, suffices to light it up very clearly, and a number of persons may conveniently assemble round it, and witness the effects produced.

Fig. 128.—M. Reynaud’s Praxinoscope.

Fig. 129.

Besides the attractions offered by the animated scenes of the Praxinoscope, the apparatus may also be made the object of useful applications in the study of optics. It permits an object, a drawing, or a colour, to be substituted instantaneously in experiments on secondary or subjective images, etc., on the contrast of colours or the persistence of impressions, etc. We can also make what is called a synthesis of movements by placing before the prism a series of diagrams of natural objects by means of photography.

M. Reynaud has already arranged an apparatus which exhibits in the[Pg 125] largest dimensions the animated reflection of the Praxinoscope, and which lends itself to the demonstration of curious effects before a numerous auditory. The ingenious inventor has recently contrived also a very curious improvement in the original apparatus. In the Praxinoscope Theatre he has succeeded in producing truly ornamental tableaux, as on a small Lilliputian stage, in the centre of which the principal object moves with startling effect. To obtain this result, M. Reynaud commences by cutting out in black paper the different figures, the whole of which will form an object animated by the[Pg 126] rotation given to the Praxinoscope. To supply the decorations, he arranges on the black foundation the image of an appropriate coloured design by means of a piece of glass. It is well known that transparent glass possesses the property of giving a reflection of the objects on the nearest side as well as on the farthest. We may recall the applications of this optical effect in theatres, and also in courses of physics, under the title of impalpable spectres. It is also by reflection on thin, transparent glass, that M. Reynaud produces the image of the ornamentations in the Praxinoscope Theatre. The decorations are really placed in the lid, which is held by a hook in a vertical position, thus forming the front side of the apparatus (fig. 130). In this side a rectangular opening is made, through which the spectator (using both eyes) perceives at the same time the animated reflection of the Praxinoscope, and the immovable image of the decorations reflected in the transparent glass. The position of the latter and its distance from the coloured decorations are arranged so that the reflection is thrown behind the moving figure, which consequently appears in strong relief against the background, the effect produced being very striking. It is evident that to change the decorations it is only necessary to place in succession on a slide the different chromos representing landscapes, buildings, the interior of a circus, etc. It is easy to choose an arrangement suitable for each of the moving figures[Pg 127] placed in the Praxinoscope. By this clever and entirely novel optical combination, the mechanism of the contrivance is entirely lost sight of, leaving only the effect produced by the animated figures, which fulfil their different movements on the little stage. The Praxinoscope Theatre can also be used as well in the evening as in the daytime. By daylight, it is sufficient to place it before a window, and in the evening the same effects may be produced, perhaps in even a more striking manner, by simply placing a lamp on the stand, with a small plated reflector, and a lamp-shade. The illusion produced by this scientific plaything is very complete and curious, and[Pg 128] M. Reynaud cannot be too much commended for so cleverly applying his knowledge of physics in the construction of an apparatus which is at the same time both an optical instrument and a charming source of amusement.

Fig 130.—The Praxinoscope Theatre.

Fig. 131.—The Dazzling Top.

Amongst the toys founded upon the persistency of impressions upon the retina we may instance the “Dazzling Top” (fig. 131). This remarkable invention is quite worthy of a place in every cabinet, and is an ingenious specimen of a perfected Helmholtz top. It is a metallic toy put in motion by means of a cord wound round a groove. The axis is hollow, admits a metallic stem, and fits into a handle which is held in the hand. The top is placed upon a little cup in an upright position, and it is then set spinning in the usual way with the cord. The stem and handle are then withdrawn, and as the top will continue to spin for a long time, discs and various outline shapes can be fixed upon it, and various objects will be shadowed thereon. Cups, bowls, candlesticks, and jugs can be seen plainly revolving as the top carries the wire representation in outline rapidly past the eyes. Coloured cardboard can be worked into various patterns, and much amusement will be created amongst children and young people.

[Pg 129]

CHAPTER XII.

OPTICAL ILLUSIONS CONTINUED—EXPERIMENTS—THE TALKING HEAD—GHOST ILLUSIONS.

The enumeration of optical illusions is so considerable that we have no intention of describing them all, and will merely cite a few other examples. The following facts have been communicated to us by M. Nachet:—

Fig. 132.—Hexagonal appearance formed by circles joined together.

When examining algæ under the microscope, we notice the spaces which separate the streaks ornamenting the silicious covering of these various organisms, and it is explained that they are formed by hexagons visible only when we examine the object with a powerful microscope. “For a long time,” says M. Nachet, “I occupied myself with the examination of the hexagonal appearance of the points constituting the streaks. Why should these hexagons show themselves, and how could they be other than the visible base of small pyramids piled very closely one on the other; and if this were the case, why were not the points of the little pyramids visible? Or, was the structure before me analogous to that of the eyes of insects? Then the carapace would be but a surface of perforated polygonal openings. This latter hypothesis was attractive enough, and would have explained many things; but some careful observations with very powerful object[Pg 130]-glasses, quite free from blemishes, had shown me that these hexagons had round points, contrary to the descriptions of micrographs. These observations, corroborated by the micrographic photographs of Lackerbauer, the much-regretted designer, and by Colonels Woodward and Washington, left not the slightest doubt that it was necessary to discover why the eye persistently saw hexagons where there were circles. To elucidate this point, it was necessary to find some means of reproducing artificially what nature had accomplished with so much precision on the surfaces of algæ. After many fruitless attempts, I decided on making a trial of a stereotype plate covered with dots arranged in quincunxes, very close together” (figs. 132 and 133). “The result was more successful than I had hoped; the effect produced is exactly that of the arrangement of the so-called hexagons of the most beautiful of the algæ, the Pleurosigma angulata. If these stereotypes are examined with one eye only, we shall be immediately convinced that we have to do with hexagonal polygons.” It is useless to give any long exposition of a figure so clearly explanatory; it is simply an effect of the contrast and opposition of the black and white in the sensation of the retina. This effect is particularly striking with fig. 134, a negative photograph heliographically engraved according to fig. 133. In this the white points seem to destroy the black spaces, and to approach each other tangentially, and the irradiation is so intense that the white circles appear much larger than the black of fig. 133, although of the same diameter. There are in these facts many points which may interest not only students of micrography, but also artists. As to the algæ, the origin of this investigation, it remains to be discovered if these circles which cover their silicious carapace are the projection of small hemispheres, or the section of openings made in the thick covering. Certain experiments, however, seem to prove that they are hemispheres, and the theory is also confirmed by a microscopic photograph from Lackerbauer’s collection, magnified 3,000 diameters, in which a black central point is seen in the centre of each circle, a certain reflection of the luminous source reproduced in the focus of each of the small demi-spheres which constitute the ornament of the algæ. The microscope, which has progressively shown first the streaks, then the hexagons, and then the round points, will surely clear up the point some day or other.

Fig. 133.—Another figure of the same kind.

Fig. 134.—Third figure.

Mr. Silvanus P. Thompson, Professor of Physics at University College,[Pg 131] Bristol, has recently presented the French Society of Physical Science with a curious example of optical illusion, the true cause of which is not clearly known, but which we may compare with other facts made known some time ago, of which no precise explanation has been given. Let us first consider in what the effect discovered by Mr. S. P. Thompson consists, according to the description that has been given of it by M. C. M. Gariel; the illustrations here given will also allow of our verifying the truth of the statements.

Fig. 135. and Fig. 136.
Mr. Thompson’s optical illusion. Give a circular movement to these figures, and the circles will appear to turn round.

The first illustration consists of a series of concentric circles of about the width of a millimetre, separated by white intervals of the same size (fig. 135). These dimensions are not absolute; they vary with the distance, and may even be a few inches in width if it is desired to show the phenomenon to a rather numerous auditory. If we hold the design in the hand, and give it a twirl by a little movement of the wrist, the circle appears to turn round its centre, and the rotation is in the same direction, and is equally swift; that is to say, the circle appears to accomplish a complete turn, whilst the cardboard really accomplishes one in the same direction. For the second effect we draw a dark circle, in the interior of which are placed a number of indentations at regular intervals (fig. 136). Operating in the same manner as described above, this notched wheel appears to turn round its centre, but this time in a different direction from the real movement. In this, however, as in the other design, the effect is more satisfactory if we do not look directly at it; the movements also are particularly striking in combinations such as that represented in fig. 137, in which the multiplicity of circles does not allow us to fix one specially. We may add that the same effects may be obtained with eccentric wheels, or even with other curves than circles. By means of a photograph on glass, Mr. Thompson has been able to reflect these designs on a screen where they were obtained on a large scale; a circular movement was communicated to the photographic plate, so that the design moved in a circular manner on the screen, and in this case also there existed the illusion that every circle seemed[Pg 132] turning round its centre. And what is the explanation of these curious effects? Mr. Thompson does not believe (and we share his opinion) that the faculty possessed by the retina of preserving images during a certain time (persistence of impressions on the retina) can entirely explain these phenomena. Without desiring to formulate a decided theory, Mr. Thompson is of opinion that we may class these facts with others which have been known for some time, and that perhaps it is necessary to attribute to the eye some new faculty which may explain the whole at once.

Fig. 137.—Another figure of Mr. Thompson’s. The different circles appear to turn round if we give the design a rotating movement.

Brewster and Adams have described phenomena which are equally curious, the principal of which we will describe, adding also some analogous investigations due to Mr. Thompson. The result seems to be that there exists in the eye a badly-defined purpose of nature, which in a certain way compensates (Brewster) for the real phenomenon, because it has a contrary effect, which will continue for some time after the cessation of the phenomena, and which gives by itself a sensation contrary to that which the real movement would have produced. Thus, after having fixed our eyes for two or three minutes on a rushing waterfall, if we suddenly turn our glance on the adjacent rocks, the latter appear to move from top to bottom. It is not a question here of the effect of the relative movement to be observed on regarding simultaneously the falling water and the rocks; if one can succeed in abstracting oneself to such an extent that the water appears motionless, the rocks appear to take a contrary movement. In the phenomenon we describe there is no simultaneous comparison; the eyes are turned successively first on the water, and then on the rocks. In a rapid river, such as the Rhine above the fall at Schaffhausen, the stream is not equally swift in every part, and the current is noticeably more rapid in the middle of the river than near the banks. If we look fixedly at the centre of the stream, and then[Pg 133] suddenly turn our eyes towards the banks, it will appear as though the river were flowing back towards its source. This kind of compensation does not only produce an apparent displacement, but also changes in size. When travelling at great speed in a railway train, the objects of the surrounding country as one flies by them gradually appear smaller and smaller. If, when this occurs, we suddenly remove our eyes to the interior of the railway carriage, and fix them on immovable objects, such as the sides of the compartment, or the faces of our travelling companions, the images on the retina will really preserve the same size, and yet the objects will appear larger. Such are some of the interesting facts among those discovered by Mr. Thompson; and though we do not intend to push the inquiry further, we think it may not be without interest to describe here another illusion of that organ whose properties are in every way so curious and remarkable.

Fig. 138.—Experiment on complementary colours.

Fig. 139.—Design for experiment or the punctum cæcum.

Another experiment to show the existence of impressions received by the retina can be made with the figure above (138). If the gaze be fixed upon the dark spot in the centre of the white figure for about half a minute, and the eyes then directed to the ceiling, or a sheet of white paper, the white figure will be reproduced in black. This result is based upon the principle of complementary colours. A red design, for instance, will be reproduced in green.

[Pg 134]

There is a dark spot in every human eye—that is, a spot which is insensible to light. The eye is generally regarded as a perfect instrument, but it is not yet so by any means. One of our great philosophers remarked that if an instrument were sent home to him so full of errors he would feel justified in returning it to the optician. But the eye has its dark place, the punctum cæcum, and it can be discovered by covering the left eye with the hand, and holding the present page at arm’s length with the other. Then fix the gaze on the small cross in the picture, and bring the book close up. At a little distance the white ball will disappear from the page (fig. 139).

Fig 140.—An optical illusion.

The illustration (fig. 140) shows us a very curious optical illusion, and one very easy to practise. Roll up a sheet of paper, and look through it, as through a telescope, with the right eye, keeping both eyes open. Then place the left hand open palm towards you against the roll of paper, you will then appear to be looking through a hole in your left hand. Sometimes the effect is produced without holding up the other hand to the roll, as shown in fig. 140.

Among optical illusions there are a great number that may be produced by means of mirrors. The divided telescope is an example. The apparatus, raised on a firm stand, allows of one apparently seeing an object through a stone or other opaque object, as shown in fig. 141. The illustration shows the arrangement of the apparatus. The observer, looking through it, plainly[Pg 135] perceives the object through the glass; the image is reflected four times before reaching his eye, by means of small mirrors concealed in the instrument.

Fig. 141.—A divided telescope.

Convex or concave mirrors distort images in a singular manner, and produce very interesting effects. Anamorphoses constitute particular objects belonging specially to the class of experiments relating to cylindrical mirrors. They are images made according to determined rules, but so distorted that when regarding them fixedly we can only distinguish confused strokes. When they are seen reflected in the curved mirrors, they present, on the contrary, a perfectly regular appearance. Fig. 142 exhibits an Anamorphosis made by a cylindrical mirror. It will be seen that the confused image of the horizontal paper is reflected in the cylindrical mirror, producing the figure of a juggler. It is easy to contrive similar designs one’s-self; and comical mirrors may also be employed which produce particular effects of a no less interesting kind. The next illustration is of a set of figures which in a cylindrical mirror look like the ten of hearts (fig. 143).

Fig. 142.—Cylindrical Mirror and Anamorphosis.

Fig. 143.—Anamorphus design for the ten of hearts.

One of the most remarkable applications of mirrors in amusing experiments is undoubtedly that of the severed and talking head. A few years back this trick obtained considerable success in Paris and a number of other towns. The spectators beheld a small space set apart, in which was placed a table on three legs; on this table was a human head, placed in cloth on a dish. The head moved its eyes and spoke; it evidently belonged to a man whose body was completely hidden. The spectators thought they saw an empty[Pg 136] space beneath the table, but the body of the individual who was really seated there was concealed by two glasses placed at an inclination of 45° to the walls on the right and left. The whole was arranged in such a manner that the reflection of the walls coincided with the visible part of the wall at the[Pg 137] back of the room. The three walls were painted in one colour, and a subdued light increased the illusion, the effect of which was remarkable (fig. 144).

Fig. 144.—The talking head.

The spectres designed by Robin also attracted considerable public attention within recent times. They were images formed by the medium of transparent glass. Glass panes often produce the phenomenon of spectres. In the evening, when it is dark out of doors, it is easy to prove that the reflection of objects in a lighted room can be reproduced behind the window panes by reason of the darkness outside. If we approach the window pane,[Pg 138] we see also the real objects outside, a balcony, tree, etc. These real objects mingle with the reflected image, and combine to produce very curious effects. In this way M. Robin has contrived the effects of the theatre. He throws on the stage the reflection of a person dressed as a Zouave, and he himself, armed with a sabre, stabs the spectre through the body. A great number of other singular effects have been obtained in the same manner. Pepper’s Ghost was managed in this way.

Fig. 145.—The ghost effect.

Within recent times, images produced in a similar way have been[Pg 139] utilized to facilitate the study of drawing. A piece of glass is fixed vertically on a black board (fig. 146). A model to copy from is placed on one side of the piece of glass, and is arranged so that the visual ray passes obliquely through the glass, and we perceive the reflection of the design very clearly on the other side. It is then very easily reproduced with a pencil on a sheet of white paper by tracing the outlines.

Fig. 146.—Drawing by reflection.

[Pg 140]

CHAPTER XIII.

VISION—THE EYE—THE STEREOSCOPE—SPECTRUM ANALYSIS—THE SPECTROSCOPE—THE TELESCOPE AND MICROSCOPE—PHOTOGRAPHY—DISSOLVING VIEWS—LUMINOUS PAINT.

The eye and vision are such important subjects to all of us that we may be excused for saying something more concerning phenomena connected with them, and the instruments we use for assisting them. We do not propose to write a treatise upon the physiology of vision, for we know the image in the eye is produced physically in the same manner as the image in a camera obscura. In the eye the sides of the box are represented by the sclerotic (see chap. x. fig. 95); the dark inner surface has its parallel in the pigment of the choroid; the opening in the box in the pupil of the eye; the convex lens in the crystalline and the cornea; and the retina receives the image. But why we see—beyond the fact that we do see—no one can explain. Science is dumb on the subject. Thought and consciousness elude our grasp; and, as Professor Tyndall says on this subject, “we stand face to face with the incomprehensible.”

But there are many interesting facts connected with our vision which may be plainly described. Some people are obliged to carry an object (or a book) to within ten inches of the eye to see it distinctly; and a person who does not possess convergent power of the eye will have to move it farther off, or use convex glasses; while a “near-sighted” person, whose eyes are too quickly convergent, will use concave glasses to bring the object near to the eye.

There is but one small place in the retina of the eye which admits of perfect vision. This, the most sensitive portion, is called the “yellow spot,” and vision becomes more and more indistinct from this point towards the circumference. This can be proved by any one; for in reading we are obliged to carry our eyes from word to word, and backwards and forwards along the lines of print. Another very important element in our vision is the contraction and enlargement of the iris around the pupil. In cases where strong light would only dazzle us the iris expands, and the pupil is contracted to a sufficient size to accommodate our vision. At night, or in a darkened room, the pupil is enlarged. This change will account for our not being immediately able to see objects when we have passed from darkness to great light, or vice versâ. The iris must have time to accommodate itself to the light.

Now, outside the small space of perfect vision above mentioned, there is a circle of considerable extent, called the “field of vision.” In man this field,[Pg 141] when the eyes are fixed, subtends an angle of about 180°, because beyond that the rays cannot enter the pupil of the eye. But in the lower animals, the fish and birds,—notably the ostrich,—the field of vision is much more extensive, because the pupils are more prominent, or the eyes are set more towards the sides of the head. The ostrich can see behind him, and fish can see in any direction without apparent limit. Man can only see indistinctly; and though he can move his eyes rapidly, he can see distinctly but a small portion of any object at a time, yet he sees with both eyes simultaneously a single object, because the two lines of vision unite at a single point, and as the two images cover each other we perceive only one image. Beyond or within this point of meeting the vision is indistinct, but the angle of convergence is always varied according to the distance of the object. If we hold up a penholder in front of us, and in a line with any other defined object,—say an ink-bottle,—we can see the penholder distinctly, and the ink-bottle indistinctly, as two images. If we then look at the ink-bottle we shall see it single, while the penholder will appear double, but with imperfect outlines.

Fig. 147.—The Stereoscope.

Fig. 148.—Mode of taking photograph for Stereoscope.

Again, if we look at a box both eyes will see it equally well, but the right eye will see a little more on its right side, and the left eye on the left. It is on this principle that the Stereoscope is constructed. Sir Charles Wheatstone was the inventor, and the instrument may be thus described:—Two pictures are taken by photography—one as the landscape is seen by the right eye, the other as it is viewed by the left; the points of view thus differing slightly. When both eyes are simultaneously applied to the instrument the view is seen exactly as it would appear to the beholder at the actual place it represents. The views are taken singly; one side at one time, and another after, as in the camera (fig. 148). A is the first view; B is kept dark; C is the shutter for A. There are Reflecting and Refracting Stereoscopes. In the former a mirror reflects the image into each eye; in the latter the views are pasted on a card, side by side, and looked at through prismatic lenses. The principles of Binocular Vision have been applied to the Microscope.

In foregoing chapters we have given many examples with diagrams of the temporary impressions made upon the retina of the eye. It is a fact that a wheel revolving at a great rate will appear to be standing still when suddenly[Pg 142] illuminated by a flash of lightning, because the eye has not time to take in the motion in the instant of time, for the spokes of the wheel are not moving fast enough to convey the impression of motion in that half second to the eye; yet the perfect outline of the wheel is distinctly visible.

Indeed, distinct vision can be exercised in a very small fraction of a second. It was calculated by Professor Rood, and proved by experiment, that forty billionths of a second is sufficient time for the eye to distinguish letters on a printed page. It this instance the illuminating power was an electric spark from a Leyden Jar.

We have remarked upon the distinctness with which we can see an object when we direct our gaze upon it, and this appears a self-evident proposition; but have any of our readers remarked the curious fact that when they want to see a faint and particular star in the sky it will at once disappear when they gaze at it? The best way to see such very faint orbs as this is to look away from them,—a little to one side or the other,—and then the tiny point will become visible again to the eye. There is also a degree of phosphorescence in the eye, which any one who receives a blow upon that organ will readily admit. Even a simple pressure on the closed lid will show us a circle of light and “colours like a peacock’s tail,” as the great Newton expressed it. There are many occasions in which light is perceived in the eye—generally the result of muscular action; and the Irish term to “knock fire out of my eye” is founded upon philosophical facts.

We are many of us aware of “spots” on our eyes when our digestion is out of order, and the inability of the eye to see figures distinctly in a faint light—within a proper seeing distance, too—has often given rise to the “ghost.” These shadowy forms are nothing more than affections of the eye, and, as well remarked in Brewster’s Letters on Natural Magic, “are always white because no other colour can be seen.” The light is not sufficiently strong to enable the person to see distinctly; and as the eye passes from side to side, and strives to take in the figure, it naturally seems shadowy and indistinct, and appears to move as our eyes move. “When the eye dimly descries an inanimate object whose different parts reflect different degrees of light, its brighter parts may enable the spectator to keep up a continued view of it; but the disappearance and reappearance of its fainter parts, and the change of shape which ensues, will necessarily give it the semblance of a living form; and if it occupies a position which is unapproachable, and where animate objects cannot find their way, the mind will soon transfer to it a supernatural existence. In like manner a human figure shadowed forth in a feeble twilight may undergo similar changes, and after being distinctly seen while it is in a situation favourable for receiving and reflecting light, it may suddenly disappear in a position before and within the reach of the observer’s eye; and if this evanescence takes place in a path or roadway where there is no sideway by which the figure could escape, it is not easy for an ordinary mind to efface the impression which it cannot fail to receive.” This will account for many so-called “ghosts.”

[Pg 143]

Accidental colours, or ocular spectra, are, so to speak, illusions, and differently-coloured objects will, when our gaze is turned from them, give us different “spectra” or images. For instance, a violet object will, when we turn to a sheet of white paper, give us a yellow “spectrum”; orange will be blue; black and white will change respectively; red will become a blue-green. From a very strong white light the accidental colours will vary.

Fig. 149.—The Solar Spectrum.

The Solar Spectrum is the name given to the coloured band formed by the decomposition of a beam of light into its elementary colours, of which there are seven. This is an easy experiment. A ray of light can be admitted into a darkened room through a hole in the shutter, and thus admitted will produce a white spot on the screen opposite, as at g in the diagram (fig. 149). If we interpose a prism—a triangular piece of glass—the “drop” of a chandelier will do—we cause it to diverge from its direct line, and it will produce a longer streak of light lower down. This streak will exhibit the prismatic colours, or the “colours of the rainbow”; viz., red (at the top), orange, yellow, green, blue, indigo (blue), and violet last. These are the colours of the Solar Spectrum. The white light is thus decomposed, and it is called mixed light, because of the seven rays of which it is composed. These rays can be again collected and returned to the white light by means of a convex lens.

“White light,” said Sir Isaac Newton, “is composed of rays differently refrangible,” and as we can obtain the colours of the rainbow from white light, we can, by painting them on a circular plate and turning it rapidly round, make the plate appear white. Thus we can prove that the seven colours make “white” when intermingled. But Newton (1675) did not arrive at the great importance of his experiment. He made a round hole in the shutter, and found that the various colours overlapped each other. But, in 1802, Dr. Wollaston improved on this experiment, and by admitting the light through a tiny slit in the wood, procured an almost perfect spectrum of “simple” colours, each one perfectly distinct and divided by black lines.

But twelve years later, Professor Fraunhofer made a chart of these lines, which are still known by his name. Only, instead of the 576 he discovered, there are now thousands known to us! To Fraunhofer’s telescope Mr. Simms added a collimating lens, and so the Spectroscope was begun; and now we use a number of prisms and almost perfect instruments, dispersing the light through each. We have here an illustration of a simple Spectroscope, which is much used for chemical analysis (fig. 150).

In the spectrum we have long and short waves of light, as we have long and short (high and low) waves in music, called notes. The long or low notes are as the red rays, the high notes as the blue waves of light. (Here we have another instance of the similarity between light and sound.) But[Pg 144] suppose we shut out the daylight and substitute an artificial light. If we use a lamp burning alcohol with salt (chloride of sodium), the spectrum will only consist of two yellow bands, all the other colours being absent. With lithium we obtain only two, one orange and one red. From this we deduce the fact that different substances when burning produce different spectra; and although a solid may (and platinum will) give all seven colours in its spectrum, others, as we have seen, will only give us a few, the portion of the spectrum between the colours being black. Others are continuous, and transversed by “lines” or narrow spaces devoid of light; such is the spectrum of the sun, and by careful and attentive calculation and observation we can get an approximate idea of the matter surrounding the heavenly bodies.

Fig. 150.—The Spectroscope.

We have said there are lines crossing the spectrum transversely; these are called Fraunhofer’s lines, after the philosopher who studied them; they were, however, discovered by Wollaston. These lines are caused by light from the lower portion of the sun passing through the metallic vapours surrounding the orb in a state of incandescence, such as sodium, iron, etc. One of Fraunhofer’s lines, a black double line known as D in the yellow portion of the spectrum, was known to occupy the same place as a certain luminous line produced by sodium compounds in the flame of a spirit lamp. This gave rise to much consideration, and at length Kirchkoff proved that the sodium vapour which gives out yellow light can also absorb that light; and this fact, viz., that every substance, which at a certain temperature emits light of a certain refrangibility, possesses at that temperature the power to absorb that same light. So the black lines are now considered the reversal of luminous lines due to the incandescent vapours by which the sun is surrounded. Thus the presence of an element can be found from black or luminous lines, so the existence of terrestrial elements in celestial bodies has[Pg 145] been discovered by means of preparing charts of the lines of the terrestrial elements, and comparing them with the lines of stella spectra.

We have supposed the beam of light to enter through a slit in the shutter, and fall upon a screen or sheet. The solar spectrum shown by the passage of the beam through a prism is roughly as below—

Fig. 151.—Example of the Spectrum.

Fraunhofer substituted a telescope for the lens and the screen, and called his instrument a Spectroscope. He then observed the lines, which are always in the same position in the solar spectrum. The principal of them he designated as A, B, C, D, E, F, G, H. The first three are in the red part of the spectrum; one in the yellow, then one in the green; F comes between green and blue, G in the indigo-blue, and H in the violet. But these by no means exhaust the lines now visible. Year by year the study of Spectrum Analysis has been perfected more and more, and now we are aware of more than three thousand “lines” existing in the solar spectrum. The spectra of the moon and planets contain similar dark lines as are seen in the solar spectrum, but the fixed stars show different lines. By spectrum analysis we know the various constituents of the sun’s atmosphere, and we can fix the result of our observations made by means of the Spectroscope in the photographic camera. By the more recent discoveries great studies have been made in “solar chemistry.”

What can we do with the Spectroscope, or rather, What can we not do? By Spectroscopy we can find out, and have already far advanced upon our path of discovery, “the measure of the sun’s rotation, the speed and direction of the fierce tornados which sweep over its surface, and give rise to the ‘maelstroms’ we term ‘sunspots,’ and the mighty alps of glowing gas that shoot far beyond the visible orb, ever changing their form and size; even the temperature and pressure of the several layers and their fluctuations are in process of being defined and determined.” This is what science is doing for us, and when we have actually succeeded in ascertaining the weather at various depths in the atmosphere of the sun, we shall be able to predict our own, which depends so much upon the sun. Last year (1880) Professor Adams, in his address to the British Association, showed that magnetic disturbances, identical in kind, took place at places widely apart simultaneously. He argues that the cause of these identical disturbances must be far removed from the earth.

“If,” he says, “we imagine the masses of iron, nickel, and magnesium in the sun to retain even in a slight degree their magnetic power in a gaseous state, we have a sufficient cause for all our magnetic changes. We[Pg 146] know that masses of metal are ever boiling up from the lower and hotter levels of the sun’s atmosphere to the cooler upper regions, where they must again form clouds to throw out their light and heat, and to absorb the light and heat coming from the hotter lower regions; then they become condensed, and are drawn back again towards the body of the sun, so forming those remarkable dark spaces or sunspots by their down rush to their former levels. In these vast changes we have abundant cause for those magnetic changes which we observe at the same instant at distant points on the surface of the earth.” So we are indebted to the Spectroscope for many wonderful results—the constitution of the stars, whether they are solid or gaseous, and many other wonders.

The manner in which we have arrived at these startling conclusions is not difficult to be understood, but some little explanation will be necessary.

The existence of dark lines in the solar spectrum proves that certain rays of solar light are absent, or that there is less light. When we look through the prism we perceive the spaces or lines, and we can produce these ourselves by interposing some substance between the slit in the shutter before mentioned and the prism. The vapour of sodium will answer our purpose, and we shall find a dark line in the spectrum, the bright lines being absorbed by the vapour. We can subject a substance to any temperature we please, and into any condition—solid, liquid, or gaseous; we can also send the light the substance may give out through certain media, and we can photograph the spectrum given out under all conditions. The distance of the source of the light makes no difference. So whether it be the sun, or a far-distant star, we can tell by the light sent to us what the physical condition of the star may be. It was discovered in 1864 that the same metallic body may give different spectra; for instance, the spectrum might be a band of light,—like the rainbow,—or a few isolated colours; or again, certain detached lines in groups. The brightness of the spectrum lines will change with the depth of the light-giving source, or matter which produces it.

We have become aware by means of the Spectroscope that numerous metals known to us on earth are in combustion in the sun, and new ones have thus been discovered. In the immense ocean of gas surrounding the sun there are twenty-two elements as given by Mr. Lockyer, including iron, sodium, nickel, barium, zinc, lead, calcium, cobalt, hydrogen, potassium, cadmium, uranium, strontium, etc. Not only is the visible spectrum capable of minute examination, but, as in the case of the heat spectrum already mentioned when speaking of Calorescence, the light spectrum has been traced and photographed far beyond the dark space after the blue and violet rays, seven times longer than the visible solar spectrum—a spectrum of light invisible to our finite vision. Although a telescope has been invented for the examination of these “ultra violet” rays, no human eye can see them. But—and here science comes in—when a photographic plate is put in place of the eye, the tiniest star can be seen and defined. Even the Spectroscope at length fails, because light at such limits has been held to[Pg 147] be “too coarse-grained for our purposes”! “Light,” says a writer on this subject “we can then no longer regard as made of smooth rays; we have to take into account—and to our annoyance—the fact that its ‘long levelled rules’ are rippled, and its texture, as it were, loose woven”!

Twenty years ago Professors Kirchkoff and Bunsen applied Fraunhofer’s method to the examination of coloured flames of various substances, and since then we have been continually investigating the subject; yet much remains to be learnt of Spectrum Analysis, and Spectroscopy has still much to reveal. From Newton’s time to the present our scientists have been slowly but surely examining with the Spectroscope the composition of spectra, and the Spectroscope is now the greatest assistant we possess.

“Spectrum Analysis, then, teaches us the great fact that solids and liquids give out continuous spectra, and vapours and gases give out discontinuous spectra instead of an unbroken light” (Lockyer). We have found out that the sunlight and moonlight are identical, that the moon gives us spectrum like a reflection of the former, but has no atmosphere, and that comets are but gases or vapours. The most minute particles of a grain of any substance can be detected to the millionth fraction. The 1/1000 of a grain of blood can be very readily distinguished in a stain after years have passed. The very year of a certain vintage of wine has been told by means of “absorption,” or the action of different bodies on light in the spectrum. It is now easy, “by means of the absorption of different vapours and different substances held in solution, to determine not only what the absorbers really are, but also to detect a minute quantity.” The application of this theory is due to Dr. Gladstone, who used hollow prisms filled with certain substances, and so thickened the “absorption lines.” By these lines, or bands, with the aid of the Spectrum Microscope, most wonderful discoveries have been made, and will continue to be made. We will close this portion of the subject with a brief description of the Spectroscope in principle.

The instrument consists of two telescopes arranged with two object-glasses on a stand (fig. 150). A narrow slit is put in place of the eye-piece of one, the arrangement admitting of the slit being made smaller or larger by means of screws. The glass to which the slit is attached is called the collimating lens. The light, at the end of the slit seen from the other telescope, being separated by the prisms between the two telescopes, will produce the spectrum. The Spectroscope is enclosed, so that no exterior light shall interfere with the spectra the student wishes to observe. This merely indicates the principle, not the details, of the Spectroscope, which vary in different instruments.

We may now pass from the Spectroscope to the Telescope and the Microscope, instruments to which we are most largely indebted for our knowledge of our surroundings in earth, air, and water.

The word Telescope is derived from the Greek tele, far, and skopein, to see; and the instrument is based upon the property possessed by a convex lens[Pg 148] or concave mirror, of converging to a focus the rays of light falling on it from any object, and at that point or focus forming an image of the object. The following diagram will illustrate this. Let VW be a lens, and AB an object between the glass, and F the focus. The ray, Ac, is so refracted as to appear to come from a. The ray from b likewise appears in a similar way, and a magnified image, ab, is the result (fig. 152).

The ordinary Telescope consists of an object-glass and an eye-lens, with two intermediates to bring the object into an erect position. A lens brings it near to us, and a magnifier enlarges it for inspection. We will now give a short history of the Telescope and its improved construction.

Roger Bacon was supposed to have had some knowledge of the Telescope, for in 1551 it was written: “Great talke there is of a glass he made at Oxford, in which men see things that were don.” But a little later, Baptista Porta found out the power of the convex lens to bring objects “nearer.” It was, however, according to the old tale, quite by an accident that the Telescope was discovered about the year 1608.

Fig. 152.—Converging rays to a focus.

In Middleburg, in Holland, lived a spectacle-maker named Zachary Jansen, and his sons, when playing with the lenses in the shop, happened to fix two of them at the proper distance, and then to look through both. To the astonishment of the boys, they perceived an inverted image of the church weathercock much nearer and much larger than usual. They at once told their father what they had seen. He fixed the glasses in a tube, and having satisfied himself that his sons were correct, thought little more about the matter. This is the story as told, but there is little doubt that for the first Telescope the world was indebted either to Hans Lippersheim or Joseph Adriansz, the former a spectacle-maker of Middleburg; and in October 1608, Lippersheim presented to the Government three instruments, with which he “could see things at a distance.” Jansen came after this. The report of the invention soon spread, and Galileo, who was then in Venice, eagerly seized upon the idea, and returning to Padua with some lenses, he managed to construct a telescope, and began to study the heavens. This was in 1609. Galileo’s Tube became celebrated, and all the first[Pg 149] telescopes were made with the concave eye-lens. Rheita, a monk, made a binocular telescope, as now used in our opera- and field-glasses approximately.

But the prismatic colours which showed themselves in the early telescopes were not got rid of, nor was it till 1729 that Hall, by studying the mechanism of the eye, managed a combination of lenses free from colour. Ten years before (in 1718) Hadley had established the Reflector Telescope; Herschel made his celebrated forty-foot “reflector” in 1789.

Fig. 153.—The Microscope.

However, to resume. In 1747, Euler declared that it was quite possible to construct an arrangement of lenses so as to obtain a colourless image, but he was at first challenged by John Dollond. The latter, however, was afterwards induced to make experiments with prisms of crown and flint glass. He then tried lenses, and with a concave lens of flint, and a convex lens of crown, he corrected the colours. The question of proper curvature was finally settled, and the “Achromatic” Telescope became an accomplished fact.

There are two classes of Telescopes—the reflecting and refracting. Lord Rosse’s is an instance of the former. Mr. Grubb’s immense instrument in Dublin is a refractor.

The Microscope has been also attributed to Zacharias Jansen, and Drebbel, in 1619, possessed the instrument in London, but it was of little or no use. The lens invented by Hall, as already mentioned, gave an impetus to the Microscope. In the simple Microscope the objects are seen directly through the lens or lenses acting as one. The compound instrument is composed of two lenses (or a number formed to do duty as two), an eye-lens, and an object-lens. Between these is a “stop” to restrain all light, except what is necessary to view the object distinctly. The large glass near the object bends the rays on to the eyeglass, and a perfect magnified image is perceived. We annex diagrams, from which the construction will be readily understood.

Fig. 154.—Image on the Retina.

We have in the previous chapter mentioned the effect of light upon the eye and its direction, and when an object is placed very near the eye we know it cannot be distinctly seen; a magnified image is thrown upon the retina, and the divergency of the rays prevents a clear image being perceived. But if a small lens of a short “focal length” be placed in front of the eye, having PQ for its focus, the rays of light will be parallel, or very nearly so, and will as such produce “distinct vision,” and the image will be magnified at pq. In the Compound Refracting Microscope, BAB is[Pg 150] the convex lens, near which an object, PQ, is placed a little beyond its focal length. An inverted image, pq, will then be formed. This image is produced in the convex lens, bab′, and when the rays are reflected out they are parallel, and are distinctly seen. So the eye of the observer at the point E will see a magnified image of the object at PQ brought up to pq (fig. 155).

Fig. 155.—The Microscope lenses.

Sir Isaac Newton suggested the Reflecting Microscope, and Dr. Wollaston and Sir David Brewster improved the instrument called the “Periscopic Microscope,” in which two hemispherical lenses were cemented together by the plane surfaces, and having a “stop” between them to limit the aperture. Then the “Achromatic” instrument came into use, and since then the Microscope has gradually attained perfection.

Fig. 156.—Concave lens.

We have so frequently mentioned lenses that it may be as well to say something about them. Lenses may be spherical, double-convex, plane-convex, plane-concave, double-concave, and concave-convex. Convex lenses bring the parallel lines which strike them to a focus, as we see in the “burning-glass.” The concave or hollow lens appears as in fig. 156. The rays that follow it parallel to its axis are refracted, and as if they came from a point F in the diagram. But converging rays falling on it emerge in a parallel direction as above, or diverge as in fig. 158.

Fig. 157.
1. Focus of parallel rays. 2. Focus of divergent rays. 3. Focus of divergent rays brought forward by more convex lens.

The use of spectacles to long or short-sighted people is a necessity, and the lenses used vary. The eye has usually the capacity of suiting itself to viewing objects—its accommodation, as it is termed—near or far. But when the forepart of the eye is curved, and cannot adapt itself to distant objects, the person is said to be short-sighted. In long sight the axis of the eyeball is too short, and the focus falls beyond the retina; in short sight it is too long. In the diagrams herewith fig. 159 shows by the dotted lines the position of the retina in long sight, and fig. 160 in short sight, the clear lines showing in each case the perfectly-formed eye. For long sight and old sight the double[Pg 151]-convex glass is used, for short sight the double-concave (fig. 162). We know the burning-glass gives us a small image of the sun as it converges the rays to its focus. But lenses will do more than this, and in the Photographic Camera we find great interest and amusement.

Fig. 158.—Diverging rays.

Photography (or writing by light) depends upon the property which certain preparations possess of being blackened by exposure to light while in contact with matter. By an achromatic arrangement of lenses the camera gives us a representation of the desired object Fig. 163 shows the image on the plate, and figs. 164 and 165 the arrangement of lenses.

Fig. 159.—Hypermetropia (long sight)..   Fig. 160.—Myopia (short sight).

Fig. 161.—Concave and convex lenses.

Fig. 162.—Lenses for long and short sight.

To Porta, the Neapolitan physician, whose name we have already mentioned more than once, is due the first idea of the Photographic Camera. He found that if light was admitted through a small aperture, objects from which rays reached the hole would be reflected on the wall like a picture. To this fact we are indebted for the Camera Obscura, which receives the picture upon a plane surface by an arrangement of lenses. In fact, Porta nearly arrived at the Daguerreotype process. He thought he could teach[Pg 152] people to draw by following the focussed picture with a crayon, but he could not conquer the aërial perspective.

So the camera languished till 1820, when Wedgwood and Sir Humphrey Davy attempted to obtain some views with nitrate of silver, but they became obliterated when exposed to the daylight.

Fig. 163.—The Camera.

As early as 1814, however, M. Niepce had made a series of experiments in photography, and subsequently having heard that M. Daguerre was turning his attention to the same subject, he communicated with him. In 1827 a paper was read before the Royal Society, and in 1829 a partnership deed was drawn up between Daguerre and Niepce for “copying engravings by photography.” Daguerre worked hard, and at length succeeded in obtaining a picture by a long process, to which, perhaps, some of our readers are indebted for their likenesses forty years ago. By means of iodine evaporated on a metal plate covered with “gold-yellow,” and exposing the plate then in a second box to mercurial vapour, he marked the image in the camera, and then he immersed the plate in hyposulphate of soda, and was able to expose the image obtained to daylight.

Fig. 164. Arrangement of lenses Fig. 165.

But the mode now in use is the “collodion” process. We have all seen the photographer pouring the iodized collodion on the plate, and letting the superfluous liquid drain from a corner of the glass. When it is dry the glass-plate is dipped into a solution of nitrate of silver, and then in a few minutes the glass is ready. The focus is then arranged, and the prepared plate conveyed in a special slide—to keep it from the light—to the camera. When the “patient” is ready, the covering of the lens is removed, and the light works the image into the sensitive plate. The impression is then[Pg 153] “brought up,” and when developed is washed in water, and after by a solution which dissolves all the silver from the parts not darkened by the light. Thus the negative is obtained and printed from in the usual manner.

Instantaneous photography is now practised with great success. An express train, or the movements of a horse at full speed, can thus be taken in a second or less. These results are obtained by using prepared plates, and the “emulsion process,” as it is called, succeeds admirably. The mode of preparation is given in a late work upon the subject, and the photographic plates may also be obtained ready for use. Gelatine and water, mixed with bromide of ammonium, nitrate of silver, and carbonate of ammonium, mixed with certain proportions of water, form the “emulsion.” We need not go into all the details here. Information can easily be obtained from published works, and as the plates can be purchased by amateurs, they will find that the best way.

Aside from the art interest in the new plates is quite another, that springs from the fact that it is now possible to take pictures of men, animals, and machinery in rapid motion, thus enabling us to view them in a way that would be impossible with the unaided eye. The first experiments in this direction were applied to the movements of a horse moving at full speed. The pictures, taken in series, showed that he performed muscular actions that were not before comprehended or even imagined. These pictures at the time attracted great attention, and instantaneous pictures have been since taken of dancers in a ball-room, of vessels and steam-boats in rapid movement, of all kinds of animals in motion, and of machinery in operation. As the pictures represent the movements at one instant of time, they give, as it were, a fixed view of a motion, precisely as if it were suddenly arrested in full action. In the case of animals, the motions of the nostrils are represented in the most singular manner, and the spokes of a steam-boat’s paddle-wheel are shown apparently perfectly still while the spray and waves appear in active motion, or, rather, as they would look if they could be instantly frozen. It is clear the new process and pictures will open a wide and instructive field in art and in the study of mechanical action.

While on the subject of Photography we may mention a very ingenious little apparatus called a Scenograph, the invention of Dr. Candize. It is really a pocket-camera, and is so easily manipulated that it will be found a most pleasant and useful holiday companion. Any one may obtain good results with it, and friends of ours have had occasion to put it in practice during a series of excursions, when it was found to answer in every instance.

The Scenograph is something like a common Stereoscope in outward appearance, and would, perhaps, be at first regarded as a mere toy, did not a more intimate acquaintance prove it a great acquisition, particularly to explorers and tourists. The tripod stand which supports the apparatus is, when not in actual use in that capacity, a very excellent walking-stick, in which the two other “legs” are carried. The instrument, as will be per[Pg 154]ceived from the illustration (fig. 166), is very handy. It produces pictures about the “cabinet” size, and the whole is so arranged that it can be packed and carried in the pocket with ease.

Fig. 166.—The Scenograph.

Photography, as a rule, necessitates a dark room or cabinet, and many preparations as we all know—a “messing” about with chemicals and considerable practice before we can become proficient; so it is not surprising that few amateurs take to it—they prefer to purchase the pictures. But in the new apparatus of which we are speaking, the glass plates are already prepared to receive the image. It is not at all necessary for the operator to stain his fingers and knuckles and nails with nitrate of silver, or any other “chemicals” whatever. He just inserts the plate in the Scenograph,[Pg 155] and then his apparatus being steadily set up, he removes the covering from the lens. To develop the image (in the dusk of the evening or by candle-light) it is necessary to put some drops of ammonia in a saucer, breathe upon the plate so as to soften the collodion, and hold it above the ammonia, and then, under the influence of the vapour, the picture will appear. After this simple operation the picture will be found fixed for a lengthened period—practically indefinitely. Thus on the return of the pedestrian he can reproduce, at small expense, a whole series of little pictures faithfully representing his holiday tour. The illustration shows a small apparatus by which on thin plates small photographs can be taken and fixed till it is found desirable to enlarge them.

The Photophone, one of the most recent contributions to science, is an instrument which, in combination with the telephone principle, makes it possible to convey sounds by means of a ray of light, and by means of a quivering beam “to produce articulate speech at a distance. The success of the Photophone depends upon a rare element, selenium, which has its “electrical resistance” affected by light. Professor Adams demonstrated that the resistance of selenium was reduced just in proportion as the intensity of the light which was acting upon it. Here was the key to the Photophone as thought out by Professor Bell. He fancied that he might by means of his telephone produce sound if he could vary the intensity of the beam of light upon the selenium, which he connected with his telephone and battery.

The Photophone consists of a transmitter for receiving the voice and conveying it along the beam of light, and a receiver for taking the light and converting it into sound—the receiver being the telephone. There is a small mirror (silvered mica has been used) suspended freely for vibration. A lens is used to transmit to this the beam of light, and this beam is again reflected by another lens to the receiver, which consists of a reflector which has a cell of selenium in its focus, connected, as already stated, with the telephone and battery. The speaker stands behind the mirror, and the sound of his voice against the reverse side makes it vibrate in unison with the sounds uttered. The movements cause a quivering in the reflected beam, and this in its changing intensity acts on the selenium, which changes its resistance accordingly, and through the telephone gives forth a sound!

This is the apparently complicated but really simple, and at the same time wonderful, invention of Professor Bell. By the Photophone not only sounds but movements can be converted into sound; even the burning of a candle can be heard! The Photophone is still capable of improvement, and has not as yet arrived at its full development, for it is stated it can be made quite independent of a battery or telephone.

There are many phenomena connected with the Polarization of Light. This requires some notice at our hands. We know that a ray of ordinary light is supposed to be caused by vibrations of the highly attenuated medium, æther. These vibrations occur across the direction of the ray; but when[Pg 156] they occur only in one plane the light is said to be “polarized.” Polarization means possessing poles (like a magnet); the polarized rays have “sides,” as Newton said, or, as explained by Dr. Whewell, “opposite properties in opposite directions, so exactly equal as to be capable of accurately neutralizing each other.” There are some crystals which possess the property of “double refraction,” and thus a ray of common light passing through such a crystal is divided into two polarized rays, taking different directions. One is refracted according to the usual laws of refraction; the other is not, and the planes of polarization are at right angles. It is difficult within the limits of this chapter to explain the whole theory of Polarization. In order to account for certain phenomena in optics, philosophers have assumed that rays possess polarity; and polarized light is light which has had the property of Polarization conferred upon it by reflection, refraction, or absorption. Common light has been compared to a round ruler, and polarized light to a flat ribbon. Huygens found out, when engaged upon the investigation of double refraction, that the rays of light, divided by passing through a crystal (a rhomb) of Iceland spar, possessed certain qualities. When he passed them through a second rhomb, he found that the brightness, relatively, of the rays depended upon the position of the second prism, and in some positions one ray disappeared entirely. The light had been reduced to vibrations in one plane. In 1808, Malus, happening to direct a double refracting prism to the windows then reflecting the sunset, found that as he turned the prism round, the ordinary image of the window nearly disappeared in two opposite positions; and in two other positions, at right angles, the “extraordinary” image nearly vanished. So he found that polarization was produced by reflection as well as by transmission. The differences between common and polarized light have been summed up by Mr. Goddard as follows:—

The bundle of glass plates or the tourmaline plate is thus the test for polarized light, and is termed an analyzer.

The arrangement called a “Nichol’s prism,” made by cutting a prism of Iceland spar and uniting the halves with a cement, so that only one polarized ray can pass through it, is termed a Polarizer. It only permits one of the two rays produced by “double refraction” to pass, and the ray (as said above) will contain none but transverse vibrations. Polarized light will[Pg 157] produce beautiful colours. The whole subject is very interesting to the scientist, but rather a difficult one for the general reader to understand.

Amongst the uses to which light has been put is that of a milk-tester. The Lactoscope will show the quantity of butter contained in a certain quantity of milk, by diluting it till it displays a certain degree of transparency. There is another method, by the transmission of light.

The first test is obtained by means of a glass tube about nine inches long, closed at one end, and containing a small porcelain rod marked with black lines. A small quantity of milk is measured and placed in the tube. The black lines cannot at first be seen through the tube, but by adding water the milk is rendered transparent, and the black lines become visible. The surface of milk in the tube, by a graduated scale upon it, shows the percentage of butter.

Fig. 167.—Cut card figures.

The second method is not so simple. A short tube of tin, blackened on the inside, and supported upright, has an opening on one side, and opposite this, inside the tube, is a mirror placed at an angle of 45°. “By placing a lighted candle at a known distance opposite the opening, its light is reflected in the mirror and thrown upward through the tube. On top of the tube is placed a round vessel of glass or metal, closed at the bottom by a sheet of clear glass. The vessel is closed at the top by a cover having an opening in the centre, in which slides up and down a small tube closed at the bottom with glass, and having an eye-piece at the top. The milk to be tested is placed in this vessel on the top of the tin tube, so that the light of the candle reflected from the mirror passes upward through the milk. Then, by looking through the sliding tube and moving it up and down, a point may be found where the image of the candle in the mirror can be seen through the milk. This device depends, as will be seen, on observing the light transmitted through a film of milk, and the thickness of the film is the measure of the value of the milk. The movable tube contains a graduated scale, and by comparison of this with a printed table, the percentage of butter in the milk may be ascertained.”

In concluding this chapter we give a few hints for some pleasant[Pg 158] relaxation for young people, which has many a time created amusement. The experiment consists in cutting out in paper or cardboard certain portions of a face or figure, as per the illustration herewith. Fig. 167 gives the card as cut with the scissors, and the two subsequent faces are the result of the same held at a less or greater distance from a screen. The illustrations (fig. 168) will assist those who wish to amuse children by making rabbits, etc., on the wall. The shadows will be seen perfectly thrown if the hands be carefully fixed near a good light.

Fig. 168.—Hand-shadows on the wall.

We are all so familiar with the “Magic Lantern,” and the apparatus[Pg 159] for dissolving views by an arrangement of lenses and manipulation of slides, that we need do no more than refer to them.

Fig. 169.—Dissolving views apparatus.

The various ghost illusions, etc., produced by indirect mirrors, have already been referred to, the ghost being merely the reflection of an individual seen through a sheet of glass between the spectators and the stage. The strong light throws a reflection from a parallel mirror lower down, and this reflected image can be made to appear amongst the real actors who are behind the plate-glass in full view of the audience, who are, however, ignorant of the existence of the glass screen.

[Pg 160]

For the winter evenings one may easily procure an apparatus for dissolving views by the oxy-hydrogen light. One, as shown in the illustration herewith (fig. 169), will answer every purpose, and by this double arrangement phantasmagoria may be produced, or a fairy tale may be illustrated. The effect of gradually-approaching night may be given to the picture by means of a special glass in the lower lanthorn. The apparatus is exhibited by means of a Drummond light, and is very simple, although a certain supply of gas is necessary for the performance. But this can be easily procured by an indiarubber tube, or in a bag supplied for the purpose. Almost any objects can be used, photographs, etc., etc., and many very comical arrangements can be made.

We have lately been reading a curious method of obtaining light from oyster-shells in a Trans-Atlantic magazine. We give an extract wherewith to close this chapter. The compound is “luminous paint.”

“It has been known that certain compounds of lime and sulphur had the property of absorbing light, and giving it out again when placed in the dark. A simple way to do this is to expose clean oyster-shells to a red heat for half an hour. When cold, the best pieces are picked out and packed with alternate layers of sulphur in a crucible, and exposed to a red heat for an hour. When cold, the mass is broken up, and the whitest pieces are placed in a clean glass bottle. On exposing the bottle to bright sunshine during the day, it is found that at night its contents will give out a pale light in the dark. Such a bottle filled more than a hundred years ago still gives out light when exposed to the sun, proving the persistency of the property of reproducing light. Very many experiments have been more recently made in this direction, and the light-giving property greatly enhanced. The chemicals, ground to a flour, may now be mixed with oils or water for paints, may be powdered on hot glass, and glass covered with a film of clear glass, or mixed with celluloid, papier-maché, or other plastic materials. As a paint, it may be applied to a diver’s dress, to cards, clock dials, sign-boards, and other surfaces exposed to sunlight during the day; the paint gives out a pale violet light at night sufficient to enable the objects to be readily seen in the dark. If the object covered with the prepared paint is not exposed to the sun, or if the light fades in the dark, a short piece of magnesium wire burned before it serves to restore the light-giving property.”

[Pg 161]

CHAPTER XIV.
SPECTRAL ILLUSIONS.

A SPECTRE VISIBLE—CURIOUS ILLUSIONS—GHOSTS.

We have already given numerous examples of the effects produced by impressions on the retina by mechanical appliances. We can now in a short chapter speak of the cause of many spectral illusions, commonly supposed to be “ghosts” or “spirits.” That there are many “well-authenticated ‘ghost stories’” no one can doubt who has read the literature of the day; and we ourselves do not in any way desire to throw any doubt upon the existence of certain so-called “ghosts.” That appearances of some kind or another are seen by people we know. We ourselves have seen such, but we cannot say we believe in the popular ghost.

In ancient times mirrors were much employed by the so-called magicians, and in our day many wonderful ghost effects have been shown at the (late) Polytechnic Institution. Some people are believers in table-turning and spiritualism, and mesmerists still attract large audiences, and appear to possess extraordinary power over some individuals. But apparitions have been seen by people eminently worthy of credit. The experience of the learned Doctor, which appeared some months ago in the Athenæum, is a case in point. This narrative is concise and clear. The spectre was there. How did it get there? Was the “appearance” objective or subjective? Let us give an extract from the Reverend Doctor’s narrative, and comment upon it afterwards. We may premise that Dr. Jessopp had gone over to Lord Orford’s (Mannington Hall), and at eleven o’clock was busy writing in the library, and was “the only person downstairs.” We will give this ghost story in the Doctor’s own words. After taking up a certain volume—time about 1 a.m.:—

“I had been engaged on it about half an hour, and was beginning to think my work was drawing to a close, when, as I was actually writing, I saw a large white hand within a foot of my elbow. Turning my head, there sat a figure of a somewhat large man with his back to the fire, bending slightly over the table, and apparently examining the pile of books that I had been at work upon.”... After describing the appearance of the nocturnal visitor, Dr. Jessopp proceeds:—

“There he sat, and I was fascinated; afraid not of his staying, but lest he should go. Stopping in my writing I lifted my left hand from the paper, stretched it out to the pile of books, and moved the top one—my arm passed[Pg 162] in front of the figure, and it vanished.”... Shortly after the figure appeared again, and “I was penning a sentence to address to him, when I discovered I did not dare to speak. I was afraid of the sound of my own voice! There he sat, and there sat I. I turned my head and finished writing. Having finished my task, I shut the book, and threw it on the table; it made a slight noise as it fell;—the figure vanished.”

Now here we have a perfectly plain narrative, clear and full. A ghost appeared; he is described distinctly. How can we account for the apparition? In the first place, someone might have played a trick, but that idea was put aside by Dr. Wilks, who attempted to explain the appearances. He went fully into the question, and as it bears upon our explanation of the reality of Spectral Illusions, we may condense his evidence. It will of course be conceded that all the usual objects seen by people are material, and the image of what we look at is formed upon the retina in the manner already explained. But all images upon the retina are not immediately observed; the impression may, to a certain extent, remain. Words are often impressed upon the brain,—words which we in our sober senses would never think of repeating,—and yet when we are delirious we give vent to these expressions, of whose very nature and meaning we are perfectly unconscious. It is, according to our reference (Dr. Wilks), “quite possible for the perceptive part of the brain to be thrown into an active condition quite independent of the normal stimulus conducted to it from the retina.” If, under these circumstances, an object be viewed independently, and, as it were, unconsciously, it is merely, we believe, a parallel to the impression of words before noted. Sound and light are governed by the same laws. In fevers we fancy we see all kinds of things which have no existence. In dreams we hear noises; and many a time people dreaming have been awakened by the report of a gun, or the ringing of a bell which had no material origin,—the nerves were excited, the “perceptive centre” of the brain was moved.

But if sight and hearing thus have their origin from the brain and not from without, there must have been some predisposing cause, some excitement to induce such a condition of things. “The impressions become abnormal and subjective,—the normal condition being objective,—the impression is received from without, and impressed upon the eye.

Now, let us consider the “ghost”! Lately there have been many instances brought forward of “spiritual” appearances, but we think nobody has ever seen a “material” ghost; yet on the other hand none of us have any knowledge of anything in the likeness of a ghost, or that has not a material basis which can bring forward an image on the retina! Therefore we are brought to the conclusion that apparitions are spectres emanating from within the brain, not from any outward manifestation, because it is within the experience of everybody that in bad health, or disordered digestive functions, images are produced in the brain and nerves of the eye.

These remarks have perhaps been made before in one form or other, but[Pg 163] as much popular interest is always awakened by the supernatural, or what is supposed to be supernatural, we may go a little farther, and inquire how it was that the ghost seen by Dr. Jessopp disappeared when he raised his arm. Would any ghost be afraid of the Doctor extending his hand? The fact no doubt occurred as related. The explanation is that the narrator had been much impressed by a certain picture, which a correspondent soon identified as a portrait of “Parsons, the Jesuit Father.” The description given is that of the priest who was described by the Doctor in one of his books. The association of ideas in the library of a Norfolk house connected with the Walpoles, with whom Parsons had been a leader, gave rise, during a period of “forty winks” at midnight, to the spectre.

In the interesting letters written upon “Natural Magic” by Sir David Brewster, the subject of Spectral Illusions is treated at some length, and with undoubted authority. Sir David thought the subject worth discussing with reference to the illusions or spectres mentioned by Dr. Hibbert. Sir David Brewster gives his own experiences which occurred while he was staying at the house of a lady in the country.

The illusions appear to have affected her ear as well as the eye. We shall see in the next chapter how intimately sound and light are connected, and how the eyes and ears are equally impressed, though in a different way, by the vibration of particles. The lady referred to was about to go upstairs to dress for dinner one afternoon, when she heard her husband’s voice calling to her by name. She opened the door, and nobody was outside; and when she returned for a moment to the fire she heard the voice again calling, “Come to me; come, come away,” in a somewhat impatient tone. She immediately went in search of her husband, but he did not come in till half an hour afterwards, and of course said he had not called, and told her where he had been at the time—some distance away. This happened on the 26th December, 1830, but a more alarming occurrence took place four days after.

About the same time in the afternoon of the 30th December, the lady came into the drawing-room, and to her great astonishment she perceived her husband standing with his back to the fireplace. She had seen him go out walking a short time previously, and was naturally surprised to find he had returned so soon. He looked at her very thoughtfully, and made no answer. She sat down close beside him at the fire, and as he still gazed upon her she said, “Why don’t you say something!” The figure immediately moved away towards the window at the farther end of the room, still gazing at her, “and it passed so close that she was struck by the circumstance of hearing no step nor sound, nor feeling her dress brushed against, nor even any agitation of the air.” Although convinced this was not her husband, the lady never fancied there was anything supernatural in the appearance of the figure. Subsequently she was convinced that it was a spectral illusion, although she could not see through the figure which appeared as substantial as the reality.

[Pg 164]

Were it advisable, we could multiply instances. In the Edinburgh Journal of Science these, and many more instances of spectral illusions were narrated by the husband of the lady. She frequently beheld deceased relatives or absent friends, and described their dress and general appearance very minutely. On one occasion she perceived a coach full of skeletons drive up to the door, and noticed spectral dogs and cats (her own pets’ likenesses) in the room. There can be no doubt upon these points; the appearances were manifest and distinct. They were seen in the presence of other people, in solitude, and in the society of her husband. The lady was in delicate health, and very sensitive. The spectres appeared in daylight as well as in the dark, or by candle-light.

Let us now, guided by what we have already written, and by Sir David Brewster’s experience, endeavour to give a rational explanation of these illusions. “The mind’s eye is really the body’s eye, and the retina is the common tablet upon which both classes of impressions are painted, and by means of which they receive their visual existence according to the same optical laws.”

“In the healthy state of mind and body the relative intensity of the two classes of impressions on the retina are nicely adjusted—the bodily and mental are balanced. The latter are feeble and transient, and in ordinary temperaments are never capable of disturbing or effacing the direct images of visible objects.... The mind cannot perform two different functions at the same instant, and the direction of its attention to one of the two classes of impressions necessarily produces the extinction of the other; but so rapid is the exercise of mental power, that the alternate appearance and disappearance of the two contending impressions is no more recognized than the successive observations of external objects during the twinkling of the eyelids.”

We have before illustrated, by means of the pen and the ink-bottle, how one object is lost sight of when the other is attentively regarded, and a material picture or scene may be equally lost sight of, and a mental picture take its place in the eye, when we recall places or people we have seen or remembered.

We have all heard numerous anecdotes of what is termed “absent-mindedness.” Some people are quite absorbed in study, and can see or hear no one in the room when deeply occupied. We may be satisfied then that “pictures of the mind and spectral illusions are equally impressions upon the retina, and only differ in the degree of vividness with which they are seen.” If we press our eyes the phosphorescence becomes apparent, and we can make a picture of the sun or a lamp visible for a long time to our closed eyes if we stare at either object for a few seconds, and shut our lids. So by increasing the sensibility of the retina we can obtain the image, and alter its colour by pressure on the eye.

It is well known that poisons will affect sight, and belladonna applied to the eyes will so affect them as to render the sight nil, by enlargement of[Pg 165] the “pupil.” If one is out of health there is practically a poisoning of the system, and when we have a “bilious headache” we see colours and stars because there is a pressure upon the blood-vessels of the eye. The effects of a disordered stomach, induced by drinking too much, are well known; objects are seen double, and most ghosts may be traced to a disordered state of health of mind or body, brought on by excitement or fatigue. We could relate a series of ghost stories,—some in our own experience, for we have seen a ghost equally with our neighbours,—but this is not the place for them. Although many apparently incontrovertible assertions are made, and many spectres have been produced to adorn a tale, we must put on record our own opinion, that every one could be traced to mental impression or bodily affection had we only the key to the life and circumstances of the ghost-seer. Many celebrated conjurers will convince us almost against our reason that our pocket-handkerchief is in the orange just cut up. They will bring live rabbits from our coat-pockets or vests, and pigeons from our opera-hats. These are equally illusions. We know what can be done with mirrors. We have seen ghosts at the Polytechnic, but we must put down all apparitions as the result of mental or bodily, even unconscious impressions upon the retina of the eye. There are numerous illusions, such as the Fata Morgana, the Spectre of the Brocken, etc., which are due to a peculiar state of the atmosphere, and to the unequal reflection and refraction of light. Those, and many other optical phenomena, will, with phenomena of heat and sound, be treated under Meteorology, when we will consider the rainbow and the aurora, with many other atmospheric effects.

[Pg 166]

CHAPTER XV
ACOUSTICS.

THE EAR, AND HEARING—PHYSIOLOGY OF HEARING AND SOUND—SOUND AS COMPARED WITH LIGHT—WHAT IS SOUND?—VELOCITY OF SOUND—CONDUCTIBILITY—THE HARMONOGRAPH.

Before entering upon the science of Acoustics, a short description of the ear, and the mode in which sound is conveyed to our brain, will be no doubt acceptable to our readers. The study of the organs of hearing is not an easy one; although we can see the exterior portion, the interior and delicate membranes are hidden from us in the very hardest bone of the body—the petrous bone, the temporal and rock-like bone of the head.

Fig. 170- 1. Temple bone. 2. Outer surface of temple. 3. Upper wall of bony part of hearing canal. 4. Ligature holding “hammer” bone to roof of drum cavity. 5. Roof to drum cavity. 6. Semicircular canals. 7. Anvil bone. 8. Hammer bone. 9. Stirrup bone. 10. Cochlea. 11. Drum-head cut across. 12. Isthmus of Eustachian tube. 13. Mouth of tube in the throat. 14. Auditory canal. 15. Lower wall of canal. 16. Lower wall of cartilaginous part of canal. 17. Wax glands. 18. Lobule. 19. Upper wall of cartilaginous portion of canal. 20. Mouth of auditory canal. 21. Anti-tragus.

The ear (external) is composed of the auricle, the visible ear, the auditory canal, and the drum-head, or membra tympani. The tympanum, or “drum,” is situated between the external and the internal portions of the ear. This part is the “middle ear,” and is an air cavity, and through it pass two[Pg 167] nerves, one to the face, and the other to the tongue. The internal ear is called the “labyrinth,” from its intricate structure. We give an illustration of the auditory apparatus of man (fig. 170).

The auricle, or exterior ear, is also represented, but we need not go into any minute description of the parts. We will just name them (fig. 171).

Sound is the motion imparted to the auditory nerve, and we shall see in a moment how sound is produced. The undulations enter the auditory canal, having been taken up by the auricle; the waves or vibrations move at the rate of 1,100 feet a second, and reach the drum-head, which has motion imparted to it. This motion or oscillation is imparted to other portions, and through the liquid in the labyrinth. The impressions of the sound wave are conveyed to the nerve, and this perception of the movement in the water of the labyrinth by the nerve threads and the brain causes what we term “hearing.”

Fig. 171.—1. Pit of anti-helix. 2, 6, 10. Curved edge of the auricle. 3. Mouth of auditory canal. 4. Tragus. 5. Lobe. 7. Anti-helix. 8. Concha. 9. Anti-tragus.

Let us now endeavour to explain what sound is, and how it arises. There are some curious parallels between sound and light. When speaking of light we mentioned some of the analogies between sound and light, and as we proceed to consider sound, we will not lose sight of the light we have just passed by.

Sound is the influence of air in motion upon the hearing or auditory nerves. Light, as we have seen, is the ether in motion, the vibrations striking the nerves of the eye.

There are musical and unmusical sounds. The former are audible when the vibrations of the air reach our nerves at regular intervals. Unmusical sounds, or irregular vibrations, create noise. Now, musical tones bear the same relation to the ear as colours do to the eye. We must have a certain number of vibrations of ether to give us a certain colour (vide table). “About four hundred and fifty billion impulses in a second” give red light. The violet rays require nearly double. So we obtain colours by[Pg 168] the different rate of the impingement of impulses on the retina. The eyes, as we have already learned, cannot receive any more rapidly-recurring impressions than those producing violet, although as proved, the spectrum is by no means exhausted, even if they are invisible. In the consideration of Calorescence we pointed this out. These invisible rays work great chemical changes when they get beyond violet, and are shown to be heat. So the rays which do not reach the velocity of red rays are also heat, which is the effect of motion.

Thus we have Heat, Light, and Sound, all the ascertained results of vibratory motion. The stillness of the ether around us is known as “Darkness”; the stillness of the air is “Silence”; the comparative absence of heat, or molecular motion of bodies is “Cold”!

In the first part we showed how coins impart motion to each other.

When an impulse was given the motion was carried from coin to coin, and at length the last one in the row flew out. This is the case with sound. The air molecules strike one upon another, and the wave of “sound” reaches the tympanum, and thus the impression is conveyed to the brain. We say we hear—but why we hear, in what manner the movement of certain particles affects our consciousness, we cannot determine.

That the air is absolutely necessary to enable us to hear can readily be proved. The experiment has frequently been made; place a bell under the receiver of an air-pump, and we can hear it ring. But if we exhaust the air the sound will get fainter and fainter. Similarly, as many of us have experienced upon high mountains, sounds are less marked. Sound diminishes in its intensity, just as heat and light do. Sound is reflected and refracted, as are light and radiant heat. We have already shown the effect of reflectors upon heat. Sound is caught and reflected in the same way as[Pg 169] light from mirrors, or as the heat waves in the reflectors. We have what we term “sounding boards” in pulpits, and speaking tubes will carry sound for us without loss of power. Echoes are merely reflected sounds.

The velocity of sound is accepted as 1,100 feet in a second, which is far inferior to the velocity of light. Fogs will retard sound, while water will carry it. Those who have ever rowed upon a lake will remember how easily the sound of their voices reached from boat to boat, and Dr. Hutton says that at Chelsea, on the Thames, he heard a person reading from a distance of a hundred and forty feet. Some extraordinary instances could be deduced of the enormous distances sound is said to have travelled. Guns have been heard at eighty miles distant, and the noise of a battle between the English and Dutch, in 1672, was heard even in Wales, a distance of two hundred miles from the scene of action.

Sound always travels with uniform velocity in the air in the same temperature. But sound! What is the cause of it? How does it arise? These questions can now be fully answered with reference to the foregoing observations. Phenomena of vibration render themselves visible by light, heat, and sound, and to arrive at some definite ideas of sound vibrations we may compare them to waves, such as may be produced by throwing a stone into a pond.

There are, so to speak, “standing” waves and “progressive” waves. The former can be produced (for instance) by thrumming a fiddle-string, and when the equilibrium of the cord is disturbed, the position of the equilibrium is passed simultaneously by the string-waves. In water the waves or vibrating points pass the position of equilibrium in succession.

Waves consist of elevations and depressions alternately, and when we obtain two “systems” of waves by throwing two stones into water, we can observe some curious effects. It can be seen how one series of depressions will come in contact with the other series of depressions, and the elevations will likewise unite with the result of longer depressions and elevations respectively; or it may very well be that elevation will meet depression, and then the so-called “interference” of waves will produce points of repose. These points are termed nodes. The waves of the string proceed in the plane of its axis; water waves extend in circles which increase in circumference.

The progression or propagation of sound may be said to begin when some tiny globule of matter expands in the air. The air particles strike one against the other, and so the motion is communicated to the air waves, which in time reach the ear. But the velocity of the sound is not equal in all substances. Air will convey it around our earth at the rate of 765 miles an hour, or 1,090 feet in a second. That is, we may accept such rate as correct at a temperature of 32° Fahr., and at a pressure of thirty inches, and the velocity increases almost exactly one foot per second for each degree of temperature above 32°. Therefore on an average, and speaking in “round numbers,” the estimate of 1,100 feet in a second may be[Pg 170] accepted as correct. In hydrogen gas the rate is much higher. Through water again it is different, and still faster through iron, glass, and wood, as will be seen in the following table:—

So there is a considerable difference in the velocities of sound through the solid substances quoted, but these figures cannot be taken as exact, as different samples may give different results. In wires and bells the bodies themselves produce the sounds we hear. In wind instruments and the voice the air is the cause of the sound.

The very deepest notes are produced by the fewest vibrations. Fourteen or fifteen vibrations will give us a very low note, if not the very lowest. The pipe of sixteen feet, closed at its upper end, will produce sound waves of thirty-two feet. High notes can be formed from vibrations up to 48,000 in a second. Beyond these limits the ear cannot accept a musical sound.

Fig. 172.—The vibration of strings.

We will explain the phenomenon of the vibration of strings by means of the illustration. In the cut we find a string or wire, which can be lengthened or shortened at pleasure by a movable bridge, and stretched by weights attached to the end (fig. 172).

We can now easily perceive that the shorter and thinner the string is, and the tighter it is, the number of vibrations will be greater and greater. The density of it is also to be considered, and when these conditions are in the smallest proportion then the tone will be highest. The depth will naturally increase with the thickness, density, and length, and with a decreasing tension. But we have strings of same thickness stretched to different degrees of tension, and thus producing different notes. Some strings are covered with wire to increase their gravity, and thus to produce low notes.

When a number of separate sounds succeed each other in very rapid course they produce a sound, but to appear as one sound to the ear they must amount to fifteen or sixteen vibrations every second. The particles of matter in the air form a connected system, and till they are disturbed they remain in equilibrium; but the moment they are in any way thrown out of this state they vibrate as the pendulum vibrates. The particles thus strike[Pg 171] each other, and impart a motion to the elastic medium air, so a sound comes to us.

The intensity of sounds gets less the farther it goes from us, or the loudness of sound is less the greater its distance. The law is, that in an unvarying medium the loudness varies inversely as the square of the distance. But Poisson has shown that when air-strata, differing in density, are existing between the ear and the source of the sound, the intensity or loudness with which it is heard depends only on the density of the air at the place the sound originated. This fact has been substantiated by balloonists who heard a railway whistle quite distinctly when they were nearly 20,000 feet above the ground. It therefore follows that sound can be heard in a balloon equally well as on the earth at certain given distances. But as the density of the air diminishes the sound becomes fainter, as has been proved by the bell rung in the receiver of an air-pump. The velocity of sound, to a certain extent, depends upon its intensity, as Earnshaw sought to prove; for he instanced a fact that in the Arctic regions, where sound can be heard for an immense distance, in consequence of the still and homogeneous air, the report of a cannon two miles and a half away was heard before the loud command to “fire,” which must have preceded the discharge. Another instance showing the difference in hearing through mixed and homogeneous media may be referred to. In the war with America, when the English and their foes were on opposite sides of a stream, an American was seen to beat his drum, but no sound came across. “A coating of soft snow and a thick atmosphere absorbed the noise.” Glazed, or hard snow, would have a contrary effect. Reynault also experimentally verified his theory, that sound when passing through a space of nearly 8,000 feet lost velocity as its intensity diminished, and in that distance between its arrival at 4,000 feet and at 7,500 feet, the sound velocity diminished by 2·2 feet per second. He also tried to demonstrate that sound velocity depended upon its pitch, and that lower notes travelled with the greater speed.

The reflection and refraction of sound follows the same fundamental laws as the reflection and refraction of light. The reflection of sound is termed an Echo, which is familiar to all tourists in Switzerland and Ireland particularly. There are several very remarkable echoes in the world: at Woodstock, and at the Sicilian cathedral of Gergenti, where the confessions poured forth near the door to priestly ears were heard by a man concealed behind the high altar at the opposite end. It is curious that such a spot should have been accidentally chosen for the Confessional. The whispering gallery in St. Paul’s is another instance of the echo.

Echoes are produced by the reflection of sound waves from a plane or even surface. A wall, or even a cloud, will produce echoes. Thunder is echoed from the clouds. (The celebrated echo of “Paddy Blake,” at Killarney, which, when you say “How do you do,” is reported to reply, “Very well, thank you,” can scarcely be quoted as a scientific illustration.) And the hills of Killarney contain an echo, and the bugle sounds are beautifully[Pg 172] repeated. In the cases of ordinary echo, when the speaker waits for the answer, he must place himself opposite the rock. If he stand at the side the echo will reply to another person in a corresponding place on the farther side, for the voice then strikes the rock at an angle, and the angle of reflection is the same, as in the case of light.

But if it should happen that there are a number of reflecting surfaces the echo will be repeated over and over again, as at the Lakes of Killarney. The Woodstock Echo, already referred to, and mentioned by several writers, repeats seventeen syllables by day, and twenty by night. In Shipley there is even a greater repetition. Of course the echo is fainter, because the waves are weaker if the reflecting surface be flat. But, as in the case of the mirrors reflecting light, a circular or concave surface will increase the intensity. A watch placed in one mirror will be heard ticking in the other focus. Whispering galleries carry sound by means of the curved surface. Sir John Herschel mentions an echo in the Menai Suspension Bridge. The blow of a hammer on one of the main piers will produce the sound from each of the crossbeams supporting the roadway, and from the opposite pier 576 feet distant, as well as many other repetitions.

Refraction of sound is caused by a wave of sound meeting another medium of different density, just as a beam of light is refracted from water. One sound wave imparts its motion to the new medium, and the new wave travels in a different direction. This change is refraction. The sound waves are refracted in different directions, according to the velocity it can acquire in the medium. If a sound pass from water into air it will be bent towards the perpendicular, because sound can travel faster in water than in air. If it pass from air into water its force will cause it to assume a less perpendicular direction, there being greater velocity in water. The velocity in air is only 1,100 feet in a second in our atmosphere. In water sound travels 4,700 feet in the same time. When the wave of sound falls upon a medium parallel to the refracting surface there is, however, no refraction—only a change of velocity, not direction.

When sound waves are prevented from dispersing the voice can be carried a great distance. Speaking tubes and trumpets, as well as ear trumpets, are examples of this principle, and of the reflection of sound.

There are many very interesting experiments in connection with Acoustics, some of which we will now impart to our readers. We shall then find many ingenious inventions to examine,—the Audiphone, Telephone, Megaphone, and Phonograph, which will occupy a separate chapter. We now resume.

Amongst the experiments usually included in the course of professors and lecturers who have a complete apparatus at their command, and which at first appear very complicated and difficult, there are some which can be performed with every-day articles at hand. There is no experiment in acoustics more interesting than that of M. Lissajons, which consists, as is well known to our scientists, of projecting upon a table or other surface, with[Pg 173] the aid of oxy-hydrogen light, the vibratory curves traced by one of the prongs of a tuning-fork. We can perform without difficulty a very similar experiment with the humble assistance of the common knitting-needle.

Fix the flexible steel needle firmly in a cork, which will give it sufficient support; fasten then at the upper extremity a small ball of sealing wax, or a piece of paper about the size of a large pea. If the cork in which the needle is fixed be held firmly in one hand, and you cause the needle to vibrate by striking it, and then letting it sway of itself, or with a pretty strong blow with a piece of wood, you will perceive the little pellet of wax or paper describe an ellipse more or less elongated, or even a circle will be described if the vibrations be frequent. The effect is much enhanced if the experiment be performed beneath a lamp, so that plenty of light may fall upon the vibrating needle. In this case, the persistence of impressions upon the retina admits of one seeing the vibrating circle in successive positions, and we may almost fancy when the needle is struck with sufficient force, that an elongated conical glass, like the old form of champagne glass, is rising from the cork, as shown in the illustration annexed (fig. 173).

Fig. 173.—Experiment showing vibration of sound waves.

Acoustics may be studied in the same way as other branches of physical science. We will describe an interesting experiment, which gives a very good idea of the transmission of sounds through solid bodies. A silver spoon is fastened to a thread, the ends of which are thrust into both[Pg 174] ears, as shown in fig. 174; we then slightly swing the spoon until we make it touch the edge of the table; the transmission of sound is in consequence so intense that we are ready to believe we are listening to the double diapason of an organ. This experiment explains perfectly the transmission of spoken words by means of the string of a telephone, another contrivance which any one may make for himself without any trouble whatever. Two round pieces of cardboard are fitted to two cylinders of tin-plate, as large round as a lamp-glass, and four-and-a-half inches in length. If the two rounds of cardboard are connected by a long string of sixteen to eighteen yards, we can transmit sounds from one end to the other of this long cord; the speaker pronouncing the words into the first cylinder, and the listener placing his ear against the other. It is easy to demonstrate that sound takes a certain time to pass from one point to another. When one sees in the distance a carpenter driving in a stake, we find that the sound produced by the blow of the hammer against the wood only reaches the ear a few seconds after the contact of the two objects. We see the flash at the firing of a gun, before hearing the sound of the report—of course on the condition that we are at a fairly considerable distance, as already remarked upon.

Fig. 174.—Conductibility of sound by solid bodies.

Fig. 175.—Musical glasses.

We can show the production of the Gamut by cutting little pieces of wood of different sizes, which one throws on to a table; the sounds produced vary according to the size of the different pieces. The same effect may be[Pg 175] obtained much better by means of goblets more or less filled with water; they are struck with a short rod, and emit a sound which can be modified by pouring in a greater or less quantity of water; if the performer is gifted with a musical ear, he can obtain, by a little arrangement, a perfect Gamut by means of seven glasses which each give a note (fig. 175). A piece of music may be fairly rendered in this manner, for the musical glasses frequently produce a very pure silvery sound. We will complete the elementary principles of acoustics by describing a very curious apparatus invented by M. Tisley, the Harmonograph. This instrument, which we can easily describe, is a most interesting object of study. The Harmonograph belongs to mechanics in principle, and to the science of acoustics in application. We will first examine the apparatus itself. It is composed of two pendulums, A and B (fig. 176), fixed to suspensions. Pendulum B supports a circular plate, P, on which we may place a small sheet of paper, as shown in the illustration. This paper is fixed by means of small brass clips. Pendulum A supports a horizontal bar, at the extremity of which is a glass tube, T, terminating at its lower extremity with a capillary opening; this tube is filled with aniline ink, and just rests on the sheet of paper; the support and the tube are balanced by a counterpoise on the right. The two pendulums, A and B, are weighted with round pieces of lead, which can be moved at pleasure, so that various oscillations may be obtained. The ratio between the oscillations of the two pendulums may be exactly regulated by means of pendulum A carrying a small additional weight, the height of which may be regulated by means of a screw and a small windlass. If we give to[Pg 176] pendulum A a slight movement of oscillation, the point of tube T traces a straight line on the paper placed in P; but if we move pendulum B, the paper also is displaced, and the point of tube T will trace curves, the shape of which varies with the nature of the movement of pendulum B, the relation between the oscillations of the two pendulums, etc. If the pendulums oscillate without any friction the curve will be clear, and the point will pass indefinitely over the same track, but when the oscillations diminish, the curve also diminishes in size, still preserving its form, and tending to a point corresponding with the position of repose of the two pendulums. The result is therefore that the curves traced by the apparatus, of which we produce three specimens (figs. 177, 178, 179), are traced in a continuous stroke, commencing with the part of the greatest amplitude.

Fig. 176.—M. Tisley’s Harmonograph.

By changing the relation and phases of the oscillations we obtain curves of infinitely varied aspect. M. Tisley has a collection of more than three thousand curves, which we have had occasion to glance over, in which we failed to meet with two corresponding figures. The ratio between these curves corresponds with some special class, of which the analyst may define the general characters, but which is outside our present subject. By giving the plate P a rotatory movement, we obtain spiral curves of a very curious effect, but the apparatus is more complicated. Considered from this point of view it constitutes an interesting mechanical apparatus, showing the combination of oscillations, and resolving certain questions of pure mechanics.[Pg 177] From the point of view of acoustics it constitutes a less curious object of study. The experiments of M. Lissajons have proved that the vibrations of diapasons are oscillations similar to, though much more rapid than those of the pendulum. We can therefore with this apparatus reproduce all the experiments of M. Lissajons, with this difference, that the movements being slower are easier to study. When the ratio between the number of vibrations—we purposely use the term vibration instead of the term oscillation—is a whole number, we obtain figs. 177 and 178. If the ratio is not exact, we obtain fig. 179, which is rather irregular in appearance, corresponding to the[Pg 178] distortions noticeable in M. Lissajon’s experiments. Fig. 178 has been traced in the exact ratio 2:3; fig. 177 in the ratio 1:2; and fig. 179 corresponds to the ratio 1:2 and a small fraction, which causes the irregularity of the figure.

Fig. 177.—Ratio 1:2. Fig. 178.—Ratio 2:3.

Fig. 179.—Ratio 1:2 and a fraction.

Fig. 180. Construction of the Harmonograph. Fig. 181.

Fig. 182.—Method of constructing an Harmonograph.

Fig. 183.—The apparatus completed.

In considering the harmony of figs. 177 and 178,—the first of which corresponds to the octave, the second to the fifth, whilst fig. 179 corresponds to the disagreeable interval of the ninth,—one is almost tempted to put a certain faith in the fundamental law of simple ratios as the basis of harmony. At first sight this appears beyond doubt, but perhaps musicians would be hardly content with the explanation. M. Tisley’s Harmonograph, it will be seen, is a rather complicated apparatus; and I will now explain how it may be constructed by means of a few pieces of wood. I endeavoured to construct as simple an apparatus as possible, and with the commonest materials, feeling that it is the best means of showing how it is possible for everybody to reproduce these charming curves of musical intervals. Also I completely excluded the employment of metals, and I constructed my apparatus entirely with pieces of wooden rulers, and old cigar boxes. I set to work in the[Pg 179] following manner: on the two consecutive sides of a drawing board I fixed four small pieces of wood (fig. 180), side by side in twos, having at the end a small piece of tin-plate forming a groove (fig. 181). In these grooves nails are placed which support the pendulums. The piece of wood is placed on the corner of the table, so that the pendulums which oscillate in two planes at right angles, are in two planes that are sensibly parallel to the sides of the table. The pendulums are made of a thin lath, with two small pieces of wood fixed to them containing some very pointed nails, on which the pendulum oscillates. Fig. 182 gives an illustration. The pendulums have a pin fixed in vertically, which passes through a piece of wood, and by means of a hinge connects the upper ends of the two pendulums. This contrivance of the pin is very useful, and if care is taken to make the hole through the hinge in the form of a double cone, as shown in fig. 182, c, it makes a perfect joint, which allows the piece of wood to be freely moved.

Fig. 184.—Details of mechanism.

To complete the apparatus, the heads of the two pendulums are united by the hinge, at the bend of which a slender glass tube is fixed, which traces the curves. The hinge is given in fig. 184, and to its two extremities are adjusted the two pins of the pendulum (fig. 183). The pendulums are encircled with round pieces of lead, which can be fixed at any height by means of a screw.

[Pg 180]

CHAPTER XVI.
ACOUSTICS (Continued).

THE TOPOPHONE—THE MEGAPHONE—THE AUTOPHONE—THE AUDIPHONE—THE TELEPHONE—THE PHONOGRAPH—THE MICROPHONE.

We propose in this chapter to give as shortly as possible a description of the various instruments lately come into use, by means of which, and electricity, sounds can be carried from place to place, and their locality identified. It is only within the last few years that these wonderful inventions have come into use, and in a measure superseded the at one time invincible electric telegraph. The Telephone is now in daily use in London and other places, and its novelty, if not all its capability, has been discounted. The Phonograph has also been frequently seen. So we will on this occasion commence with the Topophone, a rather novel instrument.

As the name indicates, the Topophone is an apparatus for discovering the position of a sound, from the Greek words signifying a “place” and “sound.” The sources of sound can be found by it, and indeed this is its actual and practical use. It is claimed for this new apparatus that it stands in the same relation to the sailor as his old and trusty friends, the compass and sextant. These in navigation inform the steersman as to his course, and tells him his position by observation. The Topophone will tell him whence a sound arises, its origin wherever it may be; and this in a fog is no mean advantage. Suppose a ship to be approaching a dangerous coast in a fog. We are all aware how deceptive sounds are when heard through such a medium. We cannot tell from what precise direction the horn, whistle, or bell is sounding. The Topophone will give us the exact spot, and we can then, from our general knowledge of the locality, work our vessel up the river, or into the harbour, in safety.

The Topophone was invented in 1880, by Professor Alfred Mayer, an American, and is based upon the well-known theory of sound waves. These, as we have already explained, exist in the air; and if the theory of sound waves has perfected the Topophone, we can fairly say that it has confirmed the supposed form of the sound waves. “Sound,” says the inventor of the apparatus, “is supposed to be a particle continually expanding in the air, composed of a wave produced by compression, and followed by rarefaction.[Pg 181] A continuous sound is a series of these particles or globules spreading and expanding as the water-rings in a pond.” This much will be at once perceived.

Now, suppose a person up to his shoulders in a pond of water, and someone throws a stone into it. If that person extend his arms and hands at right angles facing the sound, each hand would touch the edge of a ripple as it came towards him across the pond. He would then be facing the source of the ripples or waves, and look along a radius of the circle formed by the waves. But if he please, he can move his body so that both hands shall touch the same wave at the same time, or he might turn away from the source, and only one hand would touch the wave. But when both hands are actually washed by the same circular ripple he must be facing the source of it. Any position in which his fingers did not touch the ripple almost at the same instant, would not be facing the source of the wave ripples. So by turning and extending his hands, he could with his eyes shut find out whether he was or was not facing the original source of the waves.

This applied to sound waves in the air is the whole theory of the Topophone, which, however, depends for its usefulness upon the same note being sounded by all horns and whistles. One note must be better than all the others, and that note, probably C (treble), caused by about two hundred and sixty vibrations per second, has been found most applicable. If all whistles and horns can by law be compelled to adjust themselves to this note, then the Topophone will be a real and lasting benefit.

Let us now look at the apparatus itself.

It being conceded that the resonators are in the same key as the Foghorn,—and this is necessary,—they are placed upon the deck of the vessel. An ear-tube of indiarubber is carried from each of these “resonators” into the cabin. These tubes unite and again separate, ending in small pieces ready to be fitted to the ears. The apparatus is fixed on deck, and the arrangement which supports it passes into the cabin, and can be turned about in any direction. Of course in this case a dial point is necessary to indicate the direction in which the instrument is turned. If the machine be worn on the shoulders of the officer of the watch he can move as he pleases, and wants no indicator.

The Topophone when used is so constructed, that when a horn is heard, and when the listener is facing the sound, he can hear nothing! When not facing the origin of the sound he can hear the horn very well, but the moment the resonators receive the sound together as they face the source, a very low murmur is heard, or perhaps no sound at all.—Why?

A certain pitch of tone is composed of vibrations or waves of equal length. In all waves there is a hollow and a crest. One neutralizes the other. The hollow of a sound wave meeting the crest of another wave “interferes” to produce silence, stillness, a dead level. So in “light”; two rays will produce darkness. We will endeavour to explain this.

If we have two equal strings, each performing an equal number of[Pg 182] vibrations in a second, they will produce equal sound waves, and the sound produced by both together will be uninterrupted, and twice as loud as one of them. But if one string vibrate, say one hundred times, and the other one hundred and one times in a second, they will not be in unison, and one will gain upon the other string, till after it has got to fifty vibrations it will be half a vibration ahead. At that moment they will neutralize each other, and silence will ensue for an appreciable time.

Fig. 185.—The Megaphone.

In the case of light suppose a red ray strikes the eye, and another red ray to come upon it from somewhere else. If the difference between its distance and the other point from the spot in the retina on which the first ray fell, is the 258/1000 part of an inch, or exactly twice, thrice, four times as much, etc., that distance, the light will be seen twice as strong. But if the difference in the distances between the points whence the light comes be only one-half the 258/1000 part of an inch, or 1½, 2½, 3½, or 4½ times that distance, one light will extinguish the other, and darkness will be the result. Now this is precisely what happens in the case of the Topophone. To return to our simile of water waves. If two stones be cast into a pond, and two equal and similar waves produced, and if they reach a certain place at the same moment, they will make one large wave. But if[Pg 183] one followed the other a little, so that the hollow of one coincided with the crest of the other, and vice versâ, the waves would obliterate each other, and a dead level would result. One tube of the Topophone is half a wave length longer than the other, and when the resonators are in a line and receive the wave at the same time, one ear hears the elevation of the sound wave, and the other the depression,—the sound is neutralized, and comparative, if not actual, silence results. The sailor knows in what direction the land lies, and can calculate his distance, or anchor if he please.

If amongst our readers there be any who wish to make for themselves an acoustic signalling apparatus there is physically nothing to prevent them from constructing such an instrument as that shown in the annexed woodcut (fig. 185). It is founded upon the speaking-trumpet principle, which is supposed to have been originated by Samuel Markland, in 1670.

Kircher, in his “Ars magna et umbra,” and in his “Phonurgia,” mentions a kind of speaking-trumpet, or porte voix, of gigantic dimensions, and called the “Horn of Alexander.” According to Kircher, the instrument was used by Alexander the Great to summon his soldiers from a distance of ten miles. The diameter of the circumference was about eight feet, and Kircher conjectured that the instrument was mounted upon three supports.

During the last century, a German professor, named Huth, made a model of the horn, and found it answered every purpose of a speaking-trumpet with most powerful results, but we beg leave to doubt whether the instrument really carried the voice to any very great distance.

The Acoustic Cornet, which is the counterpart of the speaking-trumpet, has been made in many different forms during the last two centuries, but none of them to the present time consist of anything more intricate than a simple tube with a mouthpiece and bell-shaped orifice.

Professor Edison, however, in his researches regarding the conveyance of sounds, has made numerous and interesting experiments. On one occasion, with his Megaphone he carried on a conversation at a distance of nearly two miles, without any other assistance from instruments except a few little cornets of cardboard. These constitute the Megaphone, which may be regarded as a curiosity, considering the effects produced by such simple means. The illustration (fig. 185) represents the instrument which is (or was lately) fixed upon the balcony of Mr. Edison’s house. At a mile-and-a-half distant from the house, at a spot indicated by the two birds in the picture, another instrument was fixed, and conversation was carried on with ease.

Perhaps the present opportunity will be the most convenient to speak of the Autophone, although it is more a musical than an acoustic instrument. Until lately Barbary organs and piano organs have been the only means by which poor people have been able to hear any music, and that not of a very elevated class. Besides, there is a good deal of expense connected with the possession of an organ. But the Americans, with a view to popularize music, have invented the Autophone, which is simply a[Pg 184] mechanical accordeon, manufactured by the Autophone Company, of Ithaca, New York.

The principle of the instrument is represented in fig. 186, and is extremely simple. An upright frame carries within it on one side a bellows, and on the other a flexible air chamber, which serves as a reservoir.

The upper portion contains a set of stops like an accordeon, but the escape of the air through the small vibrating plates can only take place by the upper surface of the frame work, upon which slides a thin plate of Bristol board pierced with holes at convenient distances, and set in motion by the mechanism shown in the annexed diagram (fig. 187).

Fig. 186.—The Autophone.

The figure represents an axle furnished with a series of “washers,” which, acting upon the plate, cause it to move round. It is the bellows movement that turns the axle by the aid of two “catches,” B and C, which work upon a toothed wheel fixed upon it.

The “catch” B moves the paper on which the tune is “perforated,” when the bellows is empty, the other catch when it is distended; but a counter catch, D, represented by the dotted lines in the illustration, is so arranged that the paper cannot pass on except the tooth of the catch D is opposite a hole pierced upon the plate above. In the contrary case there is no movement of the paper during the dilatation of the bellows. The effect of this very ingenious arrangement is to give to the “musical” band of “board” an irregular movement, but it economises it in the case of sustained notes. The whole action of the instrument depends upon the correct working of the bellows.

The effect, from an artistic point of view, certainly leaves something to be desired, but the instrument is cheap, and not cumbersome, and the[Pg 185] slips of paper upon which the music is “cut out” can be made by machinery, and consequently are not dear. So far, the Autophone is fitted for popular favour and use, and may supersede the barrel organ.

Fig. 187.—Detail of the Autophone.

The Audiphone is an instrument to conduct sound to the ear, to supplement it when temporary or partial deafness has occurred. Very likely many of our readers have observed ladies carrying large black fans on occasions,—at church, or lecture, or theatre,—and wondered why, perhaps. Those “fans” are Audiphones. The instrument is made of vulcanized rubber, and consists of a long flexible disc supported by a handle. To the upper edge of the “fan” are attached cords, which pass through a clip on the handle. If the person who wishes to hear by means of the Audiphone will hold the fan against the upper teeth,—the convex side of the fan outward,—he or she will hear distinctly, for the vibrations of sound are collected and strike upon the teeth and bones, and act upon the auditory nerves from within, precisely as the vibrations act from without through the auricle. We need hardly add that if the ear be injured the Audiphone will be of no use. A writer says: “From personal observation with the Audiphone it appears to convey the sonorous vibrations to the ear through the teeth, just as a long wooden rod held in the teeth will convey the vibrations of the sounding-board of a piano, though the piano is in another room and out of hearing by the ear. In using the Audiphone during conversation there is no movement or vibration felt by the teeth; in listening to a piano there is a very faint sensation as if the Audiphone vibrated slightly, while with the handle of the Audiphone resting on the sounding-board of the piano the vibrations are so violent as to be painful to the teeth. By closing the ears a person with even acute hearing can observe the admirable manner in which the instrument conveys spoken words to the ear. The Audiphone will prove to be of great value to deaf mutes, as it enables them to hear their own voices, and thus to train them to express words, while, before, they could only make inarticulate sounds.”

[Pg 186]

We have a variation of this instrument which has been introduced employing a diaphragm held in a telephone mouthpiece, and free to vibrate under the influence of sounds. This is connected by a string to a bit of wood that may be held in the teeth. In use the hearer places the wood between his teeth, the string is drawn tight, and the speaker speaks through the telephone mouthpiece, the vibrations of the diaphragm being then conveyed to the teeth through the stretched string. This apparatus works very successfully, and ladies use it, but it is not so convenient for general use as the Audiphone.

Fig. 188.—The Telephone.

The Telephone is now in daily use in London, and is by no means strange to the majority of our countrymen, still some description of it will probably be acceptable, and a glance at its history may prove interesting.

Fig. 189.—The “receiving” apparatus.

In speaking of the Telephone, we must not lose sight of the facts before mentioned, as regards our sense of hearing, and the manner in which the ear acts by the series of bones termed the hammer, the anvil, and stirrup. In the process of reproduction of tone in the magnetic instruments, the mechanism of the human ear was, to a certain extent, imitated, and a diaphragm, by vibrations, generates and controls an electric current.

Professor Wheatstone was the first person to employ the electric wire for the transmission of sounds, but Professor Philip Reiss, of Friedrichsdorf, was the first to make the experiment of producing musical sounds at a distance. His[Pg 187] first instrument was of a most primitive nature; subsequently he produced an instrument of which fig. 188 is the Telephone, fig. 189 the “receiver.”

In fig. 188, it will be seen that there is an aperture on the top and one at the side; the latter is the mouthpiece. The top aperture is covered with a membrane which is stretched very tightly. When a person speaks or sings into the mouthpiece his voice is at once concentrated upon the tight membrane, which it causes to vibrate in a manner corresponding with the vibrations of the voice. There are two binding screws, one at each side. To the centre of the tight membrane a piece of platinum is fixed, and this is connected with the binding screw on one side, in which a wire from the battery is fixed. On the membrane is a tripod, the feet of which (two) rest in metal cups, one of them being in a mercury cup connected with the binding screw at the opposite side to that already mentioned. The third “foot”—a platinum point—is on the platinum in the centre of the membrane or top, and moves with it. Every time the membrane is stretched by a vibration the platinum point is touched, and the closed circuit is broken by the return of each vibration.

Fig. 190.—Bell’s first Telephone (Transmitter).
a. Electro-magnet. b. Diaphragm. c. Collar. d. Collar and tube. f. Screw. g. Mouthpiece. h. Battery. i. Wire from battery to coil. k. Telegraph wire. l. Through binding screw. m. Pillar holding magnet.

The receiving instrument (fig. 189) consists of a coil enclosing an iron rod, and fixed upon a hollow sounding box. It is founded upon a fact discovered by Professor Henry, that iron bars when magnetized by an electric current become a little longer, and at the interruption of the current resume their former length. Thus in the receiver the iron will become alternately longer and shorter in accordance with the vibrations of the membrane in the box far away, and so the longitudinal vibrations of the bar of iron will be communicated to the sounding box, and become perfectly audible. This instrument, however, could only produce the “pitch” of sound, “not different degrees of intensity, or other qualities of tones.” It merely sang[Pg 188] with its own little trumpet whatever was sung into it; for all the waves were produced by an electric current of a certain and uniform strength, and therefore the sound waves were of the same size.

But in 1874, Mr. Elisha Gray, of Chicago, improved Reiss’ instrument, and discovered a method by which the intensity or loudness of tones, as well as their “pitch,” were transmitted and reproduced. In this method he employed electrical vibrations of varying strength and rapidity, and so was enabled to reproduce a tune. Subsequently he conceived the notion of controlling the vibrations by means of a diaphragm, which responded to every known sound, and by this he managed to transmit speech in an articulate manner.

Fig. 191.—Bell’s Telephone (Receiver).

In 1876, Professor Graham Bell sent a Telephone to the Centennial Exhibition at Philadelphia. Mr. Bell, according to the report, managed to produce a variation of strength of current in exact proportion to the particle of air moved by the sound. A piece of iron attached to a membrane, and moved to and fro in proximity to an electro magnet, proved successful. The battery and wire of the electro magnet are in circuit with the telegraph wire, and the wire of another electro magnet at the receiving station. This second magnet has a solid bar of iron for core, which is connected at one end, by a thick disc of iron, to an iron tube surrounding the coil and bar. The free circular end of the tube constitutes one pole of the electro magnet, and the adjacent free end of the bar core the other. A thin circular iron disc held pressed against the end of the tube by the electro-magnetic attraction, and free to vibrate through a very small space without touching the central pole, constitutes the sounder by which the electric effect is reconverted into sound. The accompanying illustrations (figs. 190, 191) show Mr. Bell’s Telephone as described.

The Telephone, subsequently simplified by Professor Bell, is shown in the two following illustrations (figs. 192, 193). The voice strikes against the[Pg 189] diaphragm, and it begins to vibrate. The sound is not conveyed by the wire; the motion is communicated, and the vibrations become sound waves again. The Telephone consists of a cylindrical magnet encircled at one end by a bobbin, on which is wound a quantity of fine insulated copper wire. The magnet and coil are contained in a wooden case, the ends of the coil being soldered to thick copper wire, which traverse the “wooden envelope,” and terminate in the binding screws. In front of the magnet is a thin circular iron plate, in which is the mouthpiece. The drawings will explain the instrument.

Fig. 192.—External appearance of Bell Telephone.

Fig. 193.—a. Bobbin of coil wire round magnet. b. Diaphragm. c. Mouthpiece. d. Permanent magnet. e. Wires to binding screws. f. Both wires as one for convenience. g. Adjusting screw-holding magnet.

Mr. Edison also invented a Telephone like Gray’s, and made the discovery, that when properly prepared, carbon would change its resistance with pressure, and that the ratio of these changes corresponded with the pressure. This solved the problem of the production of speech. The carbon is placed between two plates of platinum connected in the circuit and near the diaphragm, and the carbon receives the pressure from it by means of the mouthpiece.

When we come to Magnetism and Electricity we may have some[Pg 190]thing more to say respecting the mysteries of the Telephone and its later developments. At present we are only concerned with it as a sound conveyer, and it answers its purpose admirably, although somewhat liable to attract other sounds or vibrations from neighbouring wires.

The Phonograph, a mechanical invention of Mr. Edison, does not make use of electricity, although the vibratory motion of the diaphragm is utilized. It, in a simple form, consists of a diaphragm so arranged as to operate upon a small stylus, placed just opposite and below the centre, and a brass cylinder, six or eight inches long, by three or four in diameter, mounted upon a horizontal axis, extending each way beyond its ends for a distance about its own length.

“A spiral groove is cut in the circumference of the cylinder, from one end to the other, each spiral of the groove being separated from its neighbour by about one-tenth of an inch. The shaft or axis is also cut by a screw thread corresponding to the spiral groove of the cylinder, and works in screw bearings; consequently when the cylinder is caused to revolve, by means of a crank that is fitted to the axis for this purpose, it receives a forward or backward movement of about one-tenth of an inch for every turn of the same, the direction, of course, depending upon the way the crank is turned. The diaphragm is firmly supported by an upright casting capable of adjustment, and so arranged that it may be removed altogether when necessary. When in use, however, it is clamped in a fixed position above or in front of the cylinder, thus bringing the stylus always opposite the groove as the cylinder is turned. A small, flat spring attached to the casting extends underneath the diaphragm as far as its centre and carries the stylus, and between the diaphragm and spring a small piece of india-rubber is placed to modify the action, it having been found that better results are obtained by this means than when the stylus is rigidly attached to the diaphragm itself.

Fig. 194.—Mode of using the Telephone.

[Pg 191]

“The action of the apparatus will now be readily understood from what follows. The cylinder is first very smoothly covered with tin-foil, and the diaphragm securely fastened in place by clamping its support to the base of the instrument. When this has been properly done, the stylus should lightly press against that part of the foil over the groove. The crank is now turned, while, at the same time, someone speaks into the mouthpiece of the instrument, which will cause the diaphragm to vibrate, and as the vibrations of the latter correspond with the movements of the air producing them, the soft and yielding foil will become marked along the line of the groove by a series of indentations of different depths, varying with the amplitude of the vibrations of the diaphragm; or in other words, with the inflections or modulations of the speaker’s voice. These inflections may therefore be looked upon as a sort of visible speech, which, in fact, they really are. If now the diaphragm is removed, by loosening the clamp, and the cylinder then turned back to the starting point, we have only to replace the diaphragm and turn in the same direction as at first, to hear repeated all that has been spoken into the mouthpiece of the apparatus; the stylus, by this means, being caused to traverse its former path, and consequently, rising and falling with the depressions in the foil, its motion is communicated to the diaphragm, and thence through the intervening air to the ear, where the sensation of sound is produced.

Fig. 195.—Bell’s long-distance Telephone
a. Compound magnet. d. Diaphragm. e. Speaking tube. f. Telegraph wire. g. Line to earth. b, c. Small spaces.

“As the faithful reproduction of a sound is in reality nothing more than a reproduction of similar acoustic vibrations in a given time, it at once becomes evident that the cylinder should be made to revolve with absolute uniformity at all times, otherwise a difference more or less marked between the original sound and the reproduction will become manifest. To secure this uniformity of motion, and produce a practically working machine for automatically recording speeches, vocal and instrumental music, and per[Pg 192]fectly reproducing the same, the inventor devised an apparatus in which a plate replaces the cylinder. This plate, which is ten inches in diameter, has a volute spiral groove cut in its surface on both sides from its centre to within one inch of its outer edge; an arm guided by the spiral upon the under side of the plate carries a diaphragm and mouthpiece at its extreme end. If the arm be placed near the centre of the plate and the latter rotated, the motion will cause the arm to follow the spiral outward to the edge. A spring and train of wheel-work regulated by a friction governor serves to give uniform motion to the plate. The sheet upon which the record is made is of tin-foil. This is fastened to a paper frame, made by cutting a nine-inch disc from a square piece of paper of the same dimensions as the plate. Four pins upon the plate pass through corresponding eyelet-holes punched in the four corners of the paper, when the latter is laid upon it, and thus secure accurate registration, while a clamping-frame hinged to the plate fastens the foil and its paper frame securely to the latter. The mechanism is so arranged that the plate may be started and stopped instantly, or its motion reversed at will, thus giving the greatest convenience to both speaker and copyist.

“The articulation and quality of the Phonograph, although not yet perfect, is full as good as the Telephone was. The instrument, when perfected and moved by clock-work, will undoubtedly reproduce every condition of the human voice, including the whole world of expression in speech and song, and will be used universally.

“The sheet of tin-foil or other plastic material receiving the impressions of sound, will be stereotyped or electrotyped so as to be multiplied and made durable; or the cylinder will be made of a material plastic when used, and hardening afterward. Thin sheets of papier maché, or of various substances which soften by heat, would be of this character. Having provided thus for the durability of the Phonograph plate, it will be very easy to make it separable from the cylinder producing it, and attachable to a corresponding cylinder anywhere and at any time. There will doubtless be a standard of diameter and pitch of screw for Phonograph cylinders. Friends at a distance will then send to each other Phonograph letters, which will talk at any time in the friend’s voice when put upon the instrument.” (Scribner.)

The Microphone (an outcome of the Telephone) was discovered by Professor Hughes, of London. It is an instrument which in its main features consists of a carbon “pencil,” so suspended that one end rests upon a carbon “die.” The instrument being connected with a Telephone by the circuit wires, will reproduce faint sounds very distinctly. Once a Microphone was put into a preacher’s pulpit, and joined to a private telegraph wire which led to a gentleman’s house. The owner was thus enabled to hear the sermon. So long as it is thus connected every minute sound, even a fly’s footstep, will be faithfully reproduced.

[Pg 193]

CHAPTER XVII.

THE TUNING-FORK—THE SYREN—SOUND FIGURES—SINGING FLAMES.

We cannot close the subject of Sound without some mention of the Musical Pitch, and various instruments and experiments which have from time to time been made to discover the pitch, sound, and vibrations, and even to see Sound. To understand the vibrations or “pitch” of a musical note we may study the illustration, which shows us a tuning-fork in vibration.

You will perceive that each prong of the tuning-fork beats the air in an opposite direction at the same time, say from a to b (fig. 196). The prong strikes the air, and the wave thus created strikes again outward, and the condensation thus created travels along the back beat, rarefying the air, and both these, the rarefaction and the condensation, move with the same rapidity one behind the other.

Fig. 196. and Fig. 197.

The tuning-fork of course vibrates a very great many times in a second, every vibration generating a wave. “Pitch,” in a general sense, is the number of vibrations per second which constitute a note. For instance, the note A, the standard pitch consists of four hundred and thirty-five complete vibrations per second. Concert pitch is slightly higher, for there are a few more vibrations in the second. The lowest sound pitch is forty vibrations, the highest forty thousand. “Pitch” may be determined by an instrument termed the “Syren,” or by a tooth-wheeled apparatus.

[Pg 194]

The Syren was invented by Cagniard de Latour. It consists of a metal cylinder, a tube passes through the bottom, and through the tube air is blown into the cylinder. On the top a number of holes are drilled, while just over the cylinder top, almost in contact with it, is a metallic disc, which rotates upon a vertical axis. The disc is perforated with holes equal in number to those in the cylinder top, but the holes are not perpendicular, they slope in opposite directions. So when the air is forced through the holes in the top of the cylinder it impinges upon one side of the holes in the rotating disc, and blows it round.

The disc in one revolution will therefore open and shut as many holes as there are in the disc and cylinder, and the air blown in will escape in so many puffs—the number of puffs in a given time depending upon the rapidity of rotation. There is an arrangement to show the number of turns. By these rotations a sound is produced which rises in pitch as the revolutions are increased in number.

Fig. 198.   Sound Figures.   Fig. 199.

To determine the pitch of a certain sound we must find the number of times the plate revolves in that time, then we shall have the number of vibrations per second required to produce the note we desire. The arrangement working in a notched wheel tells us the number of rotations of the disc. Successive, and rapidly-successive puffs or beats are heard as the rotation increases, and at length the two sounds will disappear, and merge into one, which is perhaps that of the tuning-fork, whose note you require to find the “pitch” of. By maintaining this rate for a minute or less, and setting the gear to tell the revolutions, the number will be found marked on the dial of the apparatus. So by multiplying the number of revolutions of the disc by the number of the holes, and dividing the product by the number of seconds during which the disc was in connection with the recording gear, we shall have the number of vibrations per second necessary to produce the pitch corresponding to the given sound. The above is the description of one form of Syren; there are others, which, however, we need not detail.

We have seen that there are certain nodal points, or resting-places, in vibrations, and this can easily be shown upon a fiddle-string, from which paper discs will fall off except on the nodal point, showing that there is no[Pg 195] vibration there. The same experiment may be made by means of plates, which will give us what are termed Chladni’s figures. Suppose we strew a glass-plate with fine sand, and stroke the edge with a fiddle-bow. The vibrations of the plates will make certain patterns, and cast the sand upon those points of repose to form nodal lines in various directions. The plates must, of course, be held or fastened, and a variety of sound figures may be produced. (See figs. 198 and 199.)

The relation between the number of segments on the plate and the pitch of the note, can be ascertained by using a circular plate clamped in the centre. “If the finger on the plate and the fiddle-bow are one-eighth of the circumference apart, the fundamental note will be produced. If one-sixteenth apart, the higher octave will be heard.”

Sensitive flames will detect air vibrations, and flames can also be made to sing. Sensitive flames were discovered by Mr. Barrett, who noticed the effect a shrill note had upon a gas flame from a tapering jet. The flame was a very long one (fourteen inches), and when the sound was produced it shortened at once, while the upper part expanded like a fan; the same effects, in a less marked degree, were observable when the shrill sound was prolonged from a distance of forty feet. Professor Tyndall was immediately interested in this discovery, and in January 1867 he lectured upon it at the Royal Institution.

If any one wish to try the experiment, a piece of glass tubing should be obtained, and let the mouth be tapered down to a small orifice one-sixteenth of an inch in diameter. Then when the highest pressure is on for the evening, light the gas and sound a shrill whistle. The flame will sink down and spread out. The illuminating power may thus be increased, and many experiments may be made. For instance, if a person be in the room and try to read, he will probably not be able to do so at a little distance; but if his friend whistle to the gas it will so expand itself as to enable him to read, so long as the whistle lasts.

A very ingenious burglar-detector was made upon the principle of the sensitive flame, which expands at a noise and heats a welded plate of gold, silver, and platinum. The plate swerves aside, the metals being unequally affected by heat, and as it is connected with a battery, rings a bell by electricity. A small high flame has been made sensitive to the chinking of coin, or even to the ticking of a watch. We will now give some explanation, derived partly from Professor Tyndall, of the cause of sensitive flames.

A sensitive flame is one just on the point of “roaring,” and about to change its aspect. “It stands,” says Tyndall, “on the edge of a precipice. The proper sound pushes it over.... We bring it to the verge of falling, and the sonorous pulses precipitate what was already imminent.” The flame is in a state of vibration, so sounds being vibrations, practically increase the pressure; and the flame acknowledges the pressure thus invisibly applied by air waves.

Singing Flames are produced by burning hydrogen in a tube; a[Pg 196] musical note is thus produced in the same way as the air causes a note in an organ pipe. Faraday attributed the sound to rapid vibration caused by successive explosions of the burning gas. The Gas Harmonicon has been made on this principle. The air, being heated in the glass tube, ascends, and the flame is thus permitted to come up more forcibly in the tube; so violent agitation results when the air tries to get into the opening above. The size of the flame and its position in the tube will give a certain note which will be the same note as the air would emit if in a pipe, for the vibrations give the sound.

Sir Charles Wheatstone has shown by experiment how sound can be transmitted by placing a rod on a musical-box, and carrying the rod through the ceiling. When a guitar or violin was placed upon the rod, the sounds of the musical-box were distinctly heard in the upper room. A Phantom Band can be made by connecting certain instruments with others being played on under the stage. Every one will then appear to play by itself.

[Pg 197]

CHAPTER XVIII
ELECTRICITY.

DERIVATION OF ELECTRICITY—SEALING-WAX EXPERIMENT—THE ELECTROPHORUS—LEYDEN JAR—POSITIVE AND NEGATIVE—THE ELECTROSCOPE—ELECTRIC MACHINES.

We have now briefly and of course imperfectly reviewed the phenomena of Vibration, as exemplified in what we term Heat, Light, and Sound. We now come to a most mysterious servant of mankind, as mysterious as any Djinn of romance; viz., Electricity.

The term Electricity is derived from the Greek word electron, meaning “amber”; because from amber the properties of what we call “Electricity” were first discovered. Six hundred years before the Christian Era, Thales wrote concerning the attraction which amber, when rubbed, possessed for light and dry bodies. But it is to an Englishman named Gilbert that we owe the word “Electricity,” which he derived from the Greek, and in his works (about 1600 A.D.) he discusses the force of the so-called “fluid.” Otto von Guerike, of “air-pump” celebrity, and many other philosophers after him, continued the investigation of the subject. At the beginning of the last century great attention was paid to the Electric Machine. The Leyden Jar was, as its name denotes, discovered by Muschenbrock, of Leyden, (though the honour was disputed). Franklin made the first lightning conductor in 1760. Volta and Galvani, to whose invention we owe “Voltaic Electricity” and “Galvanism,” and Faraday in more modern times gave a great impetus to electrical science. The great part that electricity has been playing in the domestic history of the world since Faraday’s lamented death, is probably known to the youngest of our readers. What the future of this agent may be we can only guess, but even now we may regard electricity as only in its infancy.

There are few scientific studies more attractive to the general reader than electricity, and few admit of more popular demonstration. The success of the late electrical exhibition in Paris, and its successor in London at the present time, are proofs of the interest taken in this great and mysterious agent whose origin we are in ignorance of, and of whose nature and powers we are daily discovering more and more, and finding there is still an immense field for its application.

Some fundamental facts regarding electricity may very easily be studied with the assistance of every-day objects at hand. Amber was the[Pg 198] first substance to show attraction when rubbed, but Gilbert found out that glass and sealing-wax, etc., possessed like properties with amber.

If we rub a stick of sealing-wax with a piece of cloth, we shall see that it will attract some small fragments of paper placed near it. Nothing is easier than to construct a small pendulum to show with perfect clearness the phenomenon of electric attraction. A piece of iron is fixed on a wooden pedestal, and supports a thread of silk, to the end of which is fastened a little ball cut out of a piece of cork. The stick of sealing-wax after being rubbed with the cloth will attract the ball as shown in fig. 200.

Fig. 200.—Sealing-wax attracting a piece of cork.

By means of a piece of paper we can produce a spark. I take a large, strong sheet of drawing paper, heat it very thoroughly, and lay it on a wooden table. I rub it with a perfectly dry hand, or with a piece of woollen material until it adheres to the table. That done, I place a bunch of keys in the centre of the sheet of paper, which I raise, lifting it by two corners. If at this moment any one touches the bunch of keys with his finger, a bright spark will be elicited. The metal is charged with the electricity developed on the paper; if the weather is dry, and the paper thoroughly heated several times, the spark may attain nearly an inch in length.

We can easily construct other electrical apparatus. For instance, an “Electrophorus,” or instrument for obtaining electricity by means of induction, or a Leyden jar, can both be made at home. Let us proceed to construct the former, of which we give an illustration (fig. 201).

[Pg 199]

We take a lacquered tea-tray about a foot long, and cut out a sheet of thick wrapping paper, so that it will lie over all the level portion of the tray. At each side of this sheet of paper we fix two bands of paper, as in the illustration (fig. 201), so as to serve as handles. The tea-tray should be placed upon two tumblers to support it and to insulate it, glass being a “non-conductor.” (We will speak of conductors and non-conductors presently.) We have now our Electrophorus made ready for action; let us proceed to see how it will act.

Fig. 201—Simple Electrophorus.

First, rub the thick packing paper over a hot fire or a stove, and the friction must be continued for some time, until the paper has become thoroughly dry, and as hot as possible without charring. When this has been accomplished, place it quickly upon a wooden table, and rub it rapidly and energetically with a clothes’ brush, dry and hard as can be obtained. Place the paper upon the tray; touch the tray with the knuckle, and draw away the paper by the handles fixed to it (see fig. 201); a spark will result. Then if the paper be replaced upon the tray, and the hand again presented, the same result will follow. This experiment may be repeated five or six times, at least, with success.

We have in this tea-tray and its paper covering a real electric machine. How can we manage to provide a Leyden jar to contain our electricity? Nothing is more easy. Let us take a tumbler and partly fill it with shot;[Pg 200] insert into the glass a tea-spoon, and if all the articles are quite dry we shall possess a Leyden jar.

To charge the jar we have thus provided we must work the Electrophorus we have already described. While one person lifts off the paper as directed, another must hold the glass filled with shot close to the edge of the tray, and touch the corner with the tea-spoon; the spark will then enter the “jar” or tumbler. We can thus charge the jar as we please, and by presenting the finger as in the illustration (fig. 202), we shall obtain a discharge from it.

Fig. 202.—A Leyden jar.

Mr. Louis Figuier, in his “Merveilles de la Science,” relates that Wollaston, meeting one of his friends one evening in the streets of London, drew from his pocket a copper thimble, and proceeded to turn it into a microscopic pile.13

In order to do this he removed the bottom of the thimble, flattened it with a stone, so as to bring the two internal surfaces about on a line with each other, then placed between the copper surfaces a small strip of zinc, which was not in contact with the copper, owing to the interposition of a little sealing-wax. He then placed it in a glass cup, previously filled with the contents of a small phial of water, acidulated with sulphuric acid. He[Pg 201] next wound round the strip of zinc and its copper covering a piece of platinum wire, the wire becoming red through the electricity developed in the pile. The dimensions of this platinum wire were extremely small; it was only 30/1000 of an inch in diameter, and 1/30 of an inch in length. By reason of its small dimensions it could not only be reddened, but fused by the little battery.

Thus Wollaston’s friend, who was a witness of the experiment, was able to light a tinder at the red wire. In this little battery of Wollaston’s the copper enveloped the strip of zinc in every part; that is to say, the negative element was on a higher surface than the positive metal.

Fig. 203.-A simple compass.

After considering Electricity, it is not impossible to approach the study of Magnetism, and even to construct a mariner’s compass. We shall find the method of doing so by borrowing an interesting passage from the “Magasin Pittoresque.” Let us take a small cork and pass through it an ordinary knitting-needle (fig. 203), which we have already magnetized by placing it N.S., rubbing it gently, and always in the same direction, with one of those little iron magnets with which children amuse themselves. After the needle has been passed through the cork, we also fix into it a sewing-needle, or rather a pin, the point of which rests in one of the little holes in the upper part of the thimble. In order to balance the magnetised needle, we thrust a match into both sides of the cork, as shown in the illustration, and fasten to the ends of each a ball of wax. Thus the needle, the balls, and the pin are all balanced at once, so that the contrivance has the appearance of the illustration.

As it is very important that with such a sensitive instrument any agitation of the air should be avoided, the thimble must be placed at the bottom of a common earthen pan, B D T, which should be covered over with a piece of glass, V. To graduate the compass a circle is described on a piece of paper. On this dial we trace the divisions sufficiently close only at the north extremity of the needle, and the paper is fixed underneath, as in fig. 203. Then we fix a piece of wax at the end of the match pointing N., opposite the northern extremity of the needle inside the basin. In this way we have a very useful and inexpensive compass.

We may also magnetize a fine sewing-needle, and grease it by rubbing[Pg 202] it with a little suet. It is then capable of floating on the surface of water running in the direction of the north pole. We might go on multiplying indefinitely examples of physical experiments without apparatus, but we have probably already given a sufficient number to aid our readers in imagining others.

We have now in a simple manner shown how we can easily produce electricity. We may understand that electrical phenomena are produced—(1) friction between different bodies; (2) by placing bodies which differ in contact; (3) by the transition of bodies from one condition to another; (4) by chemical changes; (5) by animals. The two first, and the fourth, are the most usual causes.

We know that certain substances when rubbed with silk or wool acquire the property of attracting other substances. But in the case of a rod of glass or stick of wax, the attraction will only be perceived when the rubbing has been applied. But metal will behave differently. Any part of the metal rod will continue to attract. So metals are Conductors of electricity; while glass, wax, silk, amber, sulphur, etc., are bad, or Non-Conductors. Metals are the best conductors we have, but trees, plants, liquids, and the bodies of animals, including men, are all good conductors of electricity. Dry air is a bad conductor.

Fig. 204.—Attraction and repulsion.

There are two kinds of electricity, known as positive and negative (plus or minus), vitreous or resinous. We saw in fig. 200 that we can attract a small ball of pith or cork by a piece of sealing-wax rubbed with flannel. If we then present a glass rod rubbed with silk to it, it will be equally attracted, but will be at once repelled; and after being so repelled, if we put the wax to it, it will be attracted to the sealing-wax again. So wax at first attracts then repels the ball, and so does glass, but either will attract the ball if presented alternately (fig. 204). The reason for this is as follows :—

When we have rubbed the glass with silk, we charge it with positive electricity, and when the rod touches the ball, the latter imbibes that electricity, and flies away from the glass rod. The sealing-wax imparts negative electricity in the same way.

The law is, that bodies charged with the same kind of electricity repel each other, and those containing the opposite kinds attract each other. Positive repels positive; negative repels negative. But positive attracts negative, and negative attracts positive. Opposite electricities unite, and so neutralize each other that no effect is perceived; but it must be borne in mind that all bodies possess both electricities in some quantity, greater or less. By rubbing we separate these electricities, the rubber becoming negative, the rubbee positive. The friction of glass supplies positive electricity, and sealing-wax supplies negative electricity, or we can obtain the same effect by rubbing either with certain material.

The manner in which a body is electrified depends upon its nature[Pg 203] and condition; but we may accept as a general axiom,—but by no means as a law,—that when two bodies are rubbed together, that which gets the hotter in the process takes the negative kind of electricity. In the following list the substances have been so arranged that each is negatively electrified by those preceding, and positively by those succeeding it. 1, cat’s skin; 2, glass; 3, woollen stuffs; 4, feathers; 5, wood; 6, paper; 7, silk; 8, shellac; 9, rough glass. We append a list of conducting and non-conducting bodies in their order:—

Fig. 205.—Positive and negative.

Fig. 206.—The Battery.

It should be observed that the degree of value as a conductor or non-conductor depends somewhat upon the atmosphere. For instance, glass is an excellent insulator, or non-conductor, when dry, but when wet it changes to a conductor. So insulators are at times covered with a solution of shell-lac, or fat, to keep away moisture. We may reasonably conclude that bodies[Pg 204] which are good and bad conductors are good and bad conductors of electricity. Water is a good conductor, air is a bad one; were it otherwise, electricity would escape from the ground into the air; as it is, the air manages in some degree to retain the electricity at the surface of bodies, for it is on the surface that we find the electric “fluid.”

We have mentioned electrical induction in a former experiment with the tea-tray. We will now explain it more fully, as a consideration of it will bring us to the electric spark, or lightning, with the account of the discovery of the Conductor and the Electrical Machine.

Let us look at the illustration next below. A B is a cylinder supported on a glass rod, and at each extremity is a small pith ball, a and b. The cylinder is in a neutral condition, as is evidenced at first by the pellets being in a vertical position. But suppose we bring a ball, C, towards the cylinder. C is charged with positive electricity, which attracts the negative to itself, and so repels the positive away at the opposite side. So the pellet at one side will be attracted to C, and the other will fly in an opposite direction.

Fig. 208.—Induction.

Let us take another illustration. Here we have a horizontal metal rod, cc′, insulated on a glass stand. Two balls of cork are attached at both ends of the rod by metallic wires. Hold a rod of resin, r, which has been made negatively electrical, and apply it to one pair of the cork balls. The positive electricity will be attracted at c′ and the negative repulsed, and fly away at c. If we remove the resin the equilibrium will be again established, and the balls will fall to a vertical position.

Fig. 207.—Electrical induction.

We can also by drawing off the negative electricity by the finger at c, while the resin rod is still held to the other side, c′, fill the whole of the metal rod with the positive electricity when the finger and the resin have been removed respectively first and last. The balls will then fly in opposite directions again, in consequence of the repulsion exercised by the positive poles.

[Pg 205]

The “Electrophore,” or “Electrophorus,” we have already learned to make for ourselves, as also the Leyden Jar. But we give cuts of them. The former is very simple, and can be made by mixing two parts of shell-lac and one of turpentine, and pouring the mixture upon a metal plate. If this be rubbed with a cat’s skin when dry, and a metal cover with a glass handle be placed upon it, it will be found that the positive and negative electricity are collected on the lower and upper surfaces of the plate respectively, and can be drawn away with a spark as before, and made use of.

Fig. 209.—Electrophorus.

The Leyden Jar requires a little more detailed description, as it is to it we are indebted for our Battery. It is a common glass bottle or jar, coated both inside and out with tinfoil nearly as high as the shoulder, a a. The mouth should be firmly closed with a bung of wood, g g; a hole should be bored in the bung, through which a brass rod is tightly pushed. The rod, too, is topped by a brass knob, and a brass chain is attached to the other extremity. The interior of the tinfoil receives positive electricity, and the exterior negative when the jar is charged from the “Electrophorus.” To discharge the jar and create a shock it is necessary to put one hand on the outside, and the other on the knob of the jar. A brilliant spark and a severe shock will result if the jar has been fully charged. It is as well to be cautious when trying this experiment. The effect of the shock may be felt by any number of persons joining hands, if one at one end of the row, and one at the other end, touch the knob and the outside of the jar simultaneously.

Fig. 210.—The Leyden Jar.

This electric discharge is lightning in miniature, and it is to Benjamin Franklin that the world is indebted for the discovery. The philosopher was greatly interested in the science of Electricity and, having retired from business, he devoted himself to the consideration of thunderstorms. He wrote a treatise to show that points drew off electricity, and that electricity and lightning were similar. He urged that metallic rods might be attached to ships and buildings, so that during thunderstorms, or at other times, the electricity might be harmlessly carried into the ground. This suggestion he made without being able to explain why points did carry off electricity without a spark. The reason is because there is no place to store it; it runs away at once, without having time to collect, as in a ball.

Franklin made one or two experiments before his renowned kite-flying arrangement, which convinced him that electricity was by no means an agent to be played with. He endeavoured to kill a turkey by electricity, but by incautious handling of the jars in which the “fluid” was stored, he discharged them, and describes the result: “The flash was very great, and the crack was as loud as a pistol; yet my senses being instantly gone, I neither saw the one nor heard the other, nor did I feel the stroke on my hand, though I afterwards found it raised a round swelling where the fire entered[Pg 206] as big as half a pistol bullet.” On a subsequent occasion he was again struck senseless while endeavouring to administer a shock to a paralytic patient.

It was not until June 1752 that Franklin made the experiment with the kite, which resulted in such great discoveries. He made his kite of a silk pocket-handkerchief, and he fixed a pointed rod upon the upright portion of the frame at the top; the string ended in a foot or so of silk, which was held by the philosopher, and to the end of the hempen portion of the string a large key was tied. For some time, notwithstanding the approach of most unmistakable thunder-clouds, his patience was tried. But at last the strands of the hempen string began to bristle up, and soon after, when Franklin applied his knuckle to the key, a spark was obtained. The great discovery was made. Franklin subsequently obtained lightning in his own house, and performed several experiments with it.

Fig. 211.—The Electroscope.

The Electroscope (fig. 211) is an instrument by which we can ascertain whether electricity is present or not, and the nature of it. If we bring an object unelectrified close to the ball or knob on the top of the glass shade, the two needles, or strips of gold-leaf, which are often used, will remain still. But if the body has been electrified it will communicate the electricity to the rod inside, and attract to itself the fluid of opposite quality; the same kind of electricity then is in action in the gold-leaf or needles, and they fly apart—repel each other. Supposing that positive electricity were first communicated, we can cause the contraction of the leaves or wires by applying a negative kind, which, meeting the positive, neutralizes it, and the wires collapse.

If the electricity with which the instrument is charged be positive, by approaching the baton to the ball, A, we shall see the wires diverge more than before, and they will finally be discharged by the knobs within. If the electricity be contrary to that in the baton, the wires will approach each other, but by gradually withdrawing the baton they will again separate, and even to a greater distance than before.

The Electric Machine is shown in the illustration (fig. 212). It consists of a large plate of glass fixed upon a glass stand, between wooden supports. The handle is of glass; two pairs of rubbers are fastened above and below; the plate is turned between them, and becomes “positively” electrified. The rubbers are covered with leather and stuffed with horsehair, DD, and press very tightly against the glass, so that the friction is constant. The rubbers are covered with an amalgam made of mercury, zinc, and tin, two parts of the first to one each of the others. A chain (of metal) connects the machine with the ground. The conductors, PP, are united by a cross[Pg 207]-piece, Q, and sustained upon glass supports. At the end of the conductors are two curved rods, CC, which are provided with points to take the electricity from the plate, but do not touch it.

Fig. 212.—The Electric Machine.

The electricity is thus stored in the insulated conductors as the machine is turned. The negative portion is carried into the ground by the chain from the rubbers, while the positive electricity is retained. The longer we turn the more we shall obtain, and the quantity is measured by an electric pendulum on one of the conductors, which flies out by degrees as the charge increases, and indicates its power by means of a needle it works upon an ivory index.

It is not difficult to make an electric machine out of a glass bottle. This will furnish the glass cylinder. If a stick be run through it (for which purpose a hole must be drilled in the bottom of the bottle), a handle can be fixed, and the bottle mounted on a stand. A wash-leather cushion, stuffed, can be so arranged that it will press against the bottle as it is turned; a piece of silk should be permitted to hang from the cushion frame over the glass. A conductor may be made from a piece of wood neatly rounded and smoothed,[Pg 208] and coated with tinfoil. The ends should be rounded like “knobs.” Stick pins in to collect the electricity (and it will be readily obtained). The cushion should of course be well smeared with amalgam. From this, as well as from the glass-plate machine, the “positive” electricity can be drawn off and stored in a Leyden jar, and then discharged by the “discharging rod,” which is represented on the cut. It may have one or two handles, and one knob is placed outside the jar, the other near the ball surmounting it. The glass being a non-conductor saves the operator, and some long sparks and loud reports may be obtained.

Fig. 213.—Cylinder machine.

The Electric Machine is always assumed to give off positive electricity.

Sir William Armstrong’s Electric Machine is a mode of obtaining electricity by moist steam. The design is Armstrong’s, and Professor Faraday subsequently went into all the conditions to produce the “fluid” by the friction of steam. The machine was something like a small boiler supported on glass legs. A row of nozzles was fixed upon the escape pipe so as to create a great velocity and friction in the escaping steam. Round the nozzles was a box of cold water, for that fluid was found necessary for the production of electricity as demonstrated by Professor Faraday. The steam rushed against a row of points attached to the prime conductor of an electric machine, and the electricity of the steam was thus given off to the conductor. There are many other forms of electric machines, but it will serve no purpose to detail them.

Fig. 214.—Discharging rod.

The Electric Battery (see fig. 206) is formed by a collection of Leyden jars. The inside and outside coatings are connected in a box divided into partitions lined with tinfoil. The rods of the jars are also connected, as in the illustration, by brass rods, and when this battery is charged people should be careful how they handle it, for a shock may be produced which would[Pg 209] cause serious injury, if not death. The battery can be charged from the machine by a chain fastened to the central ball, while a second chain connects the exterior of the box and all the outside of the jars, by means of the handle, to the ground.

When electricity is at rest it is termed “static electricity,” and when in motion “dynamic” electricity. The latter treats of electric currents which can be sent through wires or chains. We can keep this current moving by means of a machine, and the battery called a Voltaic battery, from Volta. We will describe it presently. Electric currents can be measured, for they may be of different strengths according to the battery, and they are measured by the Galvanometer. Electricity can therefore be transferred and carried by the conducting substances, and much heat will be engendered as the “electric fluid” passes along a wire. Lightning frequently fuses bell-wires as it passes, and when we touch upon Galvanism or Dynamical Electricity we shall hear more about it.

By the Electric Machine we can obtain some very powerful currents of electricity; we can produce many pleasing effects, and perform a number of experiments, such as making balls or figures of pith dance, and several other easy and entertaining tricks, which will be found in books more specially devoted to the entertainment of young people.

Fig. 215.—Leyden Jar.

We have now given some explanation of the manner in which electrical phenomena can be produced,—viz., by the Electric Machine and by the Leyden Jar,—but we must not expect to find any electricity inside any charged body. It has been proved that all the electricity is upon the surface of bodies, even if in varying quantity, and that equal quantities of electricity are always produced when bodies are excited by friction, but the kinds are different. The rubbing body is of one kind, the body rubbed another, and consequently the forces neutralize each other. The two forces or kinds of electricity we have seen repel or attract each other, and we can imagine the farther they are apart the less will be the force, and the rate of diminution of force, according to distance, is ascertained by an ingenious apparatus called a “Torsion” Electromoter, which was constructed by Coulomb, and was frequently used by Faraday.

Perhaps some people may not be aware of the term “torsion.” It means twisting, and “the torsion of a thread suspended vertically is the force tending to twist the lower extremity when the upper end is turned through an angle.” This instrument is really an Electromoter, and is not considered suited to beginners, and it is scarcely accurate in its workings. We need not therefore describe it in detail. There are some excellent Electromoters, the Elliott being, we believe, the best for use. A full and detailed description of the Quadrant Electromoter will be found in Mr. Gordon’s treatise on Electricity.

[Pg 210]

Recapitulation of foregoing Chapter. So far, we have seen there is electricity in everything, although some bodies are termed conductors and others non-conductors; though, as in applying the terms heat and cold, we must remember that no body is entirely devoid of electricity, and no body is therefore an absolute non-conductor any more than any object is absolutely devoid of heat. Faraday, indeed, was of opinion that “conduction and insulation are only extreme degrees of one common condition”; they are identical both in principle and action, except that in conduction an effect common to both is raised to the highest degree, and in the case of insulation it occurs in an almost insensible quantity.

We have also read of positive and negative electricities, and we must not fancy there is any particular reason for this distinction. It was Du Fay, whom we have mentioned, who gave the names “vitreous” and “resinous” to the two kinds, as one was developed by rubbing glass, and the other by rubbing resin. But, as shown by our experiments, either kind of electricity can be excited in glass or sealing-wax, and both kinds are produced at once. You cannot get “positive” without negative electricity. “Positive” is the term applied to the kind produced by rubbing glass with silk or wool; “negative” is the term applied to the kind developed by rubbing sealing-wax, but the kind developed by friction depends on the rubbing substance and certain conditions.

Bodies charged with the same electricity repel; if charged with different kinds they attract each other. The more readily displaced particles when bodies are rubbed become negatively electrified as a rule.

Similar electricities repel each other with a force inversely proportional to the squares of the distance between their centres, as established by Coulomb. So if the space between any two similarly electrified bodies be reduced by say one-half, the force of the repulsion will be increased four times. The rule for attraction is similar; so when two bodies are charged with opposite electricities, and the distance between them is increased, the attractive force is diminished in proportion as the square of the distance between them. Many confirmations of this theory were made by the late friend of our boyhood, Sir W. Snow Harris, and published in the Philosophical Transactions.

The following full list of conductors and non-conductors (copied from Professor Noad’s Text-book of Electricity, and compared with De La Rive’s Treatise) may be useful:—

[Pg 211]

The following may be added to the Insulators, viz.:—

There are, as we know, two kinds of electricity, the static and dynamic; and when the latter state is instantaneous, it is referred to as the “electric discharge,” which occurs when opposite electricities seek each other, and the bodies return to a state of equilibrium or neutralization. “These bodies, if insulated, obtain no more electricity after the spark has passed; but if there be a constant source of negative electricity supplying one, and a constant source of negative electricity supplying the other, there will be a succession of sparks; and if they communicate by a conductor there will be, through this conductor, an uninterrupted neutralization of a continual reunion of the two electricities, and this is what is termed the continuous dynamic state or electric current” (De la Rive).

[Pg 212]

CHAPTER XIX.

VELOCITY OF ELECTRICITY—EXPERIMENTS—THE ELECTRIC EGG—FORCE OF THE ELECTRIC SPARK.

We are now acquainted with many facts concerning electricity, and have seen that electrical phenomena can be produced by the Electric Machine and the Leyden Jar. (An insulating stool—a stool with glass legs—is a very desirable adjunct for those who wish to experiment with the machine). Glass is a great insulator, or non-conductor, as a Russian philosopher found to his cost. He had an iron lightning-conductor from his house into his room, the end not connected with the earth but with a glass. One day the lightning came down the rod and reached the glass; had a communication been made with the earth by a chain, or directly, no mischief would have ensued. As it happened, however, the current was checked by the glass, and immediately darted towards him; it struck him in the head, and killed the poor man on the spot. If no insulating stool were used, the body charged would be discharged upon contact with the ground.

The velocity of electricity is very great, and experiments have frequently been made. Wheatstone undertook to ascertain the speed of the electric fluid, and the instrument he employed he called a “Chronoscope.” He caused a mirror to revolve with enormous velocity, and measured the speed by the vibrations of air, which produced a certain note by the same motive power. (We know already that certain notes are produced by a certain number of vibrations per second.) Wheatstone’s Chronoscope consisted of this mirror, in front of which was placed a circular block of wood, in which, in a row, were set six wires carrying small knobs; round these and over the wood he put an insulating varnish. A Leyden jar was connected outside with the first knob; between the second and third a quarter of a mile of copper wire was coiled, and a like length of wire between the fourth and fifth; the inside of the Leyden jar was then connected with the last knob, and the spark passed; it ran from knob to knob over the long coils of wire. If all the flashes over the wire and knobs occurred simultaneously the mirror would show them side by side; if not, as the mirror turned a trifle, the difference would be observed. The mirror did show a slight retardation in the passage of the flash, and from certain measurements and calculations Wheatstone estimated the velocity of the spark to be 288,000 miles a second.[Pg 213] This rate will carry electricity round the earth in about a twelfth of a second, a rate Puck never dreamed of when he promised to “put a girdle round the earth in forty minutes.”

But it appeared from investigations subsequently made that it was not possible to express the velocity of electricity with any certainty, and a number of experiments were made as per the following table, with very different results. Sir William Thomson and Faraday endeavoured to account for these stupendous discrepancies, and the principles of retardation of electricity were established. The differences are shown below:—

To account for the comparatively low velocities of the cables, Faraday proved that they act very much as a Leyden jar acts; that is, it takes time to fill, as it were, and to discharge them, the wire coating of the cable in air acting like the outer coating of the jar or the water in the case of an immersed cable, and the retardation observed is owing to resistance of conduction, and depends upon the way in which the electrical impulses traverse the wire. “There is a long, gradual swell, and still more gradual subsidence of the electric current, and the length of time that elapses between the initial impulse and the attainment of maximum strength, is proportional to the square of the length of the line.”

The duration of the electric spark has been calculated at the 1/24000 part of a second, but Professor Tyndall regards this as the longest or nearly the longest time it is perceptible; the shortest time is almost inconceivable. The brightest portion of a spark has been ascertained to last only forty-six millionths of a second, and certain experiments were made to ascertain the actual duration with various numbers of Leyden jars. It was discovered by Messrs. Lucas and Cuzin, by an application of the Vernier, with batteries consisting variously of two to eight jars, and obtained the following results14—

“So,” adds the writer, “the duration (of spark) increases in proportion with the number of the jars. It increases also with the striking distance, but[Pg 214] is independent of the diameter of the balls or globes between which the spark strikes.”

Many examples might be given of the spark discharge of the electric current. This form is seen in the blue line extending between the knob of a machine and the hand, and the duration of a spark with a jar charged with an induction coil is stated by Professor Rood to vary, but the brightest portion with a jar of 114 square inches only existed for the 175th billionth of a second, and with a smaller surface was much shorter. Such a spark may be conducted to a plate of gunpowder and will not ignite it, because the time of the duration of the “fire” is not sufficiently long; the powder will be scattered, but not ignited. If, however, a partly non-conducting medium be interposed between the jar and the powder, so that the spark be retarded a little, the gunpowder will be fired.

While speaking of electric discharge we may remark upon the beautiful effect of lightning. These discharges are sometimes miles long, and by the return stroke from the cloud may kill a person a long way from the actual discharge. This phenomenon was illustrated by Viscount Mahon in 1779, in a very interesting book on the principles of electricity.

There are different ways in which the electric discharge shows itself. We have spoken about the spark discharge which, however, is found to present very different appearances in varying conditions. Professor Faraday proved that the colour of the electric sparks showed in air, when obtained with brass balls, the intense light and blue colour so familiar to all. In nitrogen they are even bluer. In oxygen again the sparks are much brighter than in air, but not so brilliant. In hydrogen they become crimson, but the sound is almost inaudible because of the physical character of the gas. In carbonic acid gas they are almost the same as in atmospheric air, only more irregular. In dry muriatic gas they are nearly white and very bright. In coal gas the colours vary—sometimes being green and sometimes red. Occasionally the same spark will be red and green at different extremities, and even black portions have been observed. The density and pressure of the atmosphere has been proved to exercise considerable influence upon the spark discharge.

The “Brush” discharge is shown in “a series of intermittent discharges which appear continuous.” This discharge assumes the shape of a fan. “It is accompanied with a low chattering sound,” which is the result of the separate and continuous discharges, and Faraday also demonstrated that its effects varied according to the medium in which it was exhibited.

The effect of the air pressure on electricity may be observed in the following way:—

If we pass a spark through rarefied air by an apparatus known as the Electric Egg, we may obtain many curious effects. The “egg” consists of a glass globe, through which enter two rods with a knob upon each inside end. The upper rod is moveable, and held in its place by a “cap” like the lower rod. There is a stop-cock in the lower cap, so that the egg may be fastened to any plate or stand. When the egg is filled with air, the[Pg 215] electric spark passed into the glass globe has the usual appearance, but as the air is gradually rarefied by an air-pump, the spark assumes beautiful forms and colours. As the exhaustion continues, however, we shall find the spark decreasing in brilliancy, and finally the spark will cease to be visible. It thus is shown that the colour of the spark depends upon the gaseous medium and on the material of the conductors, and when the electric spark is faint this medium can be observed, for nitrogen will produce a blue tinge and carbonic acid a green; hydrogen gives us a red, as already remarked. By multiplying the number of eggs and plates of glass, and placing discs of tinfoil in various shapes at certain distances, many beautiful figures may be observed when the spark is set free.

Professor Tyndall at the Royal Institution showed a very pretty experiment. He took a funnel with a very fine bore, and permitted sand to flow from it as it will in the hour-glass. When he permitted the electric current to come in contact with the sand, however, it, instead of falling vertically to the table, spread out fan-like, each grain repelling and being repelled by its neighbour with an effect very beautiful to see. Luminous effects have frequently been produced by passing an electric spark through various bodies. For instance, a lump of sugar can be made quite brilliant in the dark by passing electricity through it, and there are other substances similarly affected. Even eggs and some fruits are thus made phosphorescent. The illumination of the “diamond” covered Leyden jar is familiar to all who ever attended a lecture on Electricity.

The various effects of the electric discharge need not here be described. We have witnessed the results of lightning, but even in our laboratory many pretty little experiments can be made, such as the perforation of a card by the electric discharge. The chemical effects are various. Decomposition of water is effected by electricity, and the discharge can also be, and has been utilized for military purposes, such as employed by Professor Abel in his fuse, and in his apparatus for firing mines. Experiments at Chatham and elsewhere have been very successful in the application of electricity to modern warfare.

We will illustrate one or two of these. A thick card should be placed, as in the illustration (fig. 216), between two insulated points, and to the lower portion of the apparatus a chain be attached, held in the hand, and wound round the Leyden jar. If then the knob of the jar and the knob above the upper point be brought together, the spark will pass through the card.

Fig. 216.—Experiment with card.

In the same manner a glass may be perforated if the current be stronger. Of course the whole apparatus—and particularly the plate—must be quite dry, and it will be better to put a drop of oil under the upper needle point so as to prevent the electricity spreading over the glass.[Pg 216] The glass will be found pierced by the electric spark when the Leyden jar is brought into requisition (fig. 217).

It was at one time contended that the sudden expansion of the air by the electric spark caused heat to be generated, and a thermometer was invented by Kinnersley to show this. The illustration (fig. 218) will explain it. When the electric spark passed between the balls in the large tube the water rose in the smaller. But the immediate return of the water to its level showed that the disturbance was only mechanical, and not owing to heat.

Fig. 217.—Experiment with glass.

Fig. 218.—Kinnersley’s thermometer.

We may now pass from the “frictional” to the other kind—viz., “dynamical” electricity, and we shall begin with the consideration of Galvani’s experiments and the Voltaic Pile.

Electric condenser.

[Pg 217]

CHAPTER XX.
GALVANISM.

GALVANI’S DISCOVERY—THE FROGS ELECTRIFIED—EXPERIMENTS—VOLTA’S PILE—THE TEST—ITS USEFULNESS—FARADAY’S “RESEARCHES.”

Galvanism owes its origin to the researches of Galvani, the celebrated professor of Bologna, and we are indebted to what was a mere “accident” for our knowledge of this science.

Before Galvani’s time there had been many instances adduced of animal electricity. The Rev. F. Lunn, in his article upon Electricity,15 mentions the fact that fire streamed from the head of Servius Tullius when about seven years of age, and Virgil we know refers to flame emitted by the hair of Ascanius—

“Lambere flamma comas, et circum tempora pasci”;

and if any one will comb his or her hair with an ebonite comb in the dark, with what is sometimes called an “india-rubber comb,” the hair will give out sufficient light to enable the operator to see himself in a looking-glass. In olden days it is related that a lady when touched with a linen cloth emitted sparks, and the same phenomenon was observable when a bookseller at Pisa removed his under-garment or vest (De Castro). We are all aware of the electricity of the cat and of certain fishes (see Electricity of Animals in sequel), and “torpedos.” Galvani had of course a knowledge of this property, and had occupied himself for some time making experiments upon the electricity in animals. He was not in his laboratory that day when the great discovery was made by means of the edible frog.

Galvani’s wife was just then in a very delicate state of health, and in accordance with usage had been ordered soup made from frogs. It is related that some of these animals, ready skinned, were lying upon the laboratory table, for the Professor had been just previously investigating the question of what he opined was “animal” electricity; that is, he fancied that muscular motion depended upon that subtle force.

The electric machine was in action, and one of the attendants happening to approach or touch one of the frogs, the man as well as Madame Galvani observed that the limbs were violently agitated. Galvani was at once informed of this, and he made repeated experiments, which showed him that the convulsive movements only took place when a spark was drawn from the[Pg 218] prime conductor of the electric machine, and while the nerve was touched by a conductor. Galvani then suspended a number of frogs to a railing by metal hooks, with a view to experiment upon them with atmospherical electricity. But the frogs’ limbs were again agitated when no electricity was apparent, and Galvani after some consideration came to the conclusion that the movement was owing to the position the animals assumed with reference to the metallic bodies. Thus when muscle and nerve were in contact with metallic bodies and connected by metal, the movements of the limbs were observable, and the greater the surface contact the greater was the convulsion. The philosopher next tried various metals, and discovered that the most powerful combination was zinc and silver.

Galvani, in 1791, published his discovery and his theory that the body acted as a Leyden jar, different parts being in a different state of electricity. No sooner were his deductions published than all Europe was in a ferment, and philosophers of all nations were discussing it. Fowler, Valli, Robison, Wells, Humboldt, etc., all were deeply interested, but none of them appear to have arrived at so correct conclusions as did Volta, the physician of Pavia. “Wherever frogs were to be found,” says Du Bois Reymond, “and where two different kinds of metal could be procured, everybody was anxious to see the mangled limbs of frogs brought to life in this wonderful way. Physiologists believed that at length they should realize their visions of a vital power, and physicians thought no cure was impossible.”

But notwithstanding the popular theory, Volta, in his letters to Carallo, while giving a full and clear account of the discovery made by Galvani and his own experiments, attacked and finally defeated the Professor. Volta quite upset Galvani’s Leyden-jar theory; Volta says that it was by accident that Mr. Galvani discovered the phenomenon, and by which he was more astonished than he ought to have been. Volta’s letters will be found in the Philosophical Transactions of the Royal Society (in French), and he attributes the effect to the metals which produced a small amount of electricity. He found that the nerve was acted upon on even parts of a muscle laid upon two different metals, and if those were united, a contraction took place.

“Many experiments were made in all parts of Europe,” says Doctor Roget, and “an opinion had been very prevalent that the real source of the power developed existed in the muscle and nerve which formed part of the circuit, and that the metals which composed the other part acted merely as the conductors by which that agency was transferred from the one to the other of these animal structures. But the discoveries of Volta dispelled the error, by proving that the sources of power were derived from the galvanic properties of the metals themselves when combined with certain fluids,” and decided that this principle was electricity. From this the “general fact” was deduced—viz., “that when a certain portion of a nerve which is distributed to any muscle is made part of a galvanic circuit, convulsions, generally of a violent and convulsive kind, are produced in that muscle.”

Volta at length made the discovery that when two metals were brought[Pg 219] together electricity was developed, and by uniting a disc of copper and one of zinc, and subjecting them to the test of an Electroscope, he found positive and negative electricity developed in the zinc and copper respectively; so Volta came to the conclusion that each metal parted with electricity, and one became all “positive” and the other all “negative.” But when he came further to consider the possibility of building up a “pile” of these metal discs sufficiently strong to produce electric effects, he found that if his theory were correct he would lose from one side of the metal all he would gain from the other, and therefore he could never obtain more than the slight effect he had originally produced.

This was at first a difficulty apparently impossible to remove. It was so self-evident that the discs of metal, if placed in a pile in a series of pairs, would continually exercise like effects to the first pair of discs, that Volta was puzzled, and for some time he could not arrive at any reasonable solution. At last it struck him that if he placed between the discs some slow-conducting substance, the electricity would not pass from disc to disc, and the force developed or set in motion would be more powerful.

He made the experiment. The result was the Voltaic pile made in 1800, of which we give an illustration (fig. 219). A communication on the subject of Electricity by contact, written by M. Volta, is to be found in the Philosophical Proceedings for the year 1800.

Fig. 219.—Voltaic Pile.

Volta constructed the pile which bears his name, on the assumption that “every two heterogeneous bodies form a galvanic circle or arc in which electricity is generated.” The “pile” consisted of a number of discs of zinc and copper separated by discs of card soaked in water. This combination of metals separated by a bad conductor, developed considerable electricity, the “positive” going to the zinc at the top, and the “negative” turning to the opposite end. By touching the zinc and copper extremities simultaneously with wetted fingers we shall experience a shock. “I don’t need your frog,” Volta said, when he had proved his theory; “give me two metals and a moist rag, and I will produce your animal electricity. Your frog is nothing but a moist conductor, and in this respect it is inferior to my wet rag!”

After this discovery the theory of animal electricity died away for many years, till in 1825, Nobili, and afterwards Matteucci, proved the existence of galvanic currents in muscles.

After Volta had succeeded in obtaining a shock from his “pile,” he proceeded to the construction of another instrument, or rather apparatus,[Pg 220] which he denominated “Couronne des tasses” (fig. 220). It consisted of a series of small glasses containing water or a saline solution. He then procured a number of “metallic arcs,” partly composed of zinc and partly of copper; these were inserted into the glasses, so that every glass contained the zinc of one and the copper of another arc, not in contact, but one at the right hand the other at the left. The electro motion, supposed to be the primary cause of the galvanic action, was thus produced as well as from the “pile.” The principle was just the same in both apparatus, the metals being divided by the water in one case, and by a wet card or cloth in the other.

Volta, in 1800, addressed to the Royal Society his celebrated letter upon electricity excited by contact of conducting substances, and then the English philosophers proceeded to make further experiments. It was Fabroni of Florence who had just before suggested that chemical action was really the cause of the phenomena exhibited. Sir Humphrey Davy warmly advocated this theory, and made numerous experiments with the view to establish it. Nicholson, Carlisle, and Cruickshank also paid great attention to the subject. Volta, although he had laid the foundation, did not venture to build upon it. Messrs. Nicholson and Carlisle found the two kinds of electricity in the pile, the zinc being positive and the silver negative. They also found that the water was decomposed both in the circuit and in the body of the pile. Subsequently Cruickshank confirmed Nicholson’s observations, and made use of what is termed the “trough” apparatus. He found that hydrogen was emitted from the silver or upper end, and oxygen from the other.

Fig. 220.—Volta’s couronne des tasses.

These discoveries opened up a wide field. “The power of the pile in decomposing chemical substances was now established.” Dr. Henry employed galvanism for analysis, and Sir Humphrey Davy invented new combinations of substances. He formed a pile of charcoal and zinc, and found out that a pile could consist of only one metal, different fluids being applied to the opposite surfaces separated by water, and one fluid “capable of oxidating the metal, the other of preventing the effect of oxidation.” Soon after a pile was made of charcoal.

In 1806, Sir H. Davy gave the results of his researches to the world upon the electro-chemical action of bodies. In the course of his experiments he found out the chemical constituents of the alkalies, and a surprising number of new things were brought to light, and chemical science received a most astonishing ally. Sir W. S. Harris says: “A series of new substances[Pg 221] were speedily discovered, the existence of which had never before been imagined. Oxygen, chlorine, and acids were all dragged, as it were, to the positive pole, while metals, inflammable bodies, alkalies, and earths became determined to the negative pole of the (galvanic) battery. When wires connected with each extremity of the new battery were tipped with prepared and well-pointed charcoal, and the points brought near each other, then a most intense and pure evolution of light followed, which on separating the points extended to a gorgeous arc.” So the elements of all bodies were separated and the composition of their compounds closely investigated.

Michael Faraday threw himself con amore into the question. He set about to classify the pile phenomena, and arranged them with appropriate terms, and in a series of papers, between the years 1830 and 1840 (see his “Experimental Researches”), he explained the chemical effects of voltaic electricity and electro-magnetic induction. He showed that the electricities obtainable from the voltaic pile and the electrical machine are essentially the same in their action. He proved that the theory held respecting the necessity for the presence of water in electro-chemical composition was erroneous, and that many other fluids and compounds were equally effective. We have not space at our disposal to include a digest of his various lectures and papers. He calculated that as much electricity is employed in holding the gases oxygen and hydrogen together in a grain of water, “as is present in a discharge of lightning.” When water is decomposed by the electric current, the force which determines the oxygen and acid matter held in solution to the positive, while the hydrogen passes to the negative pole, is not in the poles, but in the body decomposed, he says. “The poles,” writes Faraday, “are merely the surfaces or doors by which the electricity enters into or passes out of the decomposing substance. They limit the extent of that substance in the course of the electric current, being its termination in that direction. Hence the elements evolved passed so far and no farther.” Faraday named the poles “electrodes”—the way (in or out) of electricity.

Fig. 221.—The Galvanic Pile.

A very simple voltaic pile may be constructed with “gold-leaf” paper. Take two sheets of the gold paper and paste them back to back, and two of silver paper; cut them into discs about the size of a five-shilling-piece (or even of half-a-crown), and place them one on the top of the other, so as the gold and silver may be alternate; press the discs together slightly when a good many layers have been piled up, and introduce them into a glass tube; close the ends of the tubes with corks through which wires are passed from the discs top and bottom. It will be found that the ends are charged with opposite electricities. This is the Zamboni pile, or the dry pile, which was constructed of hundreds of paper discs “tinned on one side, and covered with binoxide of manganese on the other,” put into a tube, and closed with brass stoppers. The electricity will last a long time in a dry pile.

In the accompanying illustration of the Galvanic Pile a disc of copper[Pg 222] is at the bottom and a disc of zinc at the top. The latter, P, is the positive pole; the former, N, the negative. When the wires are united the current is closed, and no sign of disturbance can be detected, although the action, of course, is proceeding within the pile. The opposite kinds of electricity neutralize each other, and if a continuous supply were not kept up the electricity would disappear; but as it is, a powerful current is obtained, and if the wire be divided a spark will be observed.

Fig. 222.—Bunsen Battery.

There are many forms of galvanic batteries. The Trough Battery or Cruickshank has been mentioned. There is Wollaston’s Pile, Bunsen’s Battery, Grove’s Battery, and Daniell’s, called the “Constant” Battery. In this last a porous earthenware cell is placed within a cylinder of copper; in the cell a rod of zinc is inserted, the cell being filled with diluted sulphuric acid,—one part of acid to ten parts of water,—and in the outer cylinder is a solution of sulphate of copper.

Fig. 223.—Daniell’s Battery.

Fig. 224.—Grove’s Battery.

The cut above illustrates Daniell’s Battery (fig. 223) with connectors.

In Bunsen’s Battery (or the Zinc-Carbon Battery), which is very like the “Daniell” arrangement, as will be seen from the plates (figs. 222, 223), the porous cell has a prism of carbon immersed in it, and is apparently a modification of the powerful “Grove” Battery (fig. 224). This consists of slips of platinum, h, placed in porous cells, g, each cell being surrounded by a glass cylinder. The outer (glass) cells are filled, or nearly filled, with diluted sulphuric acid; nitric acid is used in the porous cells, and a platinum plate inserted. The chemical action of the Grove cell is thus explained by[Pg 223] Professor Stewart: “The zinc dissolves in the dilute sulphuric acid, and during this process hydrogen gas is given off. But this hydrogen does not rise up in the shape of bubbles; it finds its way into the porous vessel which contains the strong nitric acid. It there decomposes the acid, taking some oxygen to itself, so as to become water (hydrogen and oxygen forming water), and thereby turning the nitric into nitrous acid, which shows its presence by strong orange-coloured fumes.” By this decomposition of the nitric acid the polarization of the platinum (due to hydrogen) is avoided. The porous cell, while keeping the liquids apart, does not interfere with the chemical action.

A great number of cells are used in the Grove Battery; perhaps even a hundred may be employed.

Smee’s Battery consists of a plate of platinized silver, S, with a bar of wood to prevent contact with the zinc on each side, Z. These are immersed in a glass jar, A, which contains dilute sulphuric acid. The current is obtained by metallic communication with the binding-screws on the top. This battery has much the same general arrangement as Wollaston’s—the position of the plates being, however, reversed; in the latter there are two negative plates to one positive. In Smee’s Battery there are two positive (zinc) plates to one negative plate.

Fig. 225.—Smee’s Cell.

Fig. 226.—Smee’s Battery.

It will now be understood how an electric current is produced; the electricity passing through the cells, etc., to wires, confers certain properties upon the wires, and we can ascertain the effect of the current by means of a Galvanometer, an instrument used to detect the strength and direction of electric currents. The current will evolve heat and light; it will excite muscular action, and will decompose substances into their constituent elements. The deflection of the magnetic needle by the electric current is considered the best evidence of its power; it is on this that the Galvanometer is based.

We can perform a few simple experiments with the current. Suppose, for instance, that a piece of fine wire be fixed between the pole wires of the battery; it will be heated “white hot.” Or if two carbon points be approached in a glass of water, as in the illustration (fig. 227), they will emit a brilliant light in the fluid from the voltaic arc which has given us the electric light. The current is the passage of electricity along the wire, and continues[Pg 224] until the working power or “potential” of one conductor is equal to that of the other. When they become equal of course the action ceases, as there is equilibrium. But when an apparatus like the galvanic battery is brought to bear so that the force of electricity from one conductor is made always greater than that of the other conductor, we have a continuous flow while the action of the battery goes on. One view of the principle is thus expressed by Professor Gordon:16

“If two metals be placed near together, but not in contact, in a liquid which acts chemically more upon one than upon the other, the metals become charged, so that the one least acted on is of higher potential than the one most acted on. The difference of potential produced depends only upon the nature of the metals and of the liquid, and not on the size or position of the plates. As soon as the difference of potential has reached its constant value the chemical action ceases.

Fig. 227.—The Voltaic arc.

“If now the metals are connected by a wire outside the liquid the difference of potential begins to diminish, and an electric current flows through the wire. As soon as the difference of potential becomes less than the maximum for the metals and liquid, chemical action recommences and brings it up to the maximum; and thus if no disturbing cause interferes the current will continue until the metal most acted on is entirely dissolved.”

The metal most acted on is considered the “generating plate,” and is “positive.” The other attacked less is “negative,” and is known as the “collecting plate,” and the zinc is the positive plate. Sir W. Thomson has shown that the electrical movement in the galvanic circuit is entirely due to the electrical difference produced at the surfaces of contact of the dissimilar metals. The electro-motive force obtained is not the same with all metals. We have mentioned that some are electro-positive and some electro-negative, and it is with reference to each other that the metals are considered to be endowed with these properties respectively. It all depends how the metals are arranged or coupled. With reference to their behaviour in this respect scientists have arranged them in a series, as follows:—

1. Zinc.
2. Cadmium.
3. Tin.
4. Lead.
5. Iron.
6. Nickel.
7. Bismuth.
8. Antimony.
9. Copper.
10. Silver.
11. Gold.
12. Platinum.
13. Graphite.

[Pg 225]

Each metal in the list is arranged so that it is electro-positive to any one below, and electro-negative to any one above it.

There is another curious fact which should be mentioned. In associating these metals it has been found that when two are brought into contact the electro-motive force becomes greater the more distant they are in the series given above; in other words, the force between any two is equal to the sum of the forces between those intervening between those two. So when zinc is used with copper its force is not so great as when used with platinum.

It was Herr G. S. Ohm who laid down the law that the strength of the electric current is equal to the electro-motive force divided by the resistance, for he proved that the “resistance was inversely proportional to the strength of a current.”

There are two other laws respecting currents; viz.,—

(1.) Parallel currents in the same direction attract each other.

(2.) Parallel currents in opposite directions repel each other.

Fig. 228.   Chemical action of electricity.   Fig. 229.

Upon these two hang all the varied phenomena of electro-dynamics. That chemical action develops electricity we can perceive with the aid of the two cuts (figs. 228 and 229). If the wires be attached to the collecting-plate of a condenser of electricity and the metal plate of a cell, as shown in the figure (fig. 228), the electricity on the plate will be negative. If the operation be reversed, and the plate be put in connection with the acid, and the metal with the earth, the instrument will be charged with positive electricity. In the other case, when two cups are used, united by a magnet so that the solutions (one acid and the other alkaline) can by capillary attraction unite upon the binding of the magnet, and we place the wires as in fig. 229, the charge on the plate will be positive if it be in connection with the acid, and negative if in communication with the alkaline solution. Every time there is chemical action between two bodies in contact electricity is produced—positive on one negative on the other, and that is the fundamental principle of the voltaic pile.

[Pg 226]

The decomposition of water can also be effected by means of the electric current. If two tubes or vessels be placed in a vase of water, and the wires from the battery be inserted in them respectively, the oxygen will go to the platinum or positive pole wire, and the hydrogen to the zinc or negative pole. This decomposition or “splitting up” of components was termed Electrolysis by Faraday, who gave a series of names to the action and the actors in these phenomena (fig. 230).

Any liquid body, such as the water we have just decomposed for instance, Faraday termed an electrolyte; the surfaces where the current enters or leaves the body were called electrodes—the “ways,” from odos, a “way”; the entry is the anode; the leaving point the katode, from ana, “up,” and kata, “down.” The electrolyte is divided into two portions, “ions” (“movers”), which move towards the electrodes, which are positive and negative. In the case of the decomposition of water the hydrogen goes to the negative electrode, the oxygen to the positive.

Fig. 230.—Decomposition of water.

There are a few observations to be made respecting electrolysis. One rule is, that it “never takes place unless the electrolyte is in a liquid state.” The liquid state is essential. It is also observed that the components go to the different electrodes; such elements as go to the negative electrode are termed electro-positive, the others electro-negative; or, as Faraday termed them, “anions” or “kations:” The chemical power or electrolytic action of the current is the same at all parts of the circuit; the quantity of the substance decomposed is in exact proportion to the strength of the current. Faraday measured the strength of the electric current, and invented for the purpose an instrument called the Voltameter. We have mentioned the Galvanometer more than once, and will proceed to describe it. There are several forms of this instrument: the Tangent, the Marine, and the Reflecting Galvanometers, and the Astatic, or “Multiplier.” In the first-named the direction of the current is determined by Ampère’s rule, which is as follows:—

“Imagine an observer placed in the wire so that the current shall pass[Pg 227] through him from his feet to his head; let him turn his face to the needle: its north pole is always deflected to his left side.”

The “Tangent” Galvanometer consists of a vertical circle like an upright ring, across which is a support in the centre holding a copper wire, through which the electric current passes. On this point (where the wire is) a needle is very lightly supported, and when the instrument is to be used it is placed so that the plane of the circle is parallel to the line in which the needle points. The current passes, and the needle is deviated. By noting which side the north end of the needle goes the direction of the current is ascertained, and the length of the needle being small in comparison with the diameter of the circle through which the current passes, the strength of the current in the vertical circle is in proportion to the tangent of the angle through which the needle turns. Hence the term “Tangent” Galvanometer.

The “Reflecting” instrument is the invention of Sir William Thomson, in which a mirror is attached to the needle, and reflects a small focus of light upon a scale. The movements, however minute, are easily read. Sir W. Thomson’s Galvanometers are extremely sensitive. We need not mention any other varieties, as full descriptions can easily be obtained. We only need to indicate the mode of working.

Fig. 231.—Galvanometer.

The accompanying illustration (fig. 231) shows an Astatic Galvanometer which may be used in two ways—either to measure strength of current, or to find out a current; in the latter case it would be termed a Galvanoscope. It is a compound needle instrument, and consists of two needles placed in parallel directions with opposite poles above each other in a coil. The wire coil is wound round a bobbin, and the astatic needle is placed therein and suspended freely, as in the illustration, by a cocoon thread. The upper needle moves upon a scale, O O, and the instrument is enclosed in a glass shade. The screw, V, communicates with the upper needle, and fixes it at zero point when ready for use. The wires are fastened to the binding-screws, and the current is then sent. The needle is deflected accordingly, and the number of degrees on the scale can be read off.

The uses of the galvanic current are many. Amongst them Electroplating is perhaps the most generally useful, though Electrotyping is also a very important process in art. A visitor to Birmingham may view the process carried on there by some enterprising firms, who have succeeded wonderfully in producing electro-plate. The principle is very simple and easy to understand, but the greatest care and watchfulness are required on the part of the men employed. The principle, as we have said, is simple,[Pg 228] and consists in the fact that if a plate of metal be suspended and attached to the positive pole of a galvanic battery and immersed in a solution of the same metal, the conducting substance hung opposite at the negative pole becomes coated with the metal immersed in the solution.

Fig. 232.—Trough for electro-deposition.

Fig. 233.—Plates immersed.

Suppose we take a plate of silver, and immerse it in cyanide of silver dissolved in cyanide of potassium; a coating of silver will be deposited upon the nickel spoon or other article suspended at the other pole. But to make the coating adhere the spoons, forks, etc., are prepared for the bath by cleansing in caustic potash to remove grease, and washed in nitric acid to remove all traces of oxide, then are scoured with sand. Next, a thin coating of mercury is put on by immersion in solution of nitrate of mercury. Finally, they are hung in the bath. A metal rod is hung across the bath (fig. 232), and the plate is immersed. If the rod to which the articles are suspended be attached to the zinc or negative pole, and the plate of silver to the positive pole of the battery, decomposition begins, and the silver begins to attach itself to the suspended objects. If it be desirable to give the plated articles a thick coating, they are retained for a long time in the bath, which is of some non-conducting material. The dull appearance is easily removed by brushing and burnishing, and then the “Electro-plate” is[Pg 229] ready for the warehouse. The gilding process is performed in the same manner, a gold plate being substituted for the silver.

Fig. 234.—Medico-galvanic Battery.

Fig. 235.—Battery in case.

Electrotyping may be briefly explained as follows:—Take two vessels, A and B, and in one, A, put some dilute sulphuric acid and two plates, one of zinc, Z, the other of copper, D, but be sure they are not touching each other; each of these plates must have a piece of wire fastened, by soldering to their upper parts. In the vessel, B, put some solution of sulphate of copper and a small quantity of dilute sulphuric acid, and attach another copper plate to the wire which comes from the copper plate in the acid; this second copper plate is to be immersed in the solution of sulphate of copper, and to the wire from the zinc plate is to be fixed the object to be coated. If a medallion or other object in plaster, it should be soaked in very hot wax and then brushed over with blacklead until the surface is perfectly blackened and bright; the wire should be bound all round the margin and soldered (as it were) with melted wax to the medallion, taking care that this wax also is well coated with blacklead. If the object be now immersed in the sulphate of copper solution and kept at a short distance from the plate (it must not touch it), a coating of copper will soon cover the surface and form a perfect cast, which, when of sufficient thickness, may be removed[Pg 230] by filing the edge all round. If instead of the plaster cast a copper coin or other copper object be used, the blackleading is not required, but the surface must be first made clean and bright.

Many uses are made of the galvanic current by medical men. If the circuit of the pile is closed and we take a wire in each hand and break contact, a concussion will be felt in the joints of the arm and fingers, and a certain contraction of the muscles. The currents of electricity cause the shocks, and by a peculiar arrangement by which the circuit can be closed or broken at pleasure, a series of shocks can be sent through the body when it forms the connection between the poles of the battery. We give illustrations of a medico-galvanic machine. In fig. 235 there are two batteries, A and B, with cells, C D. Each battery consists of a central plate of platinized silver separated from the zinc plates by a piece of wood, E and F; the binding-screws are fastened to the silver plates, and G H retain the zinc plates; I is a copper band connecting the zinc plate of one battery with the silver plate of the other. At Z and opposite are wires leading to the coil machine. The quantity and intensity of the current are regulated respectively by the indicator, O, and the wires, Q. There is a point, R S, for the breaking of the contact; P N are screws retaining the wires which lead to the handles, U V, grasped by the patients.

Fig. 236.   Fig. 237.   Fig. 238.
Horse-shoe magnets.

The electric current is employed in many diseases, and is of great use in some cases, but the further consideration of it with reference to its medical applications does not fall within the scope of our present work. We will now pass on to one of the most useful applications of the electric force, the Telegraph, and in dealing with it we must make a few remarks upon magnetism. First, let us make an experiment or two, and see the reciprocal action between electricity and magnetism.

(1.) If we take a piece of iron of the form of a horse-shoe (fig. 236), and wind around it copper wire, and pass through the wire an electric current from our battery, the iron will exhibit strong magnetic properties, which it will lose when the current is interrupted. The conducting wires are insulated with silk, and the current will then travel in one direction.

(2.) If we cover the ends of a non-magnetic piece of iron with coils of wire, and rotate the magnet, A B, so as to cause the poles to approach[Pg 231] each end of the iron alternately, an electric current will be established in the wire.

(3.) Referring to the first experiment, if we bring a needle in contact with the iron horse-shoe, while the current is passing through the wire we shall find that the needle has become a magnet; i.e., that it will point due north and south when suspended.

We will now see what a Magnet is, and why it has obtained this name.

Fig. 239.—Magnetic attraction.

In Magnesia, in Lydia, in olden times was found a stone of peculiar attributes, which had the property of attracting small portions of iron. The Chinese were acquainted with it, and nowadays it is found in many places. In our childhood we have all read of it in the story of “Sinbad the Sailor.” Popularly it is known as the loadstone; chemists call it magnetic oxide of iron (F₂O₃). This stone is a natural magnet. In Sweden it exists in great quantities as “magnetic iron,” for it has a great affinity for that metal.

If we rub a piece of steel upon the loadstone we convert the former into a magnet—an artificial magnet as it is called, and the magnetic needle so useful to us in our compasses and in the working of one form of the electric telegraph is thus obtained. Let us see how this needle acts.

Fig. 240.—Simple touch.

Fig. 241.—Double touch.

Take a magnetic needle and dust upon it some iron filings. You will observe that the filings will be attracted to both ends of the magnet, but the centre will remain uncovered. The ends of a magnet are termed “poles,” the centre the equator. So one end is north and the other south, and we might perhaps imagine that the same characteristics would abide in the bar when it is cut in two. But we find that as when a worm is divided, each portion gets a new head or tail, and makes a perfect worm, so in the magnet each divided half becomes a perfect magnet with separate poles, one of which always points to the north.

[Pg 232]

The poles of the magnet display the same phenomena as regards attraction and repulsion, as do the opposite kinds of electricity. If we suspend a magnet and bring the north pole of another to the north pole of the suspended magnet, the latter will turn away; but if we apply the north pole of one to the south pole of the other they will be attracted just as opposite electricities attract each other.

Magnetization is the term applied to the making of artificial magnets, which act is accomplished by bringing the needle in contact with other magnets, or sometimes by means of the electric current. If we carefully stroke the needle with the magnet, always in the same direction, lifting the magnet and beginning afresh every time, we shall magnetize the needle, but with a different polarity from the pole it was rubbed with. A magnet rubbing its north pole against a needle will make the latter’s point south, and vice versâ.

Now that we have seen how the “magnetic needle” is arrived at, we can proceed to explain the electric telegraph. The term telegraph is derived from the Greek words tele, “far,” and graphein, “to write,” and now includes all modes of signalling. Signalling, or telegraphing, is of very ancient origin; the Roman generals spelt words by fire. The beacons fired on the hills, the “Fiery Cross,” and other ancient modes are well known. The semaphore and flags have long been and are still used as modes of signalling, while the flashing of the heliograph “telegraphs” to a distant camp.

The Semaphore was invented by Chappé, and was really the first practical system of telegraphy. It was adopted in 1794, but before this, in 1753, a letter appeared in the Scots Magazine, by Charles Marshall, suggesting that signals should be given by means of electric wires, equal in number to the letters of the alphabet. Soon afterwards Lesage, of Geneva, made an electric telegraph to be worked by frictional electricity, and many ingenious attempts were subsequently made to utilize electricity for signalling purposes, but without any permanent success; indeed, the British government were quite content with their semaphores, for they wrote that “telegraphs of any kind are now wholly unnecessary, and no other than the one now in use will be adopted”!

The Electric Telegraph has had considerable antiquity claimed for it, but it is pretty certain that the discovery made by Doctor Watson, in 1747, that electricity would pass through wires, and that the earth would complete the circuit, gave the first impetus to the Electric Telegraph. Doctor Watson was enabled to transmit shocks across the Thames, and made experiments at Shooters Hill. Franklin did likewise across the Schuykill in 1748, and De Luc performed the same experiments on the Lake of Geneva. Both Lesage and Lomond caused pith balls to diverge at distant points, and in 1794 Reizen made use of the electric spark for transmitting signals, and made strips of foil show out certain letters when the spark passed. He had a wire and a return wire for each letter of the alphabet.

These were all slow advances, and subsequently many learned men in[Pg 233] Europe sought to improve upon the ideas then promulgated. We read of telegraphs constructed at Madrid by Salvá and Betancourt in 1797 and 1798, one extending for more than twenty miles. The first-named gentleman finally proposed to substitute the Voltaic pile for the usual machine, and Ronalds and Dyar in England and New York respectively employed frictional electricity with some success. The latter sent charges of frictional electricity through a wire, and they were recorded by being made to pass through litmus paper. The distances between the discharges were intended to indicate the letters of the alphabet, but even if the experiment was fairly tried it failed, for little was heard of the result.

After the invention of Volta’s pile, which Salvá wished to adopt, Sömmering began his experiments. He used thirty-five wires, set up vertically at the bottom of a glass reservoir of water, and terminating in gold points. These wires ended in the opposite direction in brass plates attached to a bar of wood. At one end the points and at the other the plates bore the same letters respectively; hydrogen at one gold point, and oxygen at another, and two different letters were indicated when the current was sent through any two plates. This arrangement was afterwards improved upon, and only two wires retained.

It was not until electro-magnetism had been developed, however, that Œrstead found out the power of electricity to deflect the magnetised needle, and in 1820, Scheweigger added a “multiplier.” Then came Arago into the field with his discovery, that a “wire carrying a current could magnetise a steel rod.” Ampère substituted a helix for a straight wire, and Sturgeon used soft iron for steel, and developed the electro-magnet. Daniell’s battery, and Faraday’s discoveries of magneto-electricity and the induction coil were the means of putting a constant supply of electricity at the service of the telegraph and so on, till 1830 brought out a more practical method introduced by Schilling.

In that year Baron Schilling made a telegraph, and exhibited it in 1832 at Bonn. This invention, with five vertical needles, was shown to Mr. Cooke in 1836. But in 1834, Gauss and Weber had succeeded in sending signals by means of a voltaic current acting upon a magnetised needle, and this apparatus was really the first practical electric telegraph in use, and it was much improved by Professor Steinheil of Munich. They employed a magnetic-electro machine, and caused a bar to move in certain directions to indicate certain letters of the alphabet. This was really of value, but Steinheil, the pupil of Gauss, assisted by his government, employed only a single wire, and made the earth complete the circuit for him instead of having a return wire as his predecessors had. This telegraph was perfected by a series of bells, which gave different tones for different letters, and he also caused the needle to make certain tracings as it moved upon a paper slip, something like the Morse pattern, which was completed in 1837.

Professor Morse, in 1832, conceived the idea of an electric telegraph but his claim was disputed by a Doctor Jackson, who was on the same vessel[Pg 234] when the subject was discussed. We need not enter into the details of the controversy. Mr. Morse won the day, and patented his invention.

“It was once a popular fallacy in England and elsewhere that Messrs. Cooke and Wheatstone were the original inventors of the electric telegraph. The electric telegraph had, properly speaking, no inventor.... Messrs. Cooke and Wheatstone were, however, the first who established a telegraph for practical purposes comparatively on a large scale, and in which the public were more nearly concerned.... Therefore it was that the names of these enterprising and talented inventors came to the public ear, while those of Ampère and Steinheil remained comparatively unknown.17 The telegraph, as used in Great Britain, was the result of the co-operation of Professors Cooke and Wheatstone.

Mr. Cooke, in 1836, having seen the needle telegraph when in Heidelberg, made certain designs, and soon entered into partnership with Professor Wheatstone for the application of electric telegraphs to railways. Their apparatus with five needles and five wires was put up on the London and North-Western (then London and Birmingham) and Great Western lines, but proved too expensive. The instrument was subsequently modified, and is used on the English railways still.

Fig. 242.—Cell.

We may now proceed to look at the Wheatstone needle telegraph and see the method of working it. We know already that when a pair of metallic plates are immersed in a fluid which acts chemically more rapidly on the one than the other, and a wire connects the upper parts of these plates, this wonderful agency is set in motion, and circulates from the one plate to the other (fig. 242). This arrangement may be best shown by using one plate of zinc and the other of copper, and a dilute solution of sulphuric acid for the liquid; this, however, produces by far too little of the agent to be used on a telegraphic line, there are therefore combinations of such pairs of plates, so arranged that the power of one pair shall be added to the next in such a way that at the end of the series (called a “battery”) there shall be a great increase of the power accumulated; this arrangement is shown in fig. 244. Now (if the power be sufficient) it does not signify what length of wire there may be between the two ends of this arrangement or “battery”; whether they be connected by a few feet or many hundred miles, the electricity passes instantaneously from one end to the other; and furthermore, it has been found in practice, that this electrical influence can be transmitted through the earth in one direction if sent by a wire in the other; for instance, if a wire from one end of the battery be carried on from London to Liverpool, instead of having another from Liverpool to London, to connect the two ends of the battery, it is found to answer the same purpose if the end of the wire at Liverpool be fastened to a plate of metal buried beneath the surface of the earth, and the other end of the battery at London furnished with a similar plate, also buried. In this arrangement,[Pg 235] the electricity will pass beneath the surface of the earth from Liverpool to London, and through the wire from London to Liverpool, thus completing the circuit. The end from which the electricity passes is called the “positive electrode,” that to which it returns, the “negative electrode.” Fig. 245 will show this arrangement.

Fig. 243.—The Needle Telegraph.

Fig. 244.—The Battery.

Fig. 245.—The circuit.

Fig. 246.   Fig. 247.   Fig. 248.
Magnetic needles.

If a bar-magnet be suspended on a pivot so that it may turn freely, it will (as is well known) turn with one end to the north, which is owing to a current of natural electricity passing round the earth in the direction of east and west, the magnet crossing the current at a right angle; and if a coil of wire coated with silk (to keep one part of the coil from another) be placed round, above, and below the long axis of a bar of steel, as shown at fig. 246, and a current of electricity passed through the wire, the steel becomes a magnet, and will take a direction similar to the natural magnet, more or less, at right angles to this coil, as in fig. 247, according to the intensity of the current; and the instant this electrical current is stopped, it will resume its former direction. This fact has been made use of to form the principal feature of all English telegraphs; such a needle is mounted in an upright position, and instead of its tendency to turn to the north, a tendency to maintain the upright position is given to it by having one of the arms of the magnet a little heavier than the other; such a magnet having a coil of[Pg 236] wire surrounding it. When the electric current passes through the coil, it will turn out of the upright position to either one side or the other, according to the direction of the current, from its tendency to assume a position at an angle thereto (fig. 248); if the current be stopped even for an instant, then the needle, or magnet, will again assume its upright position. The pivot of this magnet is brought forward, and has on its front part another needle, which turns with it; this is visible on the outside of the apparatus, and is looked at to ascertain the movement of the one within. There is also an arrangement called a “commutator,” so contrived, that by moving a handle to the right or left, a connection shall be made with either end of the battery, and thereby cause the direction of the current and needle to be changed at pleasure; also by moving the handle into an upright position the current shall be stopped; and finally, by a third movement, a bell shall be rung. Now, as has already been explained, when the current goes in one direction, the magnetic needle is deflected in that direction; and when the current is reversed the position of the needle is also reversed, and when the current is cut off the needle will resume its perpendicular position. If two such needles and two such handles be at each station, when the handles at one station are moved, the needles at the other station will take on a similar movement; and when the handles at that station are moved, the needles at the first station will be moved to correspond. This constitutes the system of communication kept up by the electric telegraphs in England; but it remains to be shown how all the letters of the alphabet and numerals can be represented by the movements of the two handles. These handles can be placed in eight positions (besides the upright one) by a single movement of each hand, as may be seen in fig. 249; and these eight signals if repeated, or made twice in rapid succession will make eight more, and by being repeated three times will constitute a third eight, making twenty-four; finally, by a rapid motion right and left, they may be caused to signify a[Pg 237] fourth eight, or thirty-two signals, which are found to be sufficient for every purpose, and by practice may be both produced and read off with facility. Before a message is about to be delivered the commutator is so placed as to ring a bell, which is done by the same arrangement as in a common alarm-clock, but the action is set in motion by a peculiar contrivance, which depends upon the property a bar of soft iron has of becoming magnetic when a wire is wound round it and a current of electricity passed through this wire; this magnetic property exists only as long as the current passes, and stops the instant it is cut off. The catch of the alarm is disengaged by the movement of a bar of iron being drawn to the magnet while the current passes, and forced back again by a spring when it is stopped, thus setting in action the mechanism of the alarm; or in some cases there is a simple contrivance for causing a rapid flow and stoppage of the electricity, so that the bar is alternately attracted by the magnet and released by the spring, and this motion rings the bell as long as it is continued. The bell is always rung to give notice that a message is about to be sent, and at the station where it rings, the bell at the former station is rung in return, to show that they are prepared to receive the message: which is then spelt, letter after letter, by moving the handles into the proper positions, and as it is being sent, the eye is kept on the dials, certain single signs are made and recognised, which will communicate any reply from the station to which the message is being sent, such as “repeat,” or “not understood.” The wires which convey the electricity are made of galvanised iron (iron coated with zinc), and as they must be kept from all communication with the earth by some substance incapable of conducting it, they are therefore stretched between wooden poles (fig. 250), and rest upon sockets or supports of glass or glazed earthenware, which are both substances incapable of conducting the electricity[Pg 238] to the earth (fig. 251), and in order that these may be quite dry, an inverted cup of metal, glass, or earthenware is placed over it, or the whole is blown or moulded in one piece. If the support for the wires were not kept from the rain, the wet would form a conducting surface, and allow the electricity to escape into the earth.

Fig. 249.—Handles and needles of telegraph.

Fig. 250.—Telegraph wires.

Fig. 251.—Insulator.

The Telegraph Alphabet, in the two-needle instrument, now not generally used in England, is given below.

In the single needle instrument the letters are indicated by right and left vibrations, from A one right, to B one right and left, and so on, increasing to Z. This mode is now generally used.

The manner in which the current passes is shown by the following illustrations (figs. 252, 253).

Fig 252.—Passage of the current (1).

Fig. 253.—Passage of the current (2).

For the sake of clearness, the diagram has been drawn with simple lines only. In the real needle-machine the construction is much more complicated; perspective drawings of it may be seen in Lardner’s “Electric Telegraph,” and numerous other works. In fig. 1, B is a single cell of a battery containing a plate of copper, C, and a plate of zinc, Z, immersed in sulphuric acid and water. H is the handle of the instrument, turning from left to right, and vice versâ, like the handle of a door, consisting of two pieces of brass insulated from each other by being inserted in an axis of ivory. To the ends of the two pieces of brass are fixed the wires, CW and ZW, leading from the copper and zinc ends of the cell respectively. Fixed[Pg 239] on each side of the handle are two plates of metal, which may be called LP, the left plate, and RP, the right plate. They are connected with the needle wire, NW, which passes before and behind the magnetized needle, N, suspended perpendicularly on its axis, with its north pole upwards. As long as the wires, CW and ZW, remain insulated from each other, no current passes from the cell; but as soon as the handle, H, is turned, so that the copper end touches the left plate (fig. 2), and the zinc end touches the right, communication is established between the plates of the cell, and the current commencing at the copper passes along CW, the top half of H, into LP, along N W, travelling up before and down behind the needle, causing it to deflect to the observer’s left, according to the rule given above. Reaching RP, it passes downwards to the cell to Z, and so on to C, continuing its travels and keeping the needle deflected as long as the handle remains in contact with the plates. If it is required to deflect the needle to the right, the handle is turned to the right, bringing its copper end in contact with the right plate, causing the current to travel in the opposite direction. By following the current from the copper to the zinc, as indicated by the arrows in fig. 3, it will be seen that it now travels up behind and down before the needle, deflecting it to the observer’s right. Thus, by causing a current of voltaic electricity to pass alternately up before and down behind, and up behind and down before, the needle is moved to the left or the right at will. The way in which the current is made to act on a distant needle is now simple. The following figure (fig. 253) shows the arrangement. The left portion of the figure represents an instrument at London, that on the right an instrument at York. The needle-wire, instead of being continued directly to the zinc plate of the battery, passes away from the needle over poles to York, where it joins an instrument similar in all respects to that at London. It passes similarly before and behind a magnetized needle, joining the right plate of the instrument. As long as the two plates are unconnected, no current can pass. The current is therefore completed by a contrivance which may be represented by the semicircular piece of metal, K. In practice[Pg 240] the two plates or springs, which, when not in use, are always pressed against the connector, K, which is a cross-piece on the top of the handle, keeping the London needle in circuit with the York battery, and vice versâ. As soon as London uses his handle, it presses the spring-plate, and puts his needle out of the York circuit, the current he sets up sending York needle to the right or left, as the case maybe. The second wire connecting the left York plate with the right London plate is, it will be seen, not carried along like the first wire. Use is made in this case of the conducting power of the earth itself, plates from the wires being buried many feet below the surface at London and York. When London wishes to speak to York, he first signifies his intention of so doing by ringing York’s alarum. This he effects by sending a current through an electro-magnet placed above York’s instrument. The armature is attracted, and frees the detent of the alarum, setting it ringing until York signals ready. London then stops the bell, and commences his message. By following the direction of the current, when the handle is turned to the left, as in fig. 4[within fig. 253], it will be readily seen how this is effected: commencing with London’s copper, LC, it passes up before and down behind London’s needle, flowing along the wire between the two cities to York’s needle, up before and down behind which it travels, sending it also to the left. It then passes to York’s right plate, through the connector to the left plate, and so on to earth at York, coming to the surface again at London, passing through London’s right plate and through the lower part of the handle to the zinc of the battery. The reverse current may be easily followed. Any number of instruments with similar needles may be interposed along the course of the wire. When the operator wishes to speak to any particular one, he rings all the bells for attention, and then signals Derby or Nottingham, as the case may be. They all then throw their instruments out of current except the one required. The mode by which the needle movements are converted into language is simple. A is signalled by causing the needle to vibrate once to the right, B once right and once left, C once right and twice left; and so on, as arranged.

[Pg 241]

The following is the alphabet (with numbers) once in use on the South-Eastern Railway for the double-needle instrument. The table is taken from Mr. Walker’s translation of De la Rive’s work on Electricity and Magnetism.

The Morse system of telegraphy was first brought out in 1844, and was worked by means of a Voltaic battery, an electro-magnet being used at the receiving station. This magnet attracted an “armature,” and by it dots or lines are marked on a moving paper band by a point at the other end of the wire, on the register in which the paper is carried by rollers which move out by clockwork. The lever being “tapped” down in fast or slow pressures will give a corresponding series of dots or lines (according as the pressure is long or short) upon the moving strip of paper at the receiving station. Three taps will give C, one tap and a pause will make A. The dots are “taps” on the key, the lines brief “rests” on it, as will be seen from the alphabet below, which is given as a specimen.

[Pg 242]

The various stops are also indicated in the same manner by combinations of dots and lines.

The Atlantic telegraph cables and similar enclosed wires between other countries are too well known to need detailed description. There is a great variety of telegraphic instruments. The dial, and other arrangements, are very common, and the Wheatstone Key instrument is supplied to private firms as being the most handy. It requires but a very short apprenticeship, and any person who is handy can easily learn to work it in a few minutes. The apparatus consists of a dial upon which the letters of the alphabet are printed, each letter being supplied with a key or stop. A pointer is placed in the centre, as in the wheel barometer, and there is a handle beneath. In front, upon a sloping board, is another dial plate and pointer; thus we have the receiver and transmitter before us in a very small space.

Fig. 254.—Receiver.       Dial Telegraph.       Fig. 255.—Manipulator.

When it is necessary to work the instrument a bell is rung by turning the handle rapidly. To speak by the instrument it is necessary to keep turning the handle with the right hand while the fingers of the left are employed in pressing down in as rapid succession as practice will permit the keys corresponding to the letters on the dial while the handle is kept turning. When a word is completed the operator must stop at the + at the top, and then begin again, stopping after each word. When all is said, a couple of rapid turns of the dial will signify that you have ended.

There are many other systems of telegraph, but all are dependent upon the same principles. The accompanying illustrations (figs. 254, 255) show a dial telegraph of a simple kind, which almost explains itself.

The first figure is the receiver, on which is a pointer fixed to a dial-plate having the letters of the alphabet inscribed around it. When the manipulator is being worked the dart points to the letters in succession of the words used, and they are separately spelt. The manipulator (fig. 255), by closing and opening the circuit, works the needle.

In the manipulator we have a wheel with an index point fixed above it. In this wheel are thirteen teeth, with the openings between them filled[Pg 243] with ivory. The axis of the wheel is in contact with the wire from the positive pole, p, and a spring attached to the wire or by the binding-screw, t, presses against the circumference of the wheel, and completes the circuit. When the wheel is placed so that the arrow point is above the +, the needle of the receiver is also at +. By turning the wheel to bring the needle to A, the spring on the circumference is passed from an ivory “tooth” to a “metal” one; the circuit is closed, the point of the receiver also turns to A, and so on through the word by successive closing and breaking of the circuit.

As there are a great many other applications of electricity of which we have to treat,—the Electric Light, and Mr. Edison’s other inventions,—our space will not permit a much more detailed account of the telegraph, but there are some incidents connected with its progress which it would be as well to mention.

Fig. 256.—Electric cable.

Fig. 257.—Ocean cable.

Alexander Bain, about 1840, attempted to produce a printing telegraph, and in 1846 he actually accomplished a registering apparatus, which was an application of the principles of Dyar and Davy. But although Bain’s system was good, Morse had the advantage of possession in the United States, where it was tried, and Bain went out of fashion. Bain’s system was, in fact, the present chemical “automatic” telegraph, which has been perfected for rapid transmission.

Bakewell’s instrument, which has been improved upon by later electricians, is termed the fac-simile telegraph. The message to be sent is written with a pen which has been dipped in varnish (for ink), and the characters are inscribed upon prepared tinfoil. The message is then put upon a cylinder covered with prepared paper, and has a pointer attached. There is a precisely similar cylinder at the receiving station. When the cylinders are simultaneously set going, the point at one will trace a spiral line as the first (transmitting) point passes round its cylinder. However, as the latter “stylus” meets the varnish letters a break occurs, and these spaces are exactly reproduced as blanks at the other end, and the form of the letters can be seen. Coselli, in his adaptation, caused dark letters to be registered on a white ground, and thus simplified matters. Since then we have had printing telegraphs, and dials, and writing machines, one of which will be described presently.

Submarine telegraphs were, it is said, first suggested by Salvá in[Pg 244] 1797, and Wheatstone, in 1840, declared that it was quite possible to connect England and France by wire. Morse and Calt experimented with submarine cables in America, and Lieutenant Siemens first applied gutta-percha to the wires as an insulator in the Prussian line of telegraph across the Rhine. The English laid a wire between Dover and Calais, which was broken, but successfully relaid. And so on, till in 1857 the great project of the Atlantic cable was broached. We give illustrations of the cables; the circumstances connected with the laying of which, and the enthusiasm over the successful accomplishment of the task, must be in the memory of all. The readings of the messages were shown by delicate galvanometers, the beam of light being reflected from a mirror. This cable was lost, and in 1862 Mr. Field came over to urge the importance of the submarine cable between this country and America. The cable was shipped on the Great Eastern in 1865, and was 2,186 miles long. It consisted of seven copper wires twisted, and covered with gutta-percha. The outside coating consists of ten iron wires surrounded by manilla yarn. But this cable broke, and a third was made and laid in 1866. The old cable was then recovered and spliced. There are some two hundred cables now in existence, the last being the Cape cable, laid when the Boer War was engaging our attention. The transmitting apparatus of Mr. Varley and Sir W. Thomson has greatly accelerated the rapidity of messages, and Thomson’s syphon recorder farther increased the speed.

The following description of a new system is from Scribner’s Magazine for 1880:—

“New Telegraphic System.

“A new system of sending and receiving electrical impulses over an insulated wire has recently been brought into successful operation, that seems to promise not only a radical change in the present methods of telegraphing, but a material gain in the speed and cost of sending messages by wire. It is founded on a union of the so-called “automatic” and “chemical” systems of telegraphy. The first of these employs a strip of paper having, by some mechanical means, a series of small holes punched in it, the design being to pass the perforated strip under a needle, or stylus, in electrical connection with the line. This stylus, on passing over the paper, opens the circuit, but in passing one of the holes, drops through and closes it,—this alternate making and breaking of the circuit transmitting the message. The chemical telegraph records any electrical impulses sent over a line by staining a strip of prepared paper passing under it. This is founded on the fact that electricity has the power of decomposing certain chemicals, and if paper is soaked in these chemicals and submitted to the action of electricity, it will be discoloured wherever the current passes. While both of these systems have been used, neither has been able to compete with the more simple Morse key and sounder, and it has remained for the new system to bring them to a position where they may come into general use. The new system[Pg 245] is a modification and combination of the automatic and chemical systems, the transmitting being performed by means of a perforated strip of paper, and the receiving of the message being recorded by the discolouration of chemically prepared paper. The process is entirely mechanical and chemical, the telegraph operator having no direct control over the message, either by sight, sound, or touch. The written message is sent to the operating-room, and given to the person using the perforating machine. This consists of a small key-board, with black and white keys, each marked with a letter or sign, and an ingenious system of levers, operated by the keys, for punching small holes in a ribbon of paper moving past the side of the machine. The machine stands upon a small table, and under it is a treadle for giving motion to the feeding apparatus for supplying the paper to the machine. The operator moves the treadle with his feet, and at the same time touches each key to spell out the message. In a very few seconds the message is imprinted on the ribbon in the form of a double row of small perforations, each group of two holes representing a dash, and each single hole a dot, as in the Morse alphabet. Each letter is separated from the next by a longer dash, and each word by a still longer dash, and each sentence by a dash of indefinite length. This spacing of the letters is performed automatically, the spacing of words and sentences is performed by the operator. The perforated slip containing the message is then sent to the transmitting machine. This consists essentially of a metallic wheel, divided into two sections by means of a thin insulation of hard rubber. One section of the wheel is connected with the positive pole of the battery, and the other section with the negative pole. A pair of fine metallic brushes, both of which are connected directly with the line, are suspended above the wheel, and are arranged so as to press lightly upon the latter, when desired. When resting on the wheel the circuit is closed, and when raised above it the circuit is broken. The perforated strip is, by a simple piece of mechanism, made to pass over the face of this wheel and under the brushes. While the paper is passing, both brushes are raised from the wheel, and slide over the paper, and the circuit is broken. On passing a hole, one of the brushes drops through and closes the circuit for an instant. On passing two or more holes, arranged in a series close together, the brush closes the circuit for a shorter or longer time, according to the number of holes, and as the perforations on the paper are arranged in two rows, alternating from one to the other, the brushes are used alternately, and the polarity of the current is continually changed with every impulse sent over the line. No special skill is required in sending a message, as the operator has only to put the perforated strip in the machine and turn a hand-crank, to cause it to pass rapidly under the brushes, and with a little practice, a young girl can send messages at the rate of one thousand words a minute, with absolute precision. The receiving apparatus consists essentially of a simple piece of mechanism for causing a strip of chemically prepared paper to pass rapidly under two small needles that are connected with the line.[Pg 246] As the paper passes the needles, the electricity sent over the line from the transmitting machine seeks the earth through the wet paper and the machine, and in passing discolours the paper, each stain representing a dot or dash, and the message is printed on the paper in a double row of marks at the same speed with which it was dispatched. In practice, a Morse key and sounder is placed at each end of the line, and on sending a message the transmitting operator calls the receiving station, and when the operator at the distant end replies, both turn the cranks in their machine swiftly, and the message is sent and received at an average speed of one thousand words a minute. The message received is given to a person using a type-writer, and at once translated into print and sent out by the messenger boy. It is found in practice that two operators, one at each end of a single wire of indefinite length, can keep fifteen operators fully employed in preparing the messages, and fifteen girls busy in translating and printing the messages for delivery. The system is of American origin.”

Of the hundred and one uses to which electric wires are now appropriated—of the alarms, fire-calls, clocks, etc.—we need not speak. We must pass on to the Writing Machine (fig. 258) before we make mention of Mr. Edison’s inventions.

The Writing Machine is as remarkable for the simplicity of its mechanism as for the facility and ease with which it can be used. It was invented by Remington, the American, whose name is so universally known in connection with a repeating rifle. He makes these writing machines in his own factory, where he associates them with rifles and sewing machines—implements for war and peace.

The appearance of the Writing Machine may be easily perceived from the illustration (fig. 258), which is drawn to scale one-fourth of the actual size. It comprises a key-board, upon which there are forty-four keys or stops, including numbers from 2 to 9, the i and o of the alphabet serving for numbers 1 and 0, and all the letters of the alphabet arranged in the manner most convenient for manipulation. There are also the various accents and stops, with note of interrogation, etc. The flat ruler at the base of the key-board is struck when it is necessary to separate one word from another.

In the interior of the apparatus every letter is attached to a small hammer, and corresponds to the pressure bestowed upon the notes, which are disposed in a circle. If A, for example, be touched upon the key-board, the hammer will bring A to the centre of the circle, and so every letter of the word will be, by such action, brought to the centre of the circle in succession. The paper upon which the letter is printed is wound upon a cylinder mounted upon a slide, as seen in the upper portion of the illustration.

When the letter is pressed down on the key-board the corresponding hammer strikes against the cylinder, between which and the hammer is a ribbon prepared with a special ink. The letter being in relief like ordinary type is impressed upon the paper. The slide upon which the paper is mounted is so arranged as to move from right to left exactly a letter-breadth[Pg 247] after each impression. Thus as every hammer strikes at the same spot a regular succession of letters are printed off, the paper moving with regularity. When the line is filled—that is, when the paper has moved across the cylinder—a bell rings, and a handle is moved by the operator who is thus warned. The lever moved brings the slide back again, and a new plain surface is ready to commence upon the cylinder moving upwards at the same time, and displaying the printed line.

Fig. 258.—Remington’s Writing Machine.

In operating both hands may be employed, but between each word care should be taken to press down the flat board at the base of the key-board, which has the effect of leaving a space upon the paper. Immediately the sound of the warning bell is heard the lever at the right-hand side must be lowered. The word can be finished in the line following if it be not concluded, the hyphen button being pressed to indicate the continuation.

The paper used must not exceed the width of the cylinder, but it may[Pg 248] be of any less width, and a post-card or any small sheet of paper may be substituted. If the width be thus limited the length may be indefinite, and a very long line of paper may be used if desirable. The cylinder being made of gutta-percha offers a soft surface to the impression of the hammer, and causes the letter to assume greater distinctness.

The inked ribbon which passes underneath the paper is so arranged that no two successive letters strike it on the same place. It moves from an ink reservoir on the right to another on the opposite side, and it can be made to return beneath the paper, thus keeping up the supply. The impression being made in copying ink, the message or letter when finished can easily be reproduced in an ordinary press. The characters are all “capitals.”

At first it may be found a slow means of writing, and the manipulator may imagine he can do better without it. But if the author be certain of what he intends to say, after a little practice at the instrument, and when he becomes accustomed to the positions of the various letters, etc., the rate at which words can be printed off will far exceed that at which even rapid writers can work. A young English lady after some days’ practice was able to write as many as ninety words a minute with this machine—a rate more than double the average writing rate of penmanship. When such a rate or an approximation to it can be attained, those who are quick in their ideas will find the machine a great saving of time, and for any one afflicted with “writer’s cramp” the gain must be enormous. We need not insist upon the advantages the adaptation of the apparatus would confer upon editors and readers of MSS. too often badly written, and to compositors the invention is a great boon.

Finally, the working of the machine could be entrusted to the blind, and by teaching them the form of letters which could be raised upon the key-board, those so sadly afflicted could write with facility. Some methods for teaching the blind to manipulate and to read from the impressions of the hammers on the paper have already been tried with success.

The Electric Pen, an invention of the fertile brain of Mr. Edison, is shown in fig. 259. The “writing” consists of a series of little holes close together, made by a fine steel point like a put-crayon. This point is thrust in and out with great rapidity, and passes quickly over the paper. If the characters cannot be formed so quickly as with an ordinary pen, the writing is very distinct.

The alternative movement is given to the pen by an electric motor at once simple and ingenious, which is placed on the top of the penholder. The general appearance of the apparatus will be understood from the cut on next page.

The point is the termination of a wire which traverses the penholder, and the upper extremity of which catches on the motor by an eccentric. This eccentric has three teeth or cogs, and it makes sixty revolutions a second, thus producing one hundred and eighty beats in that time. The axle carries a plate of soft iron, which acts like the armature of an electro[Pg 249]-magnet, before which it turns with great rapidity, the current being interrupted twice in every revolution by the commutator. The current which moves this little apparatus is furnished by a pile of two elements in bichromate of potash, according to Mr. Edison’s arrangement, which is considered very successful. Carbon and zinc are employed, and when ready for action the battery assumes the appearance of the cells in the illustration. When the operator wishes to discontinue writing, he simply raises the stem which has the electrodes attached to it, and the elements are thus preserved for a future time.

Under these circumstances the battery could be made to last several days without any renewal of the liquid, and the plates will last for weeks. Thus a very simple arrangement is at our disposal. Let us see what use can be made of it.

Fig. 259.—Edison’s Electric Pen.

When we use the electric pen we obtain a great number of small holes close to each other. Such hand-writing is not easy to decipher by mere inspection like ordinary writing. By holding it up to the light it is more easy to read, but in both instances reading is not easy, nor does it come by nature as Dogberry declares. But if we consider the paper as a “negative,” we may obtain a number of positive proofs or copies of the writing. To obtain these successfully we must use a press, as shown in the accompanying illustration (fig. 260).

The writing, or negative, is placed upon the cover to the left, where it is firmly fastened. Upon the body of the press a sheet of white paper is placed, and when the lid is shut down the negative comes in contact with the paper. By means of a roller, represented in the box, the writing is blackened,—the ink penetrates into all the holes which are upon the paper,—and after the manner of a stencil plate the impression will be found upon the paper when[Pg 250] the cover is removed. The writing will have a curious effect, but practice will speedily remove all deficiencies. The same negative will serve for a great many impressions, quite a thousand having been taken from one. By people accustomed to such work as many as six proofs a minute may be obtained. Of course a little practice will be necessary in this, as in every other case, before a correct or rapid result can be obtained, but there is no difficulty in the practice.

There are two or three other applications of electricity which we must refer to; such as the electric stamp, of which we give an illustration, and a curious method of stopping a horse by electricity. The electric stamp might be very advantageously employed in our post offices to obliterate the “Queen’s Head.” The description, with illustration of this apparatus, is annexed. (See fig. 261.)

Fig. 260.—Duplicating press.

At the lower end of the apparatus is a thin platinum wire, so arranged as to form either a design or an initial; by this the postage stamp can be defaced. The stamp being put in communication with the pile, the circuit is closed by the pressure of the finger, as shown in the illustration. The platinum grows heated and carbonises the paper, and thus proves itself an ineffaceable stamp.

This apparatus may easily be used, not only by the post office authorities, but by every one who is obliged to deface a certain number of stamps every day, and wishes to do so rapidly and without possibility of error.

An ingenious, if scarcely necessary arrangement for conquering restive horses, and frightening them into submission, is shown in the illustration (fig. 262). Many means have been tried to stop or conquer a restive horse, but the most efficacious has been designed by M. Defoy; and the director of[Pg 251] the Paris General Omnibus Company has experimented successfully, as we are informed, with the arrangement we are about to describe. A small magneto-electric machine is contained in a box beside the driver, within easy and convenient reach of his hand. The reins contain a wire, one end of which terminates in the horse’s bit, and the other in the electro-magnetic apparatus. When the electro-magnet is put in action an electric current is generated, which gives the horse a shock in the mouth, and so astonishes him that he suddenly stops in his course. If the operator have the humanity and good sense to unite kindness to the abrupt application of the electricity,—which in our opinion should be only used as a last resource,—no doubt some excellent results may be obtained even with vicious animals.

Fig. 261.—The electric stamp.

M. Bella, the Director of the Omnibus Company, has reported that the apparatus was tried in his presence and found very successful, and quite easy of application; and that even the most unruly animals have been subjected by it. On one occasion a most restive animal was thus treated on the way to the forge. He had a tremendous objection to be shod, and made no secret of his dislike. But a gentle application of the electric current put quite an opposite complexion upon the matter, and after a few minutes the animal permitted himself to be patted and caressed, and even allowed the smith to feel his legs and inspect his feet without making any objection whatever. His shoes were taken off, and the horse was re-shod without any of the dangerous demonstrations hitherto indulged in by the animal.

We may quote another instance of the efficacy of this method, which is reported from Paris by M. Camille.

[Pg 252]

Fig. 262.—Electric apparatus for checking vicious horses.

“Many experiments have been made upon horses which had been most difficult to shoe, and in each case we have succeeded when the electric apparatus has been put in requisition. One horse, in particular, nothing could subdue. He kicked and bit and jumped about in such a manner as to render all approach impossible. We had recourse at length to M. Defoy’s apparatus, and after the first application, and without any great difficulty, we were able to raise the animal’s feet; but after a second lesson we were permitted to shoe him without his offering the slightest resistance. He was completely subdued.”

[Pg 253]

M. Defoy recently made the experiment with a very dangerous animal, which he stopped instantaneously in full gallop (see fig. 262). It may be remarked that the application of the current is not sufficiently strong to stop the horse too suddenly. It merely causes a very unpleasant sensation—he is not stupefied nor galvanized by the electricity. The narrator has felt the shock applied without inconvenience, and the conclusion arrived at is, that this method of employing electricity is far superior to the violent and inhuman treatment so often employed to break horses, which renders them subsequently sulky and vindictive.

M. Defoy has completed an electric bit and an electric stick quite as ingenious as the electric rein. The modus operandi is simple and effective, the wires being insulated by leather, and terminating at the extremities of the stick. The current is induced, as before, by a small magneto-electric machine.

Electric Time Ball.

[Pg 254]

CHAPTER XXI.
MAGNETISM.

THE LOADSTONE—MAGNETIC CURVES—THE MAGNETIC NEEDLE—THE MARINER’S COMPASS—MAGNETO-ELECTRICITY.

We have already mentioned some of the properties of the loadstone or magnet; but as we are now about to enter more fully into the considerations of its attributes and of the compass, etc., we will add some further interesting particulars.

Ancient writers (Pliny, Homer, and Aristotle) mentioned the existence of the magnet, and Humboldt refers to the knowledge of it possessed by the ancients. Pliny says “the magnet-stone is found in Cantabria,” and we have heard of the loadstones that are supposed to support Mahomet’s coffin at Medina. The origin of this fable was (probably) owing to the order given by Ptolemy to his architect, Dinochares. Ptolemy wished the roof of a temple at Alexandria to be roofed with the magnet-stone, so that the own image of his sister, Arsinoe, should remain suspended therein. But the death of the king and his architect prevented the project from being carried out.

The name “magnet” is said to have been derived from a shepherd named Magnes, who, when tending his flock on Mount Ida, found that his iron crook was attracted to a certain stone; and six hundred years before the Christian era Thales wrote respecting amber and the magnet; and because they attracted various substances, he supposed they possessed life and power. They were the germs of the science now so developed in their applications, and whose full powers we are scarcely yet acquainted with.

We may remark that other bodies besides iron and steel are capable of magnetization; nickel and cobalt have the like property. Magnetism, properly so called, treats of certain bodies known as Magnets, describes the properties they possess, and the influence of magnetic force upon other substances. Electricity and magnetism are always associated, but practically the force is the same, electricity being the current or motive power, so to speak; and it is to Faraday that the world is indebted for the discovery of magneto-electricity.

Epinus’ theory of magnetism was that all bodies possessed a substance he termed magnetic fluid, the particles of which repelled each other. But[Pg 255] while supposed to repel each other, they attracted particles in other bodies. Thus they attracted iron. Coulomb asserted that there were two fluids—a north and south fluid. Ampère’s theory was that magnetic bodies are made up of molecules, round which currents are always circulating in all directions when non-magnetized, but when magnetized the currents all flow in the same direction. The space through which a magnet diffuses its influence was called by Faraday the Magnetic Field. The lines of magnetic force will be understood from the accompanying illustration (fig. 263). If we cover a magnet with a paper and scatter iron filings over it, we shall see the manner in which the filings arrange themselves. They radiate in curves from the poles of the magnet, and are dependent upon its form. If it be a straight bar magnet, evenly magnetized, they will turn inward in oval curves.

Fig. 263.—Lines of magnetic force.

Fig. 264.—Magnetization.

The manner of magnetization has already been mentioned, but here we will give further illustrations of the method of magnetization. Four magnets are used, two being placed with their opposite poles apart, and upon them is placed the bar of which a magnet is to be made. Two other magnets separated by a piece of wood are then brought near, and subsequently drawn from the centre to the ends of the bar. This is the separated touch system; the double touch of Mitchell is completed by moving the upper two magnets from end to end backwards and forwards, and finally lifting them away from the centre.

A magnet, then, is a bar of steel endowed with certain properties, such as attracting iron, etc.; and electro-magnetism is the term applied to the production of magnetism by means of electricity, the medium being the[Pg 256] electro-magnet. To understand the science it will be necessary to mention Ampère’s theory of magnetism.

It was Œrsted who observed that when a magnet is placed within reach of an electric current and free to move, it sets itself at right angles to the direction of the current; and Ampère defined the law already referred to when treating of the electric current,—viz., “that if a person be imagined as placed in the wire so that the current shall pass through him from feet to head, if he turn his face toward the magnetic needle the north pole will always be deflected to his left-hand side.” When the current is passed above the needle from south to north poles, the deflection is to the west; when from north to south the deflection is east. When the current is below the needle the contrary is the case. Ampère decided that currents circulating in the same direction attracted each other, and when running in opposite ways they repelled each other. He supposed currents to circulate within all magnetic substances, and then—that is, when the body is magnetized—these currents flow in planes parallel to each other, and the material which offers the least resistance to the circulation of these currents becomes the most magnetic.

The earth being supposed to be an immense magnet has currents circulating through it in a direction from east to west; and having the property or power of turning a magnetized bar in a direction similar to that in which the bar would be turned by a magnet, the earth is considered a magnetic mass. This influence is due to what is called “terrestrial magnetism.” If we suspend a bar by a thread it will point in no particular direction, but may be turned towards any side we please. But when once the needle is magnetized it will point north and south; or, as we say (but not correctly), the north pole of the magnet points to the north of the globe. It is really the south pole that points to the north, and the north pole of the magnet points south, as can be proved by suspending the bar over another magnetized bar. So if the earth be considered a magnet our English terms are inconsistent with our theories. Continental writers are more correct.

The line of the magnetic needle’s direction, which differs in different places, is called the magnetic meridian, and the amount of its divergence from the astronomical meridian is termed its declination or variation. When the amount of this variation is known it is allowed for, and the needle can be considered as pointing due north and south.

But the needle does not assume a position perfectly parallel to the horizon. It dips down in different hemispheres. As we approach the north pole the dip or inclination will become greater, and the same effect is observable at the south pole. Again, there are certain places on the earth where the attraction is so evenly balanced that the needle is perfectly horizontal. The line uniting these places is the magnetic equator. This does not coincide with the earth’s equator any more than the magnetic poles coincide with the geographical poles of the earth.

The declination of the needle varies from the meridian of Greenwich at[Pg 257] different times. If we travel to the west the variation increases westerly, and is greatest in the Atlantic. It then decreases; the needle points due north in North America. Going still forward the variation becomes easterly, increases, and decreases to nothing in Eastern Russia. Thence the variations are westerly. Columbus discovered the variation of the compass needle in September 1493. In places where the needle is due north and south the lines drawn through them are termed lines of no variation.

The variation, however, is not always the same in the same place. In the year 1580, in London, for instance, the variation was 11° 11´ E. A little more than one hundred years later London was on the line of no variation, and now the tendency is westerly. On the other hand, there are places where there is no deviation, and Sir John Herschell says that West India property has been saved from litigation in consequence of the invariability of magnetic declination there, for all surveys were made by the compass. Lines of equal variation are called isogonial; those of equal dip or inclination, isoclinical; and those of equal intensity, isodynamical.

As we have said, the magnetic elements are not always the same, and the variations of the compass are daily and annually observed with certain instruments. What are termed secular variations take place at long intervals, as the following table will show:—

In the year 1818 therefore the maximum declination was reached in London. In Paris the maximum was arrived at in 1814, and was 22° 34´. The rate of decrease is about 8´ a year, but varies in different periods, as may be seen. The discovery of the fact that an annual variation took place in the angle of declination, is attributable to Cassini, and the diurnal variation was discovered by Graham in 1722. From 8 o’clock in the morning, when the needle is pointing a little to the east of its “mean position,” it turns towards the west until 1 p.m. It then returns towards the east again, and passing westerly again between midnight and three o’clock a.m., settles down till eight a.m., when it begins afresh. This variation does not apply to all places.

Magnetic inclination is besides subject to changes. There are also variations of magnetic force which occur at very irregular periods, and cannot be said to follow any laws. These disturbances are called Magnetic Storms, of which the Aurora Borealis is one result.

[Pg 258]

Professor Faraday in his memorable experiments divided a long list of different substances into para-magnetic and dia-magnetic bodies. He classed them under these two heads, according as they took up a certain position parallel or perpendicular to the axial or equatorial line. This definition of “dia-magnetic” was “a body through which the lines of magnetic force are passing, and which does not by their action assume the usual magnetic state of iron or loadstone.” He concluded that all bodies were magnetic, and by suspending a great number of various substances he found they placed themselves axially,—that is, lying between the poles of the magnet, or equatorially,—viz., at right angles to that line. If the magnets be suspended at each side the same bodies will assume a position with their longest diameters between the poles, while others will be repelled by the magnets even if the poles be reversed. So those bodies which are attracted and lie in the axial line are termed para-magnetic; those repelled into the equatorial line are termed dia-magnetic. In the “Proceedings of the Royal Society for 1846,” Faraday’s account of the various experiments can be studied in detail. We can only give a brief resumé of them here; and he showed that the motions displayed by dia-magnetic bodies in a magnetic field are all reducible to one simple law—viz., that the particles of the dia-magnetic tend to move into the positions of the weakest magnetic force.

He experimented upon a large number of bodies and gases; he tested crystals, metals, liquids, and solids, and proved in whatever state a body might be in the effect was the same; whether simple or compound, it made no difference. Of course in a compound the preponderance of the dia-magnetic or para-magnetic property would influence the result, and the medium in which the body operated on was placed, was a condition in the experiment. He proved that if a body be suspended in a medium or surrounded by a medium whose power either way is stronger than the body, that body is para-magnetic or dia-magnetic, according as it is surrounded by a medium whose power is weaker or stronger than the body itself. The arrangement of the bodies is as follows, from the para-magnetic to the dia-magnetic, bismuth being the most dia-magnetic of all:—

Para-magnetic Metals.

• Iron.
• Nickel.
• Cobalt.
• Manganese.
• Chromium.
• Cerium.
• Titanium.
• Palladium.
• Platinum.
• Osmium.

Dia-magnetic.

• Bismuth.
• Antimony.
• Zinc.
• Tin.
• Cadmium.
• Sodium.
• Mercury.
• Lead.
• Silver.
• Copper.
• Gold.
• Arsenic.
• Uranium.
• Rhodium.
• Iridium, etc.

Common air was also discovered to have a magnetic action, and hot air is more dia-magnetic than cold. Oxygen is as para-magnetic in the air as iron is on the earth, and this, it was considered, may give rise to magnetic storms, and account for the declination of the needle.

We may now proceed to consider the Mariner’s Compass. The compass, or the mariner’s compass, is so common that it is scarcely necessary[Pg 259] to give a long description of it. Its history is unknown. The Chinese seem to have been aware of its usefulness long before the western nations adopted it. It was about the time of the Crusades that it was brought into western prominence, but was not generally known till the thirteenth century. Chinese writings ascribe to the compass a great antiquity; they maintain that it was discovered two thousand five hundred years before the Christian Era, and used for travelling on land. But, according to other accounts, it was not used at sea till the year 300 A.D.

The Chinese put the south first when speaking of the points of the compass, and in the Chinese empire and Thibet west goes likewise before east. So the imperial edifices in China face the south, and the needle, in their expression, points south and north—not as we say, north and south. The antiquity of the compass may be inferred from the recorded fact in Chinese chronicles, written in the second century before the Christian Era, that nine hundred years previously to the date of the chronicle the Emperor gave magnetic cars to certain ambassadors to guide them home in safety. These cars were fitted with a magnetic needle which communicated with a figure. Its outstretched hand and finger followed the compass-direction, and pointed out the way.

The Chinese subsequently (in the twelfth century) suspended the needle by a thread, and it is said their philosophers at that time noticed the variation of the needle. But Columbus first, in 1492, and Cabot, in 1540, certainly remarked it in Europe. It is to Marco Polo that we are indebted for the direct introduction of the needle into Europe, although it probably had been in use in the Levant previously, for we have seen a quotation by an Arab writer, who, in 1242, described the needle as being used at that time on his voyage from Tripoli in Syria to Alexandria, two years previously.

Friar Bacon possessed a loadstone, and there are many instances in which it is referred to in ancient writings. The inventor of the compass we cannot trace, but no doubt exists as to its being of Chinese origin.

The ordinary compass is shown in the illustration herewith (fig. 265). It consists of a magnetized needle, suspended freely, and fixed to a circular card, which is divided and subdivided into thirty-two points, as in the cut. This compass is suspended upon gimbals to keep it in an upright position when the vessel rolls or plunges. The gimbals are concentric rings, the compass being fastened to the inner one, and keeps its position in all weathers. It is then enclosed in the binnacle, a glass receptacle. The card moves with the needle which points north. There is a dark line (lubber line) which indicates the ship’s course, and when sailing the steersman must keep that line opposite the compass direction-point which indicates the course. At night a lamp is lighted in the binnacle, and the card being transparent and the points opaque they are easily seen.

The magnetism of iron ships has a tendency to disturb the needle, and many suggestions have been made and discussed with a view to obviate this. To put the compass at the mast-head was one, to surround the compass[Pg 260] with “counter-irritants” another. But the usual way is to “swing the ship,” and so adjust the compass. Swinging the ship means turning her round point by point, and marking the deflection of the needle with reference to a certain object. The amount of deflection at each point is read and noted, and subsequently taken into consideration when sailing.

The Azimuth Compass is a mariner’s compass fitted with brass uprights slit through the centre, through which the heavenly bodies may be seen. These are the sights. The card is divided into degrees and quarters. A fine wire is fixed upon one of the sights, and in the other slit is a prism to reflect the divisions of the card to the eye. The object—the azimuth distance of which it is desirable to know—is looked at through the slit, and bisected by the wire. The divisions of the scale are at the same time reflected, and the number read gives the azimuth distance required.

Fig. 265.—Compass.

The compass has led us away slightly from our consideration of the electro-magnet, but we will now examine it and its effects as briefly as possible.

An electro-magnet is formed by wrapping a copper wire round a piece of soft iron shaped like a horse-shoe; the wire should be insulated with silk. If the wire be wound round the iron in the same direction, and a current be merely sent through the coil, it will be found that the horse-shoe iron is highly magnetic, but if the current be stopped the power is lost. Such magnets will carry weights much heavier than themselves, and by careful consideration of certain laws, and with reference to the number of coils and the strength of the current, these magnets will sustain a weight some thousands of times greater than their own weight.

Fig. 266.—Electro-Magnet.

If we cover a non-magnetic piece of iron with a wire coil, and taking a magnet turn it rapidly beneath the wire-bound iron, so that the magnetic poles approach each other alternately, an electrical current will be generated in the wire. The electro-magnetic machine is thus made; but although strong currents may be generated as a source of motive power it is a failure.

To Faraday our knowledge of magneto-electricity is due. “He knew” (says Professor Tyndall in his interesting work, “Faraday as a Discoverer”) “that under ordinary circumstances the presence of an electrified body was sufficient to excite by induction an unelectrified body. He knew that the wire which carried an electric current was an electrified body, and still all attempts had failed to make it excite in other wires a state similar to its own.”

But while he was making his experiments on the induction of electric currents he noticed that at the time the current was passing from the battery[Pg 261] through the coils of wire that no motion was perceptible in the galvanometer. But when the circuit was opened, and when it was closed, there was a slight motion of the needle in the galvanometer, but in different directions. After consideration the philosopher came to the conclusion “that a battery current through one wire induced a similar current through the other, but for an instant only.”

Œrsted had already demonstrated that all magnetic effects were attributable to the attraction and repulsion of electric currents; and founding his views upon the theory of Ampère, Faraday came to the conclusion that electricity could be produced from magnetism, or that the electric current could be obtained from magnets. This he succeeded in doing. By inserting a steel magnet about half its length into a coil of wire, Faraday induced a current to pass through the wire in two directions. Thus he proceeded to solve all the mysteries of magneto-electricity, and stated that to produce currents it was only necessary to “cut appropriately the lines of magnetic force.”

The application of the magnet to the machines for electric lighting will be shown further on. Very powerful currents are obtained by the induction coil; but the currents would not be of practical service were it not for the apparatus called a Commutator, or key, which reverses the connection of the bobbins, and turns the current at every half revolution. Just as if a current were being sent across and back over a table, and when the current has reached the end, an instantaneous wheel round, or pivoting of the table, sends the current on, in continuation (but on the table all the time), because of the sudden change of its position. The back rush being on the table, the movement of the latter really makes the line continuous, and by quickly breaking and reversing the current in the commutator, the effect is gained in the machine.

Electro-magnets and bobbin, etc. (Clarke’s machine)

[Pg 262]

CHAPTER XXII.

SUNDRY ELECTRICAL APPLIANCES—MR. EDISON’S INVENTIONS—THE ELECTRIC LIGHT—THE GYROSCOPE—A NEW ELECTROPHORUS—ELECTRIC TOYS.

The Electro-Motograph—although perhaps even yet scarcely developed—has already proved a very useful invention. The idea of it first occurred to Mr. Edison in 1873, when he was prosecuting some researches in chemical telegraphy. “One day,” says Mr. Fox, in his account of the invention, “as he sat pondering over his work, he happened to take in hand the metallic point through which, as it rested upon the paper, the current was wont to pass. When again he closed the circuit to let the current through the paper, he held the metallic point loosely, and unintentionally allowed it to rest upon the paper. Every time he moved the metallic strip on the paper the latter became wonderfully smooth. Edison was determined to find the reason of this, and he decided that the electricity very much lessened the friction of the metal on the paper. He made many experiments, and brought the subject before the Royal Society in 1874, but nothing came of the idea till 1876, when Edison was perfecting his musical telephone.

“The new appliance is, in fact, the same invention revived and now perfected by the original inventor, and brought to complete practical success under the title of the ‘electro-motograph.’ The action of the ‘electro-motograph’ depends on the fact, discovered during former experiments, and employed imperfectly in the musical telephone, that the friction of moving bodies varies in greater or less degree with their electrical condition. In the electro-motograph a cylinder made of prepared chalk, and saturated with a strong solution of caustic alkali, is set upon supports, so that it can be turned upon its axis. A strip of metal fastened to the mica diaphragm rests on the cylinder, and is pressed so firmly by its spring upon the cylinder that when it is turned by means of the handle the friction of the strip on the cylinder tends to pull the diaphragm out of shape, causing it to bulge inward as long as the cylinder is in motion. If now, while this motion of the cylinder is maintained, an electric current passes through the strip of metal, and then through the chalk cylinder to earth, the amount of this friction is varied or it is destroyed altogether, and the strip slides freely on the cylinder. This was the basis of the former invention. The[Pg 263] release from friction by a change in electric condition in the first instrument failed simply from ignorance of some slight matters of detail, that in the electro-motograph are corrected and made practical. In the musical telephone the releasing of the frictional resistance by electric action caused the sounding-board of a guitar to vibrate, and thus set up sonorous vibrations. In the electro-motograph the mica disc takes the place of the guitar, and, by the improved construction of the apparatus, intricate and complex vibrations, such as are produced in speaking, are reproduced in their original or even in greater volume. When the apparatus is at rest the diaphragm is motionless, and electric currents shot through the apparatus produce no effect. In the same manner the mere turning of the cylinder without electric action produces no effect, except to pull the diaphragm slightly out of shape. If while the cylinder is being turned an electric impulse arrives, the pull on the diaphragm, caused by the friction of the strip on the cylinder, is more or less released, and the diaphragm is free to vibrate or spring back into its original condition. If now, the electric impulses follow one another in regular order in correspondence with the sonorous vibrations imparted to the transmitting telephone, the alternate slipping and catching of the metal strip on the cylinder will follow in the same order, and thus the diaphragm will be made to vibrate in unison with the original vibrations, and thus reproduce the original words. As the mica disc is much larger than the disc of the transmitting instrument, the amplitude of its swing may be much greater, and consequently it will repeat the words with greater power. The electro-motograph is practically an apparatus for transforming electric action received from a distance into mechanical work. The amount of electric action has nothing to do with the amount of the mechanical work performed, because the movement of the cylinder is controlled by power independently of the electric action, the electricity merely releasing this power by destroying the friction in greater or less degree. The electric action set up by the sonorous vibrations at the transmitting end of the line may be very slight, while the mechanical action at the distant end may be powerful, and in this manner the amplitude of the vibrations may be increased to an indefinite extent, and a whisper may reappear as a loud shout.

“The electro-motograph is not only a solution of the telephone, making it capable of sounds of every quality and pitch and in greatly increased volume, but by this conversion of electrical action into mechanical work at a distance makes it possible to unite the telephone and phonograph. Telephonic messages by the electro-motograph may be impressed upon a self-acting (clock-work) phonograph, the same current starting and stopping the phonograph after the manner of the stock-reporting machines, and afterward the phonograph may be made to repeat the message impressed upon it.”

[The above extract, which explains the principle fully, has been taken from a long article on the subject which formerly appeared in Scribner’s Magazine.]

[Pg 264]

The uses to which the electro-motograph may be applied are various. It can produce mechanical motion even at a distance, and is useful to lessen friction by machinery; and in this way its service to railways and other locomotive systems may be estimated. It is a great help to telegraphy by increasing the speed of transmission, and can ascertain the beatings of the heart of the apparently dead. It amplifies sound in a much greater degree than the microphone, by which even a fly can be heard moving. In fact, the limit of the usefulness of this wonderful machine has not been reached.

Another very ingenious apparatus has been developed by Professor Bell. This is for the purpose of ascertaining the position of bullets in the body. The following is condensed from the Times:—

“Two conductors are used, and the ball completes the circuit. Professor Bell inserts a fine needle in the suspected region. It is connected by wire with one of the binding screws of a telephone, which the surgeon holds to his ear; the other binding screw being connected with a metallic mass applied to the skin. When the needle point touches the ball, an electric couple is formed, and the current generates the sound in the telephone. The surgeon may then use his knife with confidence, guided by the needle. He may make several insertions of the needle if necessary without danger, and any pain may be obviated by etherization. This simple method (which should prove useful on the field of battle) was tried with success with a lead ball introduced into a piece of beef. Contact of the needle with bone had no effect, but a very distinct sound was heard each time the ball was reached. A modification consists in inserting a vibrator in the circuit; this gives a musical note in the telephone at each contact of ball and needle. Again, if the circuit include a battery, the telephone sounds may be heard by several persons at once. A sound is heard, in this case, whenever the needle enters the skin; but, on reaching the ball, it is much intensified, owing to lessened resistance. A galvanometer may be used in place of the telephone.”

Mr. C. Vernon Boys has exhibited and described a very ingenious new integrating machine of his invention, and its application as a measurer of the electric energy in the circuit of an electric lamp or a dynamo-electric motor. Mr. Boys’ mechanical integrator belongs to the class termed tangent machines, and consists essentially of a small disc or wheel running along the surface of a drum or cylinder. When the wheel runs straight along the drum parallel to its axis there is no rotation of the latter, but when the wheel is inclined to the axis the drum rotates, and the integral is represented by the amount of rotation. Continuous action is secured in giving the drum a reciprocating motion along its axis, so that when the wheel has travelled to one end of the cylinder it can travel back again. The new integrator is especially adapted for measuring forces which are either delicate or variable. It is applied by causing the varying force to be measured to vary in a corresponding manner the inclination of the wheel to the axis of the rotating cylinder.[Pg 265] In this way it can be used to find the work done by a fluid pressure reciprocating engine, or the energy transmitted by a shaft or belt from one part of a factory to another. By making the wheel very small and light, the strength of an electric current may be continuously measured, if the disc is inclined by means of the needle of a galvanometer in circuit. Mr. Boys has constructed on the same principle an electric energy meter, which integrates the product of the strength of current and the difference of potential between two points with respect to time. In it the current is passed through a pair of concentric solenoids or coils of wire, and in the annular space between these is hung a third solenoid, the upper half of which is wound in the opposite direction to its lower half. By the use of what Mr. Boys calls “induction traps” of soft iron, the magnetic force is confined to a small portion of the suspended solenoid, and by this means the attracting force of the fixed solenoids upon it is independent of position. The middle solenoid is hung from the end of a balance beam, and its motion is retarded by a counterweight, which admits of regulating the meter to give standard measure as a clock gives standard time. The motion of the beam is caused to incline the integrating wheel, and the rotation of the cylinder gives the energy expended in foot-pounds by means of an indicator or diagram, as the case may be. The object in giving an equal number of turns in opposite directions to the suspended solenoid is to render the instrument insensible to external magnetic forces.

We have, in a former portion of this work, explained the construction of the telephone and phonograph with other inventions to make sounds audible at a distance, so we need not repeat the explanations here. A brief reference to them will, however, be found in this chapter, in which the electro-magnet and the methods of lighting by magneto-electric machines are treated of. We will proceed to give some particulars concerning the electric light before considering the means by which it is produced, as such an arrangement is more convenient.

The light is very easily produced by uniting and then separating the terminals of a strong battery. The passage of the electric current induces intense heat and a most brilliant light. But if this were continued the wires would melt, and therefore some non-fusible substance is placed at the ends of the wires, which will be at once a conductor and infusible. Now in gas-carbon (the deposited substance found in gas retorts) we have a substance suited to these conditions. The carbon is heated to an intense brightness, and particles of it are passed across the arc of flame almost in a state of fusion. Combustion does not actually take place, because it has been proved that the wires will give out light under water, and in the vacuum of an air-pump the light is even increased, so that had the oxygen of the air any part in the production of the light it would not remain unaffected under these conditions. The heat arising from this Voltaic arc is intense, and even platinum may be fused with the assistance of the gas carbon. The carbon points are of course liable to be worn away, and one side more than another. The positive pole[Pg 266] is generally more concave than the other, for it sheds its particles in a greater degree, and is the more intensely heated. The electric light first appeared in public at the opera in Paris in 1836, to illustrate a sunrise, but it was not till 1843 that it was experimented upon in the open air. We need not trace it farther at present, for a full account of its origin, rise, and progress is published in a small shilling volume by Messrs. Ward, Lock, & Co. We will proceed to the methods of bringing out the light.

Fig. 267.—The Maxim light.

Fig. 268.—Mechanism of Maxim’s lamp.

There are various lamps, many of which required a regulator in consequence of the wearing away of the carbon points, as already explained. We append two illustrations of the Maxim lamp, the invention of Hiram Maxim, of New York. In both cuts the letters refer to the same portions.

In the first illustration (fig. 267), A and B are the positive and negative carbon-holders respectively, and the carbon points are controlled[Pg 267] by an armature, which is, in its turn, adjusted by the screw, D. When it happens that the magnetic force is reduced the spring acts and permits the points to approach again, and the light is rekindled; the carbons are then locked till required to move. The second illustration (fig. 268) shows a section of the lamp with the wheel arrangements for controlling the advance of the carbon points as they waste away.

Fig. 269.—Wallace lamp.

Fig. 270.—Houston lamp.

In the “Brush” light, which is in use in London, and is fitted for large spaces, the carbon points are held by a regulator side by side, and they last eight hours without renewal. The power is generated by an electro-dynamic engine. We give illustrations of the lamps of Wallace and Houston (figs. 269, 270). The current is conveyed through b and the magnet, m. The armature, a, separates the electrodes, and the weakened current is restored by b, and the light continues. The pillar, p, is hollow, with a wire running through it. The positive electrode is supported by J, the negative by C; V is a button which comes in contact with the lever, T, when the carbon points are exhausted, and cuts the lamp out of the circuit by passing it direct through mercury cups.

The Jablochkoff candle and chandelier are also represented (figs. 271, 272). The candles consist of carbons connected at the top, but otherwise insulated, and fixed in a socket. They do not last very long without renewal. The exhibition at the Crystal Palace will be essentially an Electric Light Exhibition, and all the latest forms can be studied there. The great attraction will doubtless be, as at Paris, the varied and numerous inventions[Pg 268] of Mr. Edison. The early career of that American “magician” is now tolerably well known; his tremendous energy and application are fully appreciated. With only a few months schooling all his life he has taken a foremost place in the scientific world. In ten years he has invented the phonograph, the electric pen, a system of fast telegraphy, the electro-motograph, the telephone, a tasimeter, and other useful applications of electricity, besides solving the problem of electric light for domestic purposes.

Mr. Edison’s electric light18 requires something more than a passing notice, and we will therefore endeavour to give a sketch of the general subject. Now that the electric light has been made available for domestic purposes, and the very simple lamp (consisting of an exhausted glass globe, two platinum wires, and a piece of charred paper) can be obtained, people will no doubt soon largely adopt electric lighting in their houses. The light has found a success at the theatre, in the streets, and in the train; there is no reason why it should not be adopted generally, being more economical and more healthy than gas.

Fig. 271. Electric candle.

Fig. 272.—Chandelier.

If we sever an electric wire, and bring the ends, tipped with carbon, into juxtaposition, we obtain a brilliant light. This is the Voltaic arc we have already mentioned, produced by the incandescence of finely-divided matter; it was the first method of illuminating by electricity, and was discovered by Sir Humphrey Davy, who obtained a very brilliant light, but at great expense—about a guinea a minute! But the Daniell and Grove batteries and generators, and modern improvements in 1860, brought the use of the electric light into prominence. Faraday lighted a lighthouse with its assistance.

But when the Gramme Generator was invented the needed impetus was applied. The Jablochkoff candles followed, and now we have the electric light in full operation. So far we have sketched the history of illumination by the Voltaic arc, and descriptions of the various apparatus will be found at the end of this chapter. But the method of lighting with an incandescent solid was introduced in 1845 by Starr and Peabody, who took out a patent for the use of platinum. Later on Drs. Draper and Despretz made experiments with platinum and carbon. The latter gentleman sealed the carbon in an exhausted globe, and then introduced nitrogen in place of the air. But the method died out and was forgotten, and in 1873 a medal was actually given by the Academy of St. Petersburg for the “discovery” to Messrs. Sawyer and Mann.

In 1878 Paris was lighted with the electric candles of Jablochkoff.[Pg 269] This application of electricity stirred up our transatlantic cousins, and Mr. Edison was requested—backed up by many influential persons—to make the investigation whether the light could be produced for domestic purposes. The celebrated electrician undertook the commission, and certainly came unprejudiced to the encounter, for he had not at that time even seen an electric light.

He perceived at once that “permanence in the lamp and the subdivision of the light” were the two desiderata. He put the Voltaic arc aside as unsuitable, and addressed himself to the problem of obtaining the desired results from an incandescent solid. The subdivision of the light is really an important point, and a comparison between divided and undivided burners is in favour of the more diffused light in a number of burners. This subdivision Edison worked hard to secure, and, as it is said of him, “With a steadfast faith in the fulness of nature, a profound conviction that if a new substance were demanded for the carrying out of some beneficial project, that substance need only be sought for, he set to work.”

Mr. Edison found difficulties in his way. One was the apparent impossibility of illuminating by means of an incandescent solid, for even platinum will melt at a heat too low for use. But this apparent impossibility was overcome by the inventor’s genius. He, after many trials, found that if he raised the platinum to a white heat in a vacuum he would practically obtain a new metal which would sustain the required heat.

Fig. 273.—Edison’s platinum lamp.

“In making an electric lamp without a regulator,” says Mr. Upton, “two things are essential,—great resistance in the wire, and a small radiating surface. Mr. Edison sought to combine these two essential conditions by using a considerable quantity of insulated platinum wire wound like thread on a spool.” This platinum, as shown in the accompanying cut (fig. 273), was suspended in a glass bulb in vacuo, the air contained in it being expelled by electricity, heating it, and suddenly cooling the platinum, and squeezing out the air by the process. But, after all, the great difficulty of the inventor was to insulate his wires so perfectly that they would not meet and become a conductor. For, to perfect his lamp, this non-conducting principle was a necessity, otherwise the current would flow across instead of going all along the wires. He had previously made many uses of carbon, which we know is infusible. He tried lampblack tar, but it contained air, and would not do.

Thread answered his purpose, but was too fragile and uneven in texture. It suddenly occurred to him that paper—charred paper—cut into a thread-like form would satisfy all his conditions.

[Pg 270]

The problem was solved—the lamp was a fact. But how can paper, so easily burned, answer? We will endeavour to explain. “A piece of charred paper, cut into horse-shoe shape, so delicate that it looked like a fine wire firmly clamped to the two ends of the conducting and discharging wires, so as to form part of the electric circuit, proved to be the long-sought combination.”

We will now explain the construction of this little lamp, which is shown in the illustration (fig. 274) one-half of its actual size. The illuminating is equal to ten or twelve ordinary gas jets.

Fig. 274.—Edison’s electric lamp.

The manner in which the paper is prepared is, like many other very important inventions, extremely simple, and, we may add, almost costless. Cardboard will furnish us with the loops, and these “horseshoes” are placed in layers in an iron box with tissue paper between each. The box is then hermetically sealed, and made red hot. The carbonized paper remains till all the air has been got rid of, and although it will burn freely to ashes in atmospheric air, in the vacuum prepared for it it is never consumed. That is the plain fact—the secret of the Edison lamp.

A vacuum can now be produced almost perfect. It is of course impossible to extract every tiny particle of air from the globes, but by the Sprengel pump, in which mercury is employed, excellent vacuums are obtained. Several very curious phenomena have been observed in these vacuums, and the Royal Society has been engaged upon their consideration. Another advantage of the vacuum, as applied by Edison, is that little or no heat is generated. The electricity is all, or very nearly all, converted into light. Thus the glass globes remain almost unheated, and are unbroken.

The electric current passes along the wire, W, and at a certain place marked B, the copper is soldered to a platinum wire, which enters at C, and so by platinum clamps into the horse-shoe, L. The return wire is similarly arranged; the carbon is enclosed in a glass bulb, GG, and all the air is extracted by the pumps; the end is then sealed up by melting it at F.

The world is now in possession of a lamp for household use, and we are surprised that it is not more extensively adopted in England. There are some Swan lamps used in parts of the British Museum, and when we have explained the application of the light, and the uses to which the motive power can be applied, we shall, we believe, convince the most conservative gas bill advocate that Edison’s lamp is cheaper, safer, and far better in illuminating power than gas, if the success of the electric lamp can be assured.

[Pg 271]

We need not dwell upon the construction of the “pumping station,” for that is virtually what the magneto-electric generator is. Several of these stations can be established in various parts of the city, and each station will supply a district with electricity. The wires are laid in a tight box along the street, beneath the footpath, or other convenient position, and we are informed that the frost rather improves their electrical condition. Here is one advantage over gas.

From the main wires smaller ones enter the houses, and are carried through a “meter” containing a safety valve. There are two wires—a distributing wire and a waste—coloured, one red and the other green, which communicate respectively with the main supply and return wires to the “pumping station” or generator. The electricity is admitted between carbon points and flows round a magnet, the armature of which is held above it by a spring. If too much force be put on and any danger incurred, the magnet will attract the armature, and the current will cease. A snap connected by a small wire will then be closed by the electricity, and melting from the heat will cut off all the current. In ordinary circumstances the electricity passes through regulators (wire wound on spools) and on to a copper plate, “through a solution of copper salt.” Thus for every unit of current a certain quantity of copper is deposited. A certain standard amount represents five cubic feet, and the bills, based on the accumulation of copper, are made out like gas bills.

When the lamp is required a small handle is turned, and is instantly lighted; the reverse motion cuts off the current. “By touching a knob in the bedroom the whole house can be simultaneously lighted up” if desirable. No matches are necessary, as the lamps light themselves.

By adding a small electro-motor to the furniture of the house, and turning a handle, the sewing-machine can be worked by electricity, or lathes turned; and any business operations, such as lifting by cranes, etc., can be easily carried on.

The Swan electric lamps, which, with Mr. Edison’s, were exhibited in Paris, and will be found at Sydenham, give about twelve candle-power light. Edison’s lamps are made in two sizes, and vary accordingly. The Swan lamps give a very soft light, and are as easily manipulated as Edison’s. The Siemens system of lighting was also well seen in Paris, and the Faure storage system enables our trains to be lighted instantaneously by simply turning a handle. A full description of the Faure battery was given in the Times by Sir William Thomson, and in his address to the British Association at York in September last. He pointed out that in the accumulators of M. Faure,—which can be seen at 446, West Strand, London,—by means of a large battery it is quite easy to draw off electricity and to apply it as Edison proposed to do, in lighting our houses and do any little service. The electricity thus stored would be always ready for use, and would be supplied and paid for. It can be applied to any purpose, and locomotion by its means will ere long become more general. In Paris Dr. Siemens exhibited[Pg 272] his electric tramway. This was an improvement upon the first Berlin tramway, for in it the horses frequently received shocks which they resented. In the later application the current comes from the generator by metal rods carried above the heads of the passengers alongside the line. Little rollers upon these are united with an electric machine in the tram-car. The current is sent along the wires, and reconverted into mechanical energy in the second machine, turns the wheels of the cars. In this way, as the car proceeds, the rollers overhead or alongside the track are kept moving by the car, and the connection is never broken.

But this is a digression. The electric light as applied to lighthouses was also exhibited, and any reader desirous to obtain full information upon the subject of lights and lighthouses will find it in a very pleasantly-written work by Mr. Thomas Stevenson, in which the various systems of lighting by electricity and otherwise are fully recounted, the conclusion being in favour of electricity, which is employed and has been used for years in France and in some English beacons. If its penetrative power can be finally established,—for some authorities maintain that the electric is more easily absorbed by fog than other light,—there is no doubt about its being universally adopted.

It is very interesting to watch the uses to which the electric light is being put. The latest experiment has been made by an Austrian, Doctor Mikerliez. Almost incredible as it may seem, the interior of the human stomach can now be illuminated by means of a wonderful little instrument called the Gastroscope, which is said to be actually in use and to have been favourably reported upon by the medical faculty of Vienna. There is at the end of a jointed flexible tube (which can be passed down the gullet) a miniature lamp, far more marvellous and mysterious than that of Aladdin, in which a strip of platinum is fixed and connected with fine wires conducting the electricity from a small battery. When contact is made, and the “light turned on,” the cavernous interior of the stomach is lit up. Still more extraordinary is the fact that the tube can be made to revolve, and the light reflected from the walls of the stomach and directed to the eye of the observer. There is necessarily a bend in the instrument, so that the light has literally to turn a corner before it reaches the surgeon’s eye; here the inventor’s skill and thorough knowledge of the laws of optics are brought into requisition. The reflected rays of light fall upon a sort of window situated a little above the lantern, and by means of prisms and a series of lenses, the light is twisted and turned about until it arrives at the eye-piece. No sensation of heat is to be feared, the little lamp being kept constantly cool by a reservoir of water.

Several contrivances have been invented within the last few years for examining the interior of the body, but they are very costly; the Gastroscope is likely to render great service to medical science.

The term “magneto-electric machine” is given to a collection of parts of mechanism intended to create or gather together induced electric[Pg 273] currents. The invention of the magneto-electric machine was by no means a sudden inspiration, but the gradual result of a series of experiments and discoveries, the first of which, dating from 1820, may be said to be Œrsted’s observation, that a magnetised needle is deflected by the approach of an electric current as well as by that of a magnet, clearly proving that magnetism and electricity have some relation to one another. In the same year Arago discovered that a coil of insulated wire wound round a core of soft iron, converts it into a powerful magnet (i.e., an electro-magnet) when a current passes through the coil. It was in 1830, however, that our countryman Faraday proved the creation of a current by the action of a magnet on a coil of wire, and his experiment proved shortly as follows:—If a coil of wire be wound on a hollow core, and a permanent bar magnet be introduced into the hollow core, whilst introducing it a current may be proved (by a galvanometer), to be induced in the coil flowing in a certain direction, A B, which ceases as soon as the magnet is at rest in the centre of coil. On the withdrawal of the magnet a second current is induced flowing in the opposite direction, B A. Therefore it is clear that if a magnet be incessantly approached to and withdrawn from a coil of wire a constant succession of currents will be produced, and if a charged coil (i.e., a coil connected with the poles of a voltaic battery) take the place of the magnet a precisely similar result will be obtained. Now it will have been noticed that two opposite currents are constantly being formed, and as the object is to obtain a continuous flow of electricity in one given direction, or, in fact, divert or reverse the current instantly on its formation to make it practically the same current, for this purpose a commutator is used, and as for most purposes a commutator is one of the essentials of a magneto-electric machine, we will here give a description thereof. (See fig. 275.) The machine is composed of a cylinder, consisting of two metallic conducting halves, separated by a non-conducting layer. Whilst it is at rest the alternating currents, from being connected with the halves by the current, will pass to the two contact springs, and thence through the circuit. Now if (as is the case) the current is constantly changing, as has been noticed, the inverse current will at the first change pass through the same channels, but in another direction; but if at the instant of the reversal of the current the cylinder be revolved, the current flowing the reverse way will be guided through other channels respectively, instead of the original channels, and the direction of the current being changed at the same moment as the current itself, the two inversions neutralize themselves, and one constant current is produced. In a magneto-electric machine the commutator revolves identically with the magnet or armature, and the point at which sparks are being constantly produced is where the contact is being continually broken and made by the passage of the friction springs from over the non-conducting layer. The first machine formed on the basis of Faraday’s experiments was Pixii’s. It was composed of two uprights and a cross bar, to which is attached, hanging poles downwards, an electro-magnet; underneath this, the[Pg 274] poles upwards, revolves a magnet. The commutator is fixed on the same axle and revolves with the permanent magnet. Saxton, and subsequently Clarke, made the obvious improvement of making the magnet less cumbrous and fixed, and causing the bobbins of the electro-magnet to revolve before or rather beside its poles; the commutator was fixed at the end of the axle on which the revolving bobbins (or armature) are fixed. Niaudat formed a compound Clarke machine, by setting two horse-shoe magnets a short distance apart. The armature revolves between them, and consists of twelve coils set between two plates; the coils are set alternately and connected,—i.e., the poles of the electro-magnets are set beside one another,—N. to S., S. to N., and so on, so that the N. pole receding produces a current; but the N. pole receding makes the S. pole approach, and produces another current, A B; in fact, a continuation of the same, for the approach of a N. pole naturally produces the same current as the recession of a S. pole; then as the S. pole in turn recedes it produces an inverse current, B A, which is in turn kept up by the approach of the next N. pole, and so on. Each coil is attached to a radiating metal bar, which conveys the current to be redirected to the commutator, which is affixed to the axle of the revolving armature as in Clarke’s machine. In 1854 Siemens completed his machine, the chief peculiarity of which was its cylindrical bobbin; the core is grooved deeply, parallel with its axis, and the wire is wound on cylindrically and covered with plates of brass; one end of the coil is fixed to the metal axis, the other to an insulated ferule at the end of the axis, where is also situate the commutator. This armature revolves between the poles by which it is closely embraced. One of the most celebrated of the magneto-electrical machines is that known as the “Alliance,” invented by Nollet, and perfected by Van Malderen. It is composed of four or six bronze discs, revolving on an axle, round the external circumference of each of which are set sixteen bobbins. This rotating compound armature revolves between four to six sets of horse-shoe magnets, which, being fixed radially to the centre, present in each set sixteen poles to the sixteen bobbins. It will be readily understood that this immense quantity of poles and bobbins produces a highly concentrated current, the ends of which proceed from the axle and an insulated ferule at its extremity.

Fig. 275.—The Wallace Machine.

In 1869 Mr. Holmes perfected his machine, which differs from all previous ones (except Pixii’s), in that the electro-magnets revolve in front of the coils instead of vice versâ; and besides magnetising his electro-magnets with part of the self-produced electricity, his bobbins are so disposed as to be able to keep several independent lights going at once. The Wylde machine consists, as it were, of two Siemens machines, one on the top of the other, the lower and larger of which is worked by an electro-magnet, which is magnetised by the action of the upper or smaller one, consisting in the ordinary way of a permanent magnet apparatus, which is termed “the exciting machine.” The longitudinal bobbin revolved between these permanent poles produces alternating currents, which are commutated (or redirected), and[Pg 275] pass to work the larger and lower electro-magnet, which is composed of two large sheets of iron connected by a plate (on which stands the exciter). Its poles are two masses of iron separated by a layer of copper, and in this armature revolves the larger longitudinal bobbin. This lower machine is called the generator. Both bobbins are simultaneously revolved, and an intense current of electricity is thereby generated. Almost simultaneously with this one Mr. Ladd invented his machine, which is distinguished from all hitherto described by being composed of two parallel bar electro-magnets, between the extremities of which are placed two Siemens armatures, one smaller than the other; both being revolved, the smaller excites the electro-magnets, and the larger generates the electricity required. The wire is wound round the magnets so that the N. and S. poles face each other at each end. The chief advantage of the Ladd machine is the conversion of dynamic force into electricity, there always being just sufficient magnetism in an iron bar (by induction from terrestrial magnetism and other causes) to produce a very feeble current in the Siemens bobbin, and the bobbin taking it up and returning it to the electro-magnet, and the electro-magnet at once giving it back to the bobbin, the current gradually increases till the maximum is reached. And when we take into consideration this modicum of utilisable terrestrial magnetism, we may truly say in the words of M. Hippolyte Fontaine, “The mind is lost in contemplation of the succession of discoveries completing one another, and showing that with apparatus of small dimensions an infinite source of electricity could be produced if matter could withstand infinite velocities.” The Lontin machine, which supplied the current for the electric light which used to make night bright outside the Gaiety, is also composed of two parts, one dividing, the other generating the electricity produced. The principle of the dividing machine is somewhat similar to the alliance, excepting that a number of electro-magnets arranged radially round a core, revolve close to a corresponding number of bobbins fixed inside an iron cylinder, outside which is the collecting and dividing apparatus.[Pg 276] The Maxim machine is constructed on the principle of sets of coils rotating between powerful electro-magnets. The Wallace machine was invented by the inventor of the Wallace-Farmer lamp. It consists of two horse-shoe electro-magnets placed side by side, the opposing poles facing each other. Each magnet has a rotating armature of twenty-five bobbins, on which the wire is wound quadruply, and the current generated by these coils is conducted away, passing through and exciting the electro-magnets, thus utilizing the residual and terrestrial magnetism before mentioned in connection with the Ladd machine; otherwise it partakes of the nature of the Niaudet machine.

Fig. 276.—The Gramme Machine.

We now come to what is perhaps the most perfect magneto-electric machine, which was first constructed by M. Gramme, a Parisian, in 1872, and differs in principle and construction from all those hitherto noticed. Its essential characteristic is a soft iron ring, round which is coiled one single continuous wire (i.e., the two ends are joined). Round the exterior surface of the wire coil a band is bared, and on this bared part two friction springs act. If the ring and coil be placed before the poles of a magnet, the ring will have two poles, S. and N., induced opposite the opposing poles N. and S. of the magnet; and if the ring revolve the poles will remain stationary, and as the coil revolves each coil of the wire will pass this induced pole, and as naturally half the coil will be inducted with one current, the other half (acted on by the other pole) will be charged with another or opposite current, which two kinds of electricity are carried away by the friction springs before mentioned. In the machine, as actually constructed, the soft iron ring is composed like the magnet or wire bundle of an induction coil, and the coils are set upon it side by side. Inside the ring are radially set insulated pieces, to each of which is attached the issuing end of one and the entering end of another bobbin; these answer the same purpose as the denudation of the external layer of wire. These pieces are bent so as to come out of the centre of the ring at right angles, and lay side by side (insulated) round a small cylinder. These, as they revolve, are touched by friction springs, which draw off the electricity induced in the coils in one continuous current. No sparks are produced at the contact of the friction springs, and there is no tendency to become heated. To obviate the inconvenience of the secondary[Pg 277] or inverted current produced by the stopping of the machine, the inventor has contrived a circuit breaker on the principle of the electro-magnet, the magnets holding the circuit breaker in contact so long as the machine is working; but the decrease of velocity lessening the attractive power of the magnet, the circuit breaker opens by its own weight (or a counter-weight), and all danger of a reverse current is obviated. Experimental machines are manufactured by Bréquet & Cie (Paris), composed of Jamin’s magnets, and turned with a handle, and produce a force of eight Bunsen cells.

A great revolution, or rather the beginning of a new era in the history of electricity, may be said to have commenced with the perfection of M. Faure’s accumulators. These are troughs containing eleven lead plates, each coated with oxide of lead and wrapped in felt, the fluid being dilute sulphuric acid. The application of them to the electric light is one of their most valuable features; at the depôt in the Strand, where they may be seen at work, there are thirty such elements, each weighing about 50 lbs. It takes a two-horse-power engine working an Edison or Gramme machine six to eight hours to charge them, and when charged they will keep almost any number of lamps of sixteen-candle-power going some eight hours. They are used on the Brighton and South Coast Railway, and seem peculiarly adapted to lighting by incandescence, by Swan, or Edison’s lamp. The elements fully charged may be carried any distance without losing their electric power. And the stored force may be used for charging the accumulators themselves afresh from the machine. These accumulators may be seen any day at 446, Strand, and are well worth a visit.

The Gyroscope, though now an instrument common and familiar to all students, is none the less the subject of a problem, the solution of which is still to seek. It has indeed been entitled the paradox of mechanics; for though it depends on gravitation, gravitation yet appears indifferent to it. In order to render the operation of the Gyroscope as continuous as possible, so as to facilitate the profound study of its working, and also to unite another influence with those of the ordinary Gyroscope, producing phenomena of which this instrument affords us the spectacle, a learned American has employed electricity as a motive power.

Fig. 277.—The Gyroscope.

The Gyroscope, shown in fig. 277, has a large, heavy pedestal, with a pointed column, which supports the instrument itself. The frame, of which the electro-magnets form a part, is connected with a rod, having at one end a hollow cavity which rests on the point of the vertical column. One of the extremities of the magnetic spool is attached to this cavity, the other end communicating with the bar which unites the two magnets. Over this bar is a spring which breaks the current, supported by an insulator in hard india-rubber; it is adjusted so that it touches a small cylinder on the axis of the wheel twice during every rotation of the latter. The wheel’s plane of rotation is at right angles with the magnets, and it carries an armature of soft iron, which rotates close to the magnet without touching it. The armature is so placed in relation to the surface of contact with the cylinder that breaks the current, that twice[Pg 278] during each rotation, as the armature approaches the magnets, it is attracted; but immediately afterwards, as the armature comes directly in front of the magnets, the current is broken, and the acquired impulsion is sufficient to move the wheel until the armature comes again under the influence of the magnet. The spring which interrupts the current is connected with a thin copper wire, which stretches back as far as the point of the column, entwining it several times to render it flexible, finally bending down and plunging into some mercury enclosed in a round vulcanite cup placed on the column near the pedestal. The pedestal also bears two small stakes[Pg 279] for receiving the wires of the battery, one connected with the column, and the other communicating by a small wire with the mercury contained in the vulcanite vessel. The magnets, the wheel, and all the connected parts can move in any direction round the point of the column. When two large Bunsen cells, or four small ones, are connected with the Gyroscope, the wheel turns with great rapidity, and allowing the magnets to operate, it not only sustains itself, but also the magnets and the other objects which are between it and the point of the column in opposition to the laws of gravitation. The wheel, besides turning rapidly round its axis, also effects a slow rotation round the column in the direction of the movement experienced by the lower part of the wheel. By placing the arm and the counterpoise of the machine as shown in fig. 277, so that the wheel and the magnets balance exactly on the pointed column, the whole machine rests stationary; but if we give the preponderance to the wheel and the magnets, the apparatus begins to rotate in a direction contrary to or following that of the upper part of the wheel.

The Gyroscope exemplifies very clearly the persistence with which a body undergoing a movement of rotation maintains itself in the plane of its rotation in spite of gravitation. It shows also the result of the combined action of two forces tending to produce rotations round two axes which are separate, but situated in the same plane. The rotation of the wheel round its axis, produced, in the present instance, by the electro-magnet, and the tendency of the wheel to fall or turn in a vertical plane, parallel to its axis, produce, as a result, the rotation of the entire instrument round a new axis which coincides with the column.

Peiffer’s Electrophorus.

It will now perhaps interest our readers to describe a charming little plaything which is a great favourite with children, and which has the incontestable merit of early initiating them into all the principal phenomena of the statics of electricity, and teaching them the science of physics in an amusing form.

It is a small electrophorus invented by M. J. Peiffer, and reduced to such a point of simplicity, that it consists merely of a thin plate of ebonite, about the size of a large sheet of letter paper. The tinned wooden disc of the electrophorus which is found described in most treatises on physics, is replaced by a small sheet of tin, about the size of a playing-card, fastened on to the surface of the ebonite. The ebonite electrophorus produces electricity with remarkable facility. It must be placed flat on a wooden table, and thoroughly rubbed with the hand; if it is then lifted, and the sheet of tin lightly touched, a spark is elicited from ¼ inch to ½ inch in length. The electrophorus is completed by the addition of a number of small accessories in the shape of small dolls made of elder-wood, which exhibit in a very amusing manner the phenomena of attraction and repulsion. After the board has been charged with electricity, place the three little[Pg 280] figures on the sheet of tin, and lift up the apparatus, so as to isolate it from any support. You will then see one little doll extending its arms, another with its silky hair standing on end, and a third, lighter than the others, leaping like a clown, and displacing as he does so the two small balls of elder-wood which have been placed on each side of him. We have given an illustration of the three figures performing at once (fig. 278), but they are generally used separately. M. Peiffer has indeed collected every known accessory for an electric machine, such as Geissler’s tube, the electric carillon, etc. These different experiments are all reduced to their simplest form, and, with their appliances, are all contained in a cardboard box. They are placed beside the electrophorus, which thus takes the place of an unwieldy electric machine. M. J. Peiffer accompanies this little portable cabinet with an exhaustive pamphlet, which is a valuable guide to the young physicist in studying the first principles of electricity.

Fig. 278.—M. J. Peiffer’s electrophorus with dolls.

“It is easy to discover in the education of children,” says M. Peiffer in his preface, “how to turn their budding faculties to the best account. Would you utilize them in a satisfactory manner?—Then put in their hands playthings which, in an attractive form, serve to familiarize them at an early age with those sciences, a knowledge of which will be at a later period absolutely indispensable to them; and they will be much more amused than with ordinary commonplace toys.”

These are sensible words, in which we heartily concur. Yes! Science properly taught, and properly understood, can indeed be brought within the range of children; it should give a lasting interest to all amusements, and form a part of the culture of the youthful mind, as at a later period it will contribute to the perfect development of the grown man.

[Pg 281]

Magic Fish.

An ingenious physicist, M. de. Combettes, who is a civil engineer at Paris, has devoted himself to constructing a number of playthings and scientific appliances for young people, among which we will describe the very curious one represented in fig. 279. A jar is filled with water, holding in suspension some fish made of tin, similar to those which children put in water and attract with a magnet. In this case, however, the mechanism is hidden, and the operator can turn the fish first in one direction and then in the other at pleasure. The secret of this experiment is easily explained by examining the illustration (fig. 279). In the wooden stand which supports the jar there is concealed a small electro-magnet which acts on the soft iron contained in the floating fish. When the current passes the small magnet turns round and attracts the little fish swimming in the water. This gyratory movement can be changed at pleasure by means of a regulator.

Fig. 279.—Experiment of magic fish set in motion by electricity.

Fig. 280.—An electric toy.

We will give an illustration of a few electric toys which M. Trouvé has found for us. In the picture (fig. 281) we see three different objects,—a rabbit beating a small bell, a representation of a bird with outstretched wings, and a pin surmounted by a skull. All these are capable of having movement imparted to them by means of electricity, although made and intended for ordinary use in the form of scarf-pin or other ornament.

Let us take the “death’s head” pin first. It is in gold, and enamelled[Pg 282] with diamond eyes and articulated jaws. The rabbit is also gold, and carries two small drumsticks, with which he can play a tiny bell. This device also can be worn as a scarf-pin.

Fig. 281.—Magic toys.

A conducting wire leads from the pin into the waistcoat pocket, where a small “pile,” about the size of a cigarette, is hidden away. If any one particularly admires the scarf-pin, all you have to do is to insinuate your fingers into your pocket, and you will, by contact, cause the electric current to act upon the pin in your scarf. The death’s head will at once begin to roll its eyes and grind its teeth, while the rabbit, under similar circumstances, will begin to play its bell with the greatest energy.

[Pg 283]

The handsome diamond bird represented in the centre of the illustration belongs to Madame de Metternich. When any lady wears it in her hair, she can, by the concealed wire, make it flap its jewelled wings, and by so doing cause much surprise amongst the spectators.

We will now endeavour to give a description of the manner in which these toys play their parts in company with the “hermetic-pile” which M. Trouvé has applied to many specialities that he has supplied to doctors, who use them largely.

This pile is formed by a “couple” of carbon and zinc hermetically enclosed in an ebonite box. The carbon and zinc only occupy one-half of the case. The liquid occupies the other. The sketch (fig. 280) on preceding page will explain the apparatus.

So long as the case is in its normal position the elements are not immersed in the solution, and consequently no electricity is developed. But as soon as the figure is placed in a horizontal or leaning position the force is generated; on readjusting the box the electric current is cut off, and all development ceases. Many curious electrical toys can be seen in Paris. Dolls are made to talk, and many other wonders for children can be easily procured.

Animal and Atmospheric Electricity.

Before concluding the subject of electricity we must devote a few pages to the consideration of the electric influence possessed by certain fishes, and to some of the phenomena of the atmosphere, especially thunderstorms. We have seen how Galvani experimented upon the limbs of frogs, and maintained that they possessed electricity; he attributed the current in the muscles to that cause. This theory Volta denied, but subsequently Nobili, in 1827, proved the existence of a current in the frog by means of a Galvanometer. This was conclusive, and the experiment was performed in the following manner:—He filled two vessels with salt and water, and into one he dipped the crucal muscles of a frog, and in the other the lumbar nerves were immersed. By putting these vessels in communication with his improved Galvanometer, which was extremely sensitive, he perceived a current passing from the feet towards the head of the animal.

It is, however, to Matteucci and Du Bois Reymond that the investigation of the phenomena of the courant propre are due. The former formed a “pile” of the thighs of frogs, and by placing the interior and exterior muscles in contact he formed a current from the inside to the outside muscles. This current is supposed to be occasioned by certain chemical changes which are continually taking place, and it continues longer in the case of a cold-blooded animal than in a warm-blooded one. There are many interesting papers on this subject included amongst the “Philosophical Transactions”; and the “Physical Phenomena of Living Beings” is fully treated in Matteucci’s lectures on that subject. In the “Transactions” for 1848 and subsequent[Pg 284] years, other experiments may be perused, but space will not permit us to dilate upon them. The fact has been established, and we are told that muscles and nerves, as well as certain glands of the body, possess certain electrical properties.

The electricity of fishes, and the power possessed by the torpedo—whose name is now chiefly known in connection with warlike appliances—and the gymnotus, have been known for a very long time. This fish, popularly known as the electric eel, inhabits the warm fresh-water lakes of Africa, Asia, and America. A specimen was exhibited at the Polytechnic some years ago. This was the fish experimented on at the Adelaide Gallery by Professor Faraday, who clearly demonstrated the fact that the electricity of the animal and the common electricity are identical. Numerous experiments were made, and the circuit shock and even sparks were obtained from the gymnotus. In fact, the gymnotus is a natural electric machine. The force of the shock given by the electric eel is very great, for Faraday has put on record that a single discharge of the eel is equal to fifteen Leyden jars charged as highly as possible. Its power does not even end there, for having shocked people to that extent, it was capable of a second and occasionally of a third shock of less violence.

Fig. 282.—Electric eel.

The manner in which the gymnotus acts is from a regular battery in the head, the sides of which are filled with a fluid. These cells are something like a honeycomb in appearance. The shock is quite voluntary on the part of the fish. Sometimes it will kill its prey, on other occasions it is merely numbed. Professor Faraday on one occasion placed a live fish in the tub with the gymnotus, which curled itself so as to enclose the unsuspecting one. In a second the prey was struck dead, and floated on the water. The gymnotus immediately devoured it, and went in quest of more. Another, but an injured fish, was then introduced, but the electric eel took no trouble about this one. It did not trouble to give it a shock, seeing it was disabled, it merely swallowed without killing it. It is also on record that on one occasion an electric eel had stunned a fish which, before he began to eat it, gave signs of returning animation; the eel immediately gave it another shock and killed it.

Fig. 283.—Large gymnotus.

There were some other curious peculiarities connected with the electric eel. It appears to be quite capable of discriminating between animate and[Pg 285] inanimate touch. For instance, when touched with a glass rod it at first gave signs of electricity, and discharged a shock at the attacking party. But on subsequent occasions, when touched with metal rods or glass, the fish declined to “shock”; nevertheless the Professor succeeded the moment he touched the animal with his hands.

The torpedo is something like the well-known skate; it is sometimes called the electric ray, and is common enough in the Bay of Biscay and in the Mediterranean Sea. It sometimes pays England a visit, or is caught by fishermen and brought in. We have seen one at Plymouth, and a very ugly-looking fish it was. Its electric power is considerable.

Fig. 284.—Ray torpedo.
c, brain; m e, spinal chord; o, eye; e, electric organs; b, gills; np, nl, nerves; n, spinal nerve.

There is yet another fish known as the malapterurus; one species is called the thunder-fish. Professor Wilson has written a paper upon the electric fishes as applied to the remedy of disease, and considers them the “earliest electric machines ever known.”

Humboldt relates that the South-American Indians capture the gymnotus by driving horses into ponds which the electric eels are known to inhabit. The result is that the fish deliver shock after shock upon the unfortunate quadrupeds. Mules and horses have frequently been killed by these powerful eels, and even Faraday experienced a very great shock when he touched the head and tail of the captive gymnotus with either hand.

The malapterurus to which we have referred is an inhabitant of the African rivers, chiefly in the Nile and Senegal. Such a fish has been known with others for some hundreds of years; its electrical powers are not great. There are one or two other species of fish which possess electrical qualities, but none apparently to the same extent as the torpedo and the gymnotus.

Fig. 285.—The Malapterurus.

The electricity of plants also is in some cases very marked. Flashes have been seen to come from some flowers in hot and dry weather. Currents of electricity have been detected, and Wartmann investigated the subject closely. He says the currents in flowers are feeble, but in succulent fruits and some kinds of grain they are very marked. These currents depend[Pg 286] upon the season, and are greatest in the spring, when the plant is bathed in sap. These experiences were confirmed by Bequerel in 1850, and he concludes that the rank vegetation in some parts of the world must exercise considerable influence on the electric phenomena of the atmosphere. M. Buff has more recently made experiments in this direction, and he examined plants and trees, and even mushrooms. M. de la Rive, after carefully summing up the various theories, comes to the conclusion that it is to chemical reactions that the traces of electricity are due.

The subject of atmospherical electricity properly belongs to meteorology, and under that heading we will treat of it more fully. But lightning is so identified with electricity, and being the most common form observable to all, we will say something about thunderstorms and the electric discharges accompanying them.

Fig. 286.—Benjamin Franklin.

Before Franklin’s ever-memorable experiment with his kite established the identity of lightning and electricity, the resemblance between the two discharges had been frequently noticed. The Etruscans pretended to bring down lightning from heaven, and Tullus Hostilius, when experimenting or performing certain “ceremonies,” was killed by the electric discharge he desired to attract. But after all, we cannot attribute any knowledge of electric science to the ancients, although they were, of course, familiar with electric phenomena.

It is to Dr. Wall that testimony points as the first person who remarked the analogy between the electric spark and lightning. This was in 1708. Grey and other philosophers supported the theory, but could not establish it. To Franklin, who in June 1752 actually brought down the[Pg 287] lightning by his kite and a key, is the actual discovery due. We have already detailed the circumstances (page 206) and need not repeat the account of the experiment.

Of course the American philosopher found numerous imitators, not always with impunity. Professor Richmann was killed by the spirit he was invoking; Lemounier and Beccaria confirmed the theory that the air was full of electricity; while Du Saussure, from his investigations on the Alps, and Volta from the invention of the pile, are most famous in the history of electricity. They applied themselves with much success to the investigation of the electric condition of the atmosphere, of which the disturbances called thunderstorms are the result.

The amount of electricity varies in the atmosphere at different times in the day and night. Towards midday and midnight the development is generally greatest, and this fact will account for the prevalence of storms during our hours of rest. Again, different kinds of clouds have different degrees of electricity, and of different kinds. Under certain conditions these clouds will give forth lightning, and a storm will begin. The more clouds the more globules, and therefore in summer, while there is more production of vapour from solutions of salts, etc., we are more likely to have the storms. We are most of us familiar with the mass of the “thunder cloud” rising in the distance, light at the upper part, very dark below, and throwing out tentacles like the octopus, coming up sometimes—frequently, indeed—“against the wind,” impelled by an upper current, or following the course of a river, which is not unusual. Below, there is perhaps an army of thin dark clouds. The nature and height of clouds have also a great deal to do with the phenomena displayed. In general, storm-clouds are positively electrified.

Fig. 287.—Cirrus cloud.

Clouds are good conductors of electricity, and yet they may be so insulated by the dry air surrounding them that they will accumulate it; and when thus charged, if they encounter other clouds charged with opposite electricity, the opposing masses will attract each other until a discharge[Pg 288] takes place. This is what we term lightning, and under such conditions electricity, though very dazzling, is harmless. It is when the cloud comes near to the earth, and a discharge is released, that lightning is so dangerous to persons who remain in the fields. Sometimes the discharge comes from the earth to meet that from the cloud. Sheep are frequently killed by ground lightning, and once, at Malvern, we had an escape from an upward stroke. The back-stroke from a cloud is also dangerous. It may happen that the cloud has discharged itself upon the earth many miles away, but a return discharge takes place at the other end, and if that end be near the earth the consequences may be serious. As a rule, the return stroke is not so violent as the first discharge.

The colour of lightning varies very much. We have the white, the blue, the violet, and red. The colour depends upon the distance and intensity of the lightning, and the more there is of it the whiter the light. We can illustrate the varied hues of the electric “fluid” by passing a spark through the receiver of an air-pump. If the air be rarefied, or there be a vacuum, we shall perceive a blue or violet light. Therefore we may conclude that the blue and violet flashes have birth in high strata of the atmosphere.

Fig. 288.—Cumulus cloud.

We have all heard how dangerous it is to stand under a tree during a thunderstorm, or rather, we should say, when the storm is approaching us nearly. The tree is a conductor, and the lightning having no better one at hand will pass through the tree on its way to the earth, and if we are standing against the tree we shall be included in the course, and die from the shock to the nerves while the lightning is passing through us. The best position in a thunderstorm, if we are in the neighbourhood of trees, is to sit or lie down on the ground some little distance from the base of the nearest tree. If the tree be sixty feet high suppose, and we sit fifty feet or less from the trunk, we shall be pretty safe, because the lightning will reach the tree top before it can reach us. We are protected by it as by a conductor, bad though it be. Standing up in a boat during a storm is not wise. Lightning has an affinity for water, and besides, if no higher objects are near, our body will act[Pg 289] as a conductor. Bed is the safest place, as blankets are non-conductors. Cellars are not the safest by any means. Lightning may, and it frequently does, strike the house and descend to the basement. If the air be very full of electricity, and a flash be near, a person running away may conduct the lightning to himself by creating a vacuum into which the flash may dart.

Fig. 289.—Nimbus, or rain cloud.

Arago classified lightning into three kinds: zig-zag, globular, and sheet. The first we call forked lightning, and frequently this kind branches out at the end, so that although there may be only one flash, it may strike out in two or three directions at the same time. This may be accounted for by the unequal power of the air strata to conduct the electricity. The forked flashes are of very great length, extending frequently for miles, and the bifurcations also are often miles apart. The duration of the flash is less than the thousandth part of a second; so instantaneous is it that no motion can be perceived even in a most rapidly-moving wheel, as proved by Professor Wheatstone. We sometimes fancy that the flash lasts longer, but the impression received by the eye quite accounts for the apparently prolonged sight of the lightning.

Sheet lightning, the faint flashes frequently seen upon the horizon, are quite harmless. Sheet lightning is that which is seen reflected behind clouds or from far-distant storms. It is sometimes very beautiful. Ball, or globular lightning, is dangerous, and globes of fire have been seen to descend, and striking the ground, bound onwards for some distance. The descent of these forms of electric discharge has given rise to the popular notion of “thunderbolts.” The “Mariner’s Lights,” or St. Elmo’s fire, is frequently observed in ships. It is usually regarded as a fortunate occurrence. It was noticed by Columbus. One voyager describes the phenomena as follows:—“The sky was suddenly covered with thick clouds.... There were more than thirty of St. Elmo’s fires on the ship. One of them occupied the vane of the mainmast. I sent a sailor to fetch it. When he was aloft he heard a noise like that which is made when moist gunpowder is burned. I ordered him to take off the vane. He had scarcely executed this order, when the fire quitted it and placed itself at the apex of the mainmast, whence it could not possibly be removed.”

[Pg 290]

Fig. 290.—Thunderstorm.

There have been occasions when the manes and tails of horses, and even the ears of human beings, have shown a phosphorescent light which emitted a hissing noise. Alpine travellers have noticed similar phenomena; and Professor Forbes, when crossing the Theodule Pass into Italy, heard[Pg 291] the hissing sound in his alpenstock. The tips of rocks and grass points were all hissing too. The party were in the midst of an electric cloud. When the Professor turned the point of his alpenstock upwards, a vivid flash was emitted, but no thunder followed. They descended as quickly as possible from such a dangerous neighbourhood.

Fig. 291.—St. Elmo’s fire.

It is observable that the properties of lightning and of the electric spark are identical—the faint crackle of the latter being magnified into the loud rolling of the thunder. The disturbance of the atmosphere is the cause of the loud reverberations, and echoes produced from clouds tend to intensify and prolong the peal. The sound rises and falls, and varies accordingly as the cloud is near or far. A smart sharp report or rattle denotes the nearness of the lightning, while the gradual swelling and subsidence, followed, mayhap, by an increasing volume of sound, which in its turn dies away, tells us that the danger is not imminent. The cause o£ this loud rolling, unless it proceeds from echoes from different clouds, has not been satisfactorily explained. Sound travels less quickly than light, and therefore we only hear the thunder some seconds after we have perceived the flash. It is therefore conceivable that we may hear the last reverberations and its echoes first, and the sound of the first disturbance with its echoes last of all. Thus there will be distinct sounds. Firstly, the actual noise we call thunder from the air strata nearest to us; secondly, the echoes of that disturbance from the clouds, of course fainter; then we hear the sound caused by the tearing asunder of the air particles farthest off, and again the echoes of that disturbance. This theory will, we think, account for the swelling peals of thunder, and the successive loud and fainter reverberations. At any rate, in the absence of any other theory, we submit it for consideration. The sound of thunder is seldom or never heard at a distance greater than fifteen miles.

[Pg 292]

Lightning conductors are such every-day objects that no description is necessary; but the reason the lightning runs along it harmlessly is because the galvanized iron rod is the best conductor in the immediate neighbourhood. Where there is not a good conductor lightning will accept the next best, and so on, any conductor being better than none. The point of the rod cannot contain any electricity, there being no room for it, and the “fluid,” as it is termed, runs down to the ground, to terminate, when possible, in water or charcoal. A great deal of electricity is no doubt carried away from the air by the numerous conductors without any spark passing. Until Sir W. Snow Harris brought forward his lightning conductors for ships, the loss was great at sea. But now we rarely hear of any vessel being disabled by lightning. We owe to Franklin the idea of the lightning conductor.

According to observations made by Mr. Crosse, the following statement shows the tendency of the atmosphere, in certain conditions, to thunderstorms. We may accept the deduction of M. Peltier that grey and slate-colour clouds are charged with negative, and white, rose-colour, and orange clouds with positive electricity. The order of arrangement in the following table places the most likely source of thunderstorms first, and the least likely source at the end, with regular rotation of intermediate probabilities intervening:—

1. Regular thunder clouds.
2. Driving fog with small rain.
3. Fall of snow, or hailstorm.
4. Smart shower on a hot day.
5. Smart shower on a cold day.
6. Hot weather after wet days.
7. Wet weather after dry days.
8. Clear frosty weather.
9. Clear warm weather.
10. Cloudy days.
11. “Mackerel” sky.
12. Sultry weather and hazy clouds.
13. Cold damp night.
14. Cold, dry north-east winds.

We have thus briefly touched upon some of the atmospherical phenomena directly attributable to electricity. In our articles upon Meteorology we will consider the aurora and many other interesting facts concerning the atmosphere, and the effects of sound, heat, and light upon the air.

Fig. 292.—Lightning conductor.

[Pg 293]

CHAPTER XXIII.
AERONAUTICS.

PRESSURE OF AIR IN BODIES—EARLY ATTEMPTS TO FLY IN THE AIR—DISCOVERY OF HYDROGEN—THE MONTGOLFIER BALLOONS—FIRST EXPERIMENTS IN PARIS—NOTED ASCENTS.

In the first part of this volume we entered into the circumstances of air pressure, and in the Chemistry section we shall be told about the atmosphere and its constituents. We know that the air around us is composed principally of two gases, oxygen and nitrogen, with aqueous vapour and some carbonic acid. An enormous quantity of carbonic acid is produced every day, and were it not for the action of vegetation the amount produced would speedily set all animal life at rest. But our friends, the plants, decompose the carbonic acid by assimilating the carbon and setting free the oxygen which animals consume. Thus our atmosphere keeps its balance, so to speak. Nothing is lost in nature.

We have illustrated the pressure of the atmosphere by the Magdeburg hemispheres, and we know that the higher we ascend the pressure is lessened. The weight of the atmosphere is 15 lbs. to the square inch at sea level. This we have seen in the barometer. Now pressure is equal. Any body immersed in a liquid suffers pressure, and we remember Archimedes and the crown. It displaced a certain amount of water when immersed. A body in air displaces it just the same. Therefore when any body is heavier than the air, it will fall just as a stone will fall in water. If it be of equal weight, it will remain balanced in the air, if lighter it will rise, till it attains a height where the weight of the atmosphere and its own are equal; there it will remain till the conditions are altered. Now we will readily understand why balloons float in the air, and why clouds ascend and descend in the atmosphere.

In the following pages we propose to consider the question of ballooning, and the possibility of flying. We all have been anxious concerning the unfortunate balloonist who was lost in the Channel, so some details concerning the science generally, with the experiences of skilled aeronauts, will guide us in our selection of material. We will first give a history of the efforts made by the ancients to fly, and this ambition to soar above the earth has not yet died out.

[Pg 294]

From a very early period man appears to have been desirous to study the art of flying. The old myths of Dædalus and Icarus show us this, and it is not to be wondered at. When the graceful flight of birds is noticed, we feel envious almost that we cannot rise from the earth and sail away at our pleasure over land and sea. Any one who has watched the flight of the storks around and above Strasburg will feel desirous to emulate that long, swift-sailing flight without apparent motion of wing, and envy the accuracy with which the bird hits the point aimed at on the chimney, however small. It is small wonder that some heathens of old time looked upon birds as deities.

The earliest flying machine that we can trace is that invented by Archytas, of Tarentum, B.C. 400. The historian of the “Brazen Age” tells us how the geometrician, Archytas, made a wooden pigeon which was able to sustain itself in the air for a few minutes, but it came down to the ground after a short time, notwithstanding the mysterious “aura spirit” with which it was supposed to be endowed. The capability of flying has for centuries been regarded as supernatural. Putting angels aside, demons are depicted with wings like bats’ wings, while witches, etc., possessed the faculty of flying up chimneys upon broomsticks. We even read in childish lore of an old woman who “went up in a basket” (perhaps a balloon-car), and attained a most astonishing altitude-an elevation no less than “seventy times as high as the moon!”

But to descend to history. It is undoubtedly true that in the time of Nero, Simon Magus attempted to fly from one house to another by means of some mechanical contrivance, and failing, killed himself. Roger Bacon, the “admirable doctor,” to whom the invention of gunpowder is generally attributed, had distinct notions of flying by means of machines, and “hollow globes,” and “liquid fire.” But he did not succeed, nor did many successive attempts succeed any better in subsequent years. Bishop Wilkins treated of the art of flying, but most, if not all who discussed the subject appear to have been indebted to Roger Bacon for the idea.

When the nature and pressure of the atmosphere by Torricelli’s experiments became better known, Father Lana, a Jesuit priest, constructed a flying machine or balloon of curious shape. He proposed to fix four copper globes, very thin, and about twenty feet in diameter, and to these he fastened a boat or car, looking very much like a basin. His idea was to empty his great copper globes, and that their buoyancy would then bear the weight of the traveller. But he overlooked or was ignorant of the effect of the atmospheric pressure, which would have speedily crushed the thin copper globes when empty. Lana’s suggestion was made in 1670, the barometer had been discovered in 1643.

There were some fairly successful experiments made in flying in 1678 and in 1709. The former attempt was made by Besmir, a locksmith of Sable, who raised himself by means of wings up to the top of a house by leaps, and then succeeded in passing from one house to another lower down[Pg 295] by supporting himself in the air for a time. He started from an elevated position, and came down by degrees. Dante, a mathematician, also tried to fly, but without great success. He broke his thigh on one occasion. Laurence de Gusman claimed an invention for flying in 1709, and petitioned for a “patent,” which was granted by the king’s letter. The machine appears to have borne some resemblance to a bird.

It was not till 1782, however, that the true art of aerial navigation was discovered. The knowledge of hydrogen gas possessed by Cavendish in 1766 no doubt led up to it, and in the year following its discovery Professor Black, lecturing in Edinburgh, stated that it was much lighter than the atmosphere, and that any vessel filled with the gas would rise in the air. We now come to the invention of the Balloon (so called from its shape being similar to a vessel used in the laboratory) by the Brothers Montgolfier.

Fig. 293.—Montgolfier balloon.

Stephen and James Montgolfier were paper-makers, and carried on their business at Annonay, near Lyons, but it was partly by accident that the great discovery was made. They had no knowledge of the buoyancy of hydrogen gas. They took their idea of the balloon (inflated) from noticing an ascending column of smoke. It occurred to Stephen that if a paper bag were filled with smoke it would ascend into the air. A large bag was made and some paper burnt beneath it in a room. When the smoke had filled the bag it was released, and immediately ascended to the ceiling. Here was[Pg 296] the germ of the Montgolfier or heated air balloon. The experiment was repeated in the open air with even greater success, and a trial upon a larger scale was immediately determined upon. A story is related of Mongolfier when prosecuting his researches, that a widow whose husband had belonged to the printing firm with whom Montgolfier was then connected in business, saw the smoke issuing from the room in which the little balloon was being filled. She entered, and was astonished to see the difficulty experienced by the experimenter in filling the balloon. It swerved aside, and increased the trouble he had to keep it above the chafing dish. Montgolfier was greatly troubled, and seeing his disappointment, the widow said, “Why don’t you fasten the balloon to the chafing dish?” This had not occurred to the experimenter, and the idea was a valuable one. That was the secret of success.

The Montgolfier Brothers determined to exhibit their successful experiment, and accordingly on the 5th of June, 1783, a great concourse assembled to see the wonderful sight. A large canvas or linen balloon was made and suspended over a fire of chopped straw. The heated air quickly filled the balloon, which rose high in the air, and descended more than a mile away. This balloon contained 22,000 cubic feet of heated air, which is lighter than cold air, and of course rising carried the globe with it. As soon as the air began to cool the balloon ceased to rise, and as it got colder descended.

Here was the actual discovery of the science of Aerostatics. The intelligence of the success achieved soon spread from France to other countries. Paris, however, was in advance, and the Brothers Robert applied hydrogen gas to a balloon which was sent up from the Champ de Mars in August 1783. There was some trouble experienced in filling it, but when the balloon was at length released it realized all expectations by remaining in the air nearly an hour. When at length it fell it met with a worse fate than it deserved, for the ignorant and superstitious peasantry at once destroyed it. After this Montgolfier exhibited his experiment next time at Versailles in the presence of the Court. The first aerial travellers appeared on this occasion—viz., a sheep, a cock, and a duck, which were secured in the car. They all descended in safety, and this success encouraged M. Pilatre de Rozier to make an attempt in a “fire balloon.” He went up first in a captive balloon, and at length he and a friend, the Marquis d’Arlandes, ascended from the Bois de Boulogne. The trip was a decided success, and the possibility of navigating the air was fully demonstrated.

Soon after this,—viz., in December 1783,—an Italian Count, named Zambeccari, made an ascent in London, and came down safely at Petworth. MM. Charles and Robert ascended from Paris in December, and in February a balloon crossed the English Channel. We must pass over some time and come to the ascents of Lunardi, which caused great excitement in London. His balloon was a very large one, and was inflated, or rather partially so, at the Artillery ground. Some delay occurred, and fearing a riot, M. Lunardi proposed to go up alone with the partially-filled balloon. A Mr. Biggin who[Pg 297] had intended to ascend was left behind. The Prince of Wales was present, with thousands of spectators. Lunardi cast off and ascended rapidly, causing great admiration from the whole metropolis. Judge and jury, sovereign and ministers, all turned out to gaze at the balloon; a guilty prisoner was acquitted hurriedly, so that no time was lost in discussion, and one lady died of excitement. Lunardi was regarded as a hero, and made many other ascents. He died in 1806.

In those earlier days one or two fatal accidents happened. Count Zambeccari and a companion were in a balloon which caught fire, and both occupants of the car leaped from it as they were descending. The Count was killed on the spot, and his companion was much injured. Pilatre de Rozier made an attempt to cross the channel to England in 1785; he had reached three thousand feet when the balloon caught fire, and the unfortunate traveller was precipitated to the ground. His associate only survived him a few minutes.

Fig. 294.—MM. Charles’ and Roberts’ balloon.

Fig. 295.—Blanchard’s balloon.

It is to the celebrated English aeronaut, Mr. Green, that the substitution of carburetted hydrogen or street gas for hydrogen is due, and since his ascent in 1821 no other means of inflation have been used. A great many quite successful and a few unsuccessful ascents have been made for pleasure or profit. Mr. Green, in the Nassau balloon, passed over to Nassau, a distance of five hundred miles, in eighteen hours. This exploit was the cause of the name being bestowed upon the balloon. The Giant of M. Nadar was exhibited in England, and it was an enormous one, being an hundred feet high, and nearly as wide in the widest part. But even this machine was outdone by[Pg 298] the Godard “Montgolfier” balloon, which was one hundred and seventeen feet high, and carried a stove. We give illustrations of these celebrated balloons, and will now pass on to the more scientific portion of the subject and the ascents of Mr. Glaisher and other aeronauts for the purpose of making meteorological observations, and the use of balloons for purposes of observation in war.

It appears that the first ascent for scientific investigation was made in the year 1803. The aeronauts were Messrs. Robertson and Lhoest. They ascended from Hamburg and came down at Hanover, and made meantime several experiments with reference to the electrical condition of the atmosphere, its influence upon a magnetic needle, and some experiments with regard to acoustics and heat. The report was presented to the St. Petersburg Academy, and contains the result of their interesting observations. The travellers ascertained that at the elevation to which they attained,—viz., 25,500 feet,—the temperature was on that July day fifty degrees colder, falling to 19·6°, while on the earth the thermometer had shown 68°. They ascertained that glass and wax did not become electric when rubbed, that the Voltaic battery lost much of its power, that the oscillation of a “dipping needle” increased as they mounted into the air, while sound was certainly less easily transmitted at that elevation, and struck them as less powerful in tone. The heat experiment was not a success, owing to the breaking of the thermometer. They wished to find the temperature of boiling water at that elevation, but when the experiment was about to be made Robertson accidentally plunged the instrument into the fire instead of into the water. So the question was not settled.

The effect upon the aeronauts was a sensation of sleepiness, and two birds died. The muscular powers of the voyagers also appear to have been much affected, and similar sensations may be experienced by travellers on high mountains who find their breath very short and a disinclination to exertion oppress them.

MM. Biot and Gay-Lussac made a very interesting ascent in 1804. We will detail their experiences at some length, for the coolness displayed and the value of the observations made are remarkable in the history of scientific ballooning. They started, at 10 o’clock a.m. on the 23rd of August, and when the balloon had carried them up to an altitude of 8,600 feet they commenced their experiments. They had some animals in the car with them, a bee amongst the number, and the insect was let go first. It flew away swiftly, not at all inconvenienced apparently. The sun was very hot at 56° Fahr. Their pulses were beating very fast, but no inconvenience was felt.

When 11,000 feet had been reached a linnet was permitted to go at large, but after a little time the bird returned to the balloon. It remained perched for a few minutes, and then dashed downwards at a tremendous pace. A pigeon was then liberated. It also appeared very uncertain, and wheeled around in circles for a time. At last it gained confidence, and[Pg 299] descended, and disappeared in the clouds beneath. They made other experiments, but descended without having obtained as accurate results as had been anticipated.

Fig. 296.—The Nassau balloon.

Fig. 297.—The “Giant” balloon of M. Nadar.

On the next occasion, however, every care was taken, and on the 15th of September the important ascent was made by Gay-Lussac alone. He fixed hanging ropes to the balloon with the view to check the rotating movements, and having provided himself with all necessary apparatus and two vacuum flasks to bring down some of the upper air, the young man started. The barometer marked 30·66°, the thermometer 82° (Fahr.). At an elevation of 12,680 feet Lussac perceived that the variation of the compass was the same as on land. Two hundred feet higher up he ascertained that a key held in the magnetic direction repelled with the lower, and attracted with its upper extremity the north pole of a needle. This experiment was repeated with the same result at an elevation of 20,000 feet, which shows how the earth exercises its magnetic influence. The temperature of the air was found to decrease in proportion as the ascent up to 12,000 feet, where the reading was 47·3". It then increased up to 14,000 feet by 6°, and then regularly diminished again as the balloon rose, till at the greatest elevation reached, 23,000 feet,[Pg 300] there was a difference of 67° in the temperature on the earth, for at the maximum height attained the thermometer stood at 14·9°.

But the most important fact ascertained, and one which set many theories at rest, was the composition of the atmosphere in those high altitudes. We mentioned that Gay-Lussac took up two empty flasks from which the air had been taken. The vacuum was almost perfect. When the aeronaut had reached 21,460 feet he opened one flask, and it was quickly filled; he secured it carefully; and when at his highest point,—four miles and a half above the sea-level,—he opened the other flask. The barometer stood at 12.95 inches, and the cold was very great. The voyager felt benumbed, and experienced difficulty of breathing; his throat was parched and dry. So Lussac determined to return, he could go no higher. He dropped gently near Rouen, and soon reached Paris. As soon as possible the air in the flasks was submitted to very delicate tests, and to the satisfaction of the scientists engaged it was found to be in exactly the same proportions as that collected near the earth—two hundred and fifteen parts of oxygen to every thousand of atmospheric air.

Messrs. Banal and Bixio, in 1850, also made some observations, and found the temperature very variable. At 23,000 feet they found the thermometer at minus 38·2° Fahr., which was much below the cold experienced by Gay-Lussac. We may still conclude that the various currents of the atmosphere cause considerable variation, and that it is impossible to lay down anything respecting the degrees of heat and cold likely to be found at certain elevations. We quote Arago’s observations upon this ascent:—

“This discovery” (the ice particles found in the air) “explains how these minute crystals may become the nucleus of large hailstones, for they may condense round them the aqueous vapour contained in the portion of the atmosphere where they exist. They go far to prove the truth of Mariotte’s theory, according to which these crystals of ice suspended in the air are the cause of parahelia—or mock-suns and mock-moons. Moreover, the great extent of so cold a cloud explains very satisfactorily the sudden changes of temperature which occur in our climates.”

M. Flammarion gives in his “Voyages” some very interesting and amusing particulars, as well as many valuable scientific observations. During one ascent he remarked that the shadow of the balloon was white, and at another time dark. When white the surface upon which it fell looked more luminous than any other part of the country! The phenomenon was an anthelion. The absolute silence impressed the voyager very much. He adds, “The silence was so oppressive that we cannot help asking ourselves are we still alive! We appear to appertain no longer to the world below.” M. Flammarion’s observations on the colour of what we term the sky are worth quoting—not because they are novel, but because they put so very clearly before us the appearance we call the “blue vault.” He says,—speaking of the non-existence of the “celestial vault,”—“The air[Pg 301] reflects the blue rays of the solar spectrum from every side. The white light of the sun contains every colour, and the air allows all tints to pass through it except the blue. This causes us to suppose the atmosphere is blue. But the air has no such colour, and the tint in question is merely owing to the reflection of light. Planetary space is absolutely black; the higher we rise the thinner the layer of atmosphere that separates us from it, and the darker the sky appears.”

Fig. 298.—The “Eagle” of M. Godard.

Some beautiful effects may be witnessed at night from a balloon, and considering the few accidents there have been in proportion to the number of ascents, we do not wonder at balloon voyages being undertaken for mere pleasure. When science has to be advanced there can be no objection made, for then experience goes hand-in-hand with caution. It is only the ignorant who are rash; the student of Nature learns to respect her, and to attend to her admonitions and warnings in time. The details of the ascents of famous aeronauts give us a great deal of pleasant and profitable reading. The phenomena of the sky and clouds, and of the heavens, are all studied with great advantage from a balloon, or “aerostat,” as it is the fashion to call it. The grand phenomena of “Ulloa’s circles,” or anthelia, which represent the balloon in air, and surrounded by a kind of glory, or aureola, like those represented behind saintly heads, appear, as the name denotes, opposite to the sun.

The various experiments made to ascertain the intensity of sounds have resulted in the conclusion that they can be heard at great distances. For instance, the steam whistle is distinctly audible 10,000 feet up in the[Pg 302] air, and human voices are heard at an altitude of 5,000 feet. A man’s voice alone will penetrate more than 3,000 feet into the air; and at that elevation the croaking of frogs is quite distinguishable. This shows that sound ascends with ease, but it meets with great resistance in its downward course, for the aeronaut cannot make himself audible to a listener on the earth at a greater distance than 300 or 400 feet, though the latter can be distinctly heard at an elevation of 1,600 feet. The diminution of temperature noted by M. Flammarion is stated to be 1° Fahr. for every 345 feet on a fine day. On a cloudy day the mean decrease was 1° for every 354 feet of altitude. The temperature of clouds is higher than the air surrounding them, and the decrease is more rapid near the surface, less rapid as the balloon ascends. We may add that at high elevations the cork from a water-bottle will pop out as if from a champagne flask.

We have hitherto referred more to M. Flammarion and other French aeronauts, but we must not be considered in any way oblivious of our countrymen, Messrs. Glaisher, Green, and Coxwell, nor of the American,—one of the most experienced of aerial voyagers,—Mr. Wise. The scientific observations made by the French voyagers confirmed generally Mr. Glaisher’s experiments. This noted air-traveller made twenty-eight ascents in the cause of science, and his experiences related in “Travels in the Air,” and in the “Reports” of the British Association, are both useful and entertaining. For “Sensational ballooning” one wishes to go no farther than his account of his experience with Mr. Coxwell, when (on the 5th of September, 1862) he attained the greatest elevation ever reached, viz., seven miles, or thirty-seven thousand feet.

We condense this exciting narrative for the benefit of those who have not seen it already.

The ascent was made from Wolverhampton. At 1.39 p.m., the balloon was four miles high, the temperature was 8°, and by the time the fifth mile had been reached the mercury was below zero, and up to this time observations had been made without discomfort, though Mr. Coxwell, having exerted himself as aeronaut, found some difficulty in breathing. About 2 o’clock, the balloon still ascending, Mr. Glaisher could not see the mercury in the thermometer, and Mr. Coxwell had just then ascended into the ring above the car to release the valve line which had become twisted. Mr. Glaisher was able to note the barometer, however, and found it marked 10 inches, and was rapidly decreasing. It fell to 9¾ inches, and this indicated an elevation of 29,000 feet! But the idea was to ascend as high as possible, so the upward direction was maintained. “Shortly afterwards,” writes Mr. Glaisher, “I laid my arm upon the table possessed of its full vigour, and on being desirous of using it I found it powerless,—it must have lost power momentarily. I tried to move the other arm, and found it powerless also. I then tried to shake myself, and succeeded in shaking my body. I seemed to have no limbs. I then looked at the barometer, and whilst doing so my head fell on my left shoulder.”

[Pg 303]

Mr. Glaisher subsequently quite lost consciousness, and “black darkness” came. While powerless he heard Mr. Coxwell speaking, and then the words, “Do try, now do.” Then sight slowly returned, and rousing himself, Mr. Glaisher said, “I have been insensible.” Mr. Coxwell replied, “You have, and I, too, very nearly.” Mr. Coxwell’s hands were black, and his companion had to pour brandy upon them. Mr. Coxwell’s situation was a perilous one. He had lost the use of his hands, which were frozen, and had to hang by his arms to the ring and drop into the car. He then perceived his friend was insensible, and found insensibility coming on himself. There was only one course to pursue—to pull the valve line and let the gas escape, so as to descend. But his hands were powerless! As a last resource he gripped the line with his teeth, and bending down his head, after many attempts succeeded in opening the valve and letting the gas escape. The descent was easily made, and accomplished in safety.

Fig. 299.—A descending balloon.

Some pigeons were taken up on this occasion, and were set free at different altitudes. The first, at three miles, “dropped as a piece of paper”; the second, at four miles, “flew vigorously round and round, apparently taking a dip each time”; a third, a little later, “fell like a stone.” On descending[Pg 304] a fourth was thrown out at four miles, and after flying in a circle, “alighted on the top of the balloon.” Of the remaining pair one was dead when the ground was gained, and the other recovered.

The observations noted are too numerous to be included here. Some, we have seen, were confirmed by subsequent aeronauts, and as we have mentioned them in former pages we need not repeat them. The results differed very much under different conditions, and it is almost impossible to decide upon any law. The direction of the wind in the higher and lower regions sometimes differed, sometimes was the same, and so on. The “Reports” of the British Association (1862-1866) will furnish full particulars of all Mr. Glaisher’s experiments.

We have scarcely space left to mention the parachutes or umbrella-like balloons which have occasionally been used. Its invention is traced to very early times; but Garnerin was the first who descended in a parachute, in 1797, and continued to do so in safety on many subsequent occasions. The parachute was suspended to a balloon, and at a certain elevation the voyager let go and came down in safety. He ascended once from London, and let go when 8,000 feet up. The parachute did not expand as usual, and fell at a tremendous rate. At length it opened out, and the occupier of the car came down forcibly, it is true, but safely. The form of the parachute is not unlike an umbrella opened, with cords attaching the car to the extremities of the “ribs,” the top of the basket car being fastened to the “stick” of the umbrella.

Mr. Robert Cocking invented a novel kind of parachute, but when he attempted to descend by it from Mr. Green’s balloon it collapsed, and the unfortunate voyager was dashed to pieces. His remains were found near Lee, in Kent. Mr. Hampton did better on Garnerin’s principle, and made several descents in safety and without injury.

The problem of flying in the air has attracted the notice of the Aeronautical Society, established in 1873, but so far without leading to practical results, though many daring and ingenious suggestions have been put forth in the “Reports.”

It is not within our province to do more than refer to the uses of the balloon for scientific purposes, but we may mention the services it was employed upon during the French war, 1870-71. The investment of Paris by the German army necessitated aerial communication, for no other means were available. Balloon manufactories were established, and a great number were made, and carried millions of letters to the provinces. Carrier-pigeons were used to carry the return messages to the city, and photography was applied to bring the correspondence into the smallest legible compass. The many adventures of the aeronauts are within the recollection of all. A few of the balloons never reappeared; some were carried into Norway, and beyond the French frontier in other directions. The average capacity of these balloons was 70,000 cubic feet.

Of course it will be understood how balloons are enabled to navigate the[Pg 305] air. The envelope being partly filled with coal-gas-heated air and hydrogen, is much lighter than the surrounding atmosphere, and rises to a height according as the density of the air strata diminishes. The density is less as we ascend, and the buoyant force also is lessened in proportion. So when the weight of the balloon and its occupants is the same as the power of buoyancy, it will come to a stand, and go no higher. It can also be understood that as the pressure of the outside becomes less, the expansive force of the gas within becomes greater. We know that gas is very compressible, and yet a little gas will fill a large space. Therefore, as the balloon rises, it retains its rounded form, and appears full even at great altitudes; but if the upper part were quite filled before it left the ground, the balloon would inevitably burst at a certain elevation when the external pressure of the air would be removed, unless an escape were provided. This escape is arranged for by a valve at the top of the balloon, and the lower part above the car is also left open very often, so that the gas can escape of itself. When a rapid descent is necessary, the top valve is opened by means of a rope, and the balloon sinks by its own weight. Mr. Glaisher advises for great ascensions a balloon of a capacity of 90,000 cubic feet, and only filled one-third of that capacity with gas. Six hundred pounds of ballast should be taken.

Fig. 300.—Filling a balloon.

[Pg 306]

A very small quantity of ballast thrown away will make a great difference; a handful will raise the balloon many feet, and a chicken bone cast out occasions a rise of thirty yards. The ballast is carried in small bags, and consists of dry sand, which speedily dissipates in the air as it falls. By throwing out ballast the aeronaut can ascend to a great height—in fact, as high as he can go, the limit apparently for human existence being about seven miles, when cold and rarefied air will speedily put an end to existence.

It is a curious fact, that however rapidly the balloon may be travelling through the air, the occupants are not sensible of the motion. This, in part, arises from the impossibility of comparing it with other objects. We pass nothing stationary which would indicate the pace at which we travel. But the absence of oscillation is also remarkable; even a glass of water may be filled brim-full, and to such a level that the water is above the rim of the glass, and yet not a drop will fall. This experiment was made by M. Flammarion. When the aeronaut has ascended some distance the earth loses its flat appearance, and appears as concave as the firmament above. Guide ropes are usually attached to balloons, and as they rest upon the ground they relieve the balloon of the amount of weight the length trailing would cause. They thus act as a kind of substitute for ballast as the balloon is descending. Most of the danger of aerial travelling lies in the descent; and though in fine weather the aeronaut can calculate to a nicety where he will descend, on a windy day, he must cast a grapnel, which catches with an ugly jerk, and the balloon bounds and strains at her moorings.

Although many attempts have been made to guide balloons through the air, no successful apparatus has ever been completed for use. Paddles, sails, fans, and screws have all been tried, but have failed to achieve the desired end. Whether man will ever be able to fly we cannot of course say. In the present advancing state of science it may not be impossible ere long to supply human beings with an apparatus worked by electricity, perhaps, which will enable them to mount into the air and sustain themselves. But even the bird cannot always fly without previous momentum. A rook will run before it rises, and many other birds have to “get up steam,” as it were, before they can soar in the atmosphere. Eagles and such heavy birds find it very difficult to rise from the ground. We know that vultures when gorged cannot move at all, or certainly cannot fly away; and eagles take up their positions on high rocks, so that they may launch down on their prey, and avoid the difficulty of rising from the ground. They swoop down with tremendous momentum and carry off their booty, but often lose their lives from the initial difficulty of soaring immediately. We fear man’s weight will militate against his ever becoming a flying animal. When we obtain a knowledge of the atmospheric currents we shall no doubt be able to navigate our balloons; but until then—and the information is as yet very limited, and the currents themselves very variable—we must be content to rise and fall in the air, and travel at the will of the wind in the upper regions of the atmosphere.

[Pg 307]

CHAPTER XXIV.
CHEMISTRY.
INTRODUCTION.

WHAT CHEMISTRY IS—THE ELEMENTS—METALLIC AND NON-METALLIC—ATOMIC WEIGHT—ACIDS—ALKALIS—BASES—SALTS—CHEMICAL COMBINATION AND STUDY.

Chemistry is the science of phenomena which are attended by a change of the objects which produce them. We know that when a candle burns, or when wood is burned, or even a piece of metal becomes what we term “rusty,” that certain chemical changes take place. There is a change by what is termed chemical action. Rust on iron is not iron; it is oxide of iron. The oxygen of the air causes it. So we endeavour, by Chemistry, to find out the nature of various bodies, their changes, and the results.

Fig. 301.—The Laboratory.

In nature we have simple and compound bodies. The former are called Elements. We must not confuse these elements with the so-called elements—earth, air, fire, and water. These are really compound bodies. An element is a substance or a gas which is not composed of more than one constituent; it is itself—a compound of perfectly identical particles. Every “compound” body, therefore, must be made up of some of the elements, of which there are sixty-five. These bodies are divided into non-metallic and metallic elements, and all bodies are composed of them, or are these bodies[Pg 308] themselves. The list is as follows. The non-metallic elements are “metalloids.” We have omitted fractions from the weights, on which chemists differ.

TABLE OF ELEMENTS WITH THEIR CHEMICAL SYMBOLS AND COMBINING WEIGHTS.

METALS.

The term “combining weight” requires a little explanation. We are aware that water, for instance, is made up of oxygen and hydrogen in certain proportions. This we will prove by-and-by. The proportions are in eighteen grains or parts of water, sixteen parts (by weight) of oxygen, and two parts (by weight) of hydrogen. These are the weights or proportions in which oxygen and hydrogen combine to form water, and such weights are always the same in these proportions. Chemical combination always occurs for certain substances in certain proportions which never vary in those compounds, and if we wish to extract oxygen from an oxide we must take the aggregate amount of the combining weights of the oxide, and we shall find the proportion of oxygen; for the compound always weighs the same as the sum of the elements that compose it. To return to the illustration of water. The molecule of water is made up of one atom of oxygen and two atoms of hydrogen. One atom of the former weighs sixteen times the atom of the latter. The weights given in the foregoing table are atomic weights, and the law of their proportions is called the Atomic Theory.

An atom in chemistry is usually considered the smallest quantity of matter that exists, and is indivisible. A molecule is supposed to contain two or more atoms, and is the smallest portion of a compound body. The[Pg 310] standard atom is hydrogen, which is put down as 1, because we find that when one part by weight of hydrogen is put in combination, it must have many more parts by weight of others to form a compound. Two grains of hydrogen, combining with sixteen of oxygen, makes eighteen of water, as we have already seen.

Take an example so plainly given by Professor Roscoe, remembering that the numbers in our table represent the fixed weight or proportion by weight in which the simple body combines. The red oxide of mercury contains sixteen parts by weight of oxygen to two hundred parts by weight of mercury (we see the same numbers in the table); these combined make two hundred and sixteen parts of oxide. So to obtain 16 lbs. of oxygen we must get 216 lbs. of the powder. It is the same all through, and it will be found by experiment that if any more parts than these fixed proportions be taken to form a compound, some of that element used in excess will remain free. Lime is made up of calcium and oxygen. We find calcium combining weight is forty, oxygen sixteen. Lime is oxide of calcium in these proportions (by weight).

When we wish to express the number of atoms in a compound we write the number underneath when more than one; thus water is H₂O. Sulphuric acid H₂SO₄. As we proceed we will give the various formulæ when considering the chief elements.

In chemistry we have acids, alkalis, and salts, with metallic oxides, termed bases, or bodies, that when combined with acids form salts. Alkalis are bases.

Acids are compounds which possess an acid taste, impart red colour to vegetable blues, but lose their qualities when combined with bases. Hydrogen is present in all acids. There are insoluble acids. Silicic acid, for instance, is not soluble in water, has no sour taste, and will not redden the test litmus paper. On the other hand, there are substances (not acids) which possess the characteristics of acids, and most acids have only one or two of these characteristics.

Thus it has come to pass that the term “acid” has in a measure dropped out from scientific nomenclature, and salt of hydrogen has been substituted by chemists. For popular exposition, however, the term is retained.

Alkalis are bases distinguished by an alkaline taste. The derivation is from Arabic, al-kali. They are characterized by certain properties, and they change vegetable blues to green, and will restore the blue to a substance which has been reddened by acid. They are soluble in water, and the solutions are caustic in their effects. Potash, soda, and ammonia are alkalis, or chemically, the oxides of potassium, sodium, ammonium, lithium, and cæsium are all alkalis. Potash is sometimes called “caustic” potash. There are alkaline earths, such as oxides of barium, strontium, etc. Bases may be defined as the converse of acids.

Acids and alkalis are then evidently opposite in character, and yet they readily combine, and in chemistry we shall find that unlike bodies are very[Pg 311] fond of combining (just as opposite electricities attract each other), and the body made by this combination differs in its properties from its constituents.

Salts are composed of acids and bases, and are considered neutral compounds, but there are other bodies not salts, which likewise come under that definition—sugar, for instance. As a rule, when acids and alkalis combine salts are found.

Chemical phenomena are divided into two groups, called inorganic and organic, comprising the simple and compound aspects of the subject, the elementary substances being in the first, and the chemistry of animals or vegetables, or organic substances, in the latter. In the inorganic section we shall become acquainted with the elements and their combinations so often seen as minerals in nature. Chemical preparations are artificially prepared. To consider these elements we must have certain appliances, and indeed a laboratory is needed. Heat, as we have already seen, plays a great part in developing substances, and by means of heat we can do a great deal in the way of chemical decomposition. It expands, and thus diminishes cohesion; it counteracts the chemical attraction. Light and electricity also decompose chemical combinations. But before proceeding it will be as well to notice a few facts showing how Nature has balanced all things.

The earth, and its surrounding envelope, the atmosphere, consist of a number of elements, which in myriad combinations give us everything we possess,—the air we breathe, the water we drink, the fire that warms us, are all made up of certain elements or gases. Water, hydrogen and oxygen; air, oxygen and nitrogen. Fire is combustion evolving light and heat. Chemical union always evolves heat, and when such union proceeds very rapidly fire is the result.

In all these combinations we shall find when we study chemistry that not a particle or atom of matter is ever lost. It may change or combine or be “given off,” but the matter in some shape or way exists still. We may burn things, and rid ourselves, as we think, of them. We do rid ourselves of the compounds, the elements remain somewhere. We only alter the condition. During combustion, as in a candle or a fire, the simple bodies assume gaseous or other forms, such as carbon, but they do not escape far. True they pass beyond our ken, but nature is so nicely balanced that there is a place for everything, and everything is in its place under certain conditions which never alter. We cannot destroy and we cannot create. We may prepare a combination, and science has even succeeded in producing a form like the diamond—a crystal of carbon which looks like that most beautiful of all crystals, but we cannot make a diamond after all. We can only separate the chemical compounds. We can turn diamonds into charcoal it is true, but we cannot create “natural” products. We can take a particle of an element and hide it, or let it pass beyond our ken, and remain incapable of detection, but the particle is there all the time, and when we retrace our steps we shall find it as it was before.

This view of chemistry carries it as a science beyond the mere holiday[Pg 312] amusement we frequently take it to be. It is a grand study, a study for a lifetime. Nature is always willing, like a kind, good mother as she is, to render us up her secrets if we inquire respectfully and lovingly. The more we inquire the more we shall find we have to learn. In these and the following pages we can only tell you a few things, but no one need be turned away because he does not find all he wants. We never do get all we want in life, and there are many first-rate men—scientists—who would give “half their kingdom” for a certain bit of knowledge concerning some natural phenomena. There are numerous excellent treatises on chemistry, and exhaustive as they are, at present they do not tell us all. Let these popular pages lead us to the study of nature, and we shall find our labour far from onerous and full of interest, daily increasing to the end, when we shall know no more of earth, or chemistry.

As a preliminary we will put our workshop aside, and show you something of Chemistry without a Laboratory.

[Pg 313]

CHAPTER XXV.
CHEMISTRY WITHOUT A LABORATORY.

We have already pointed out the possibility of going through a course of physics without any special apparatus, we shall now endeavour to show our readers the method of performing some experiments in chemistry without a laboratory, or at any rate with only a few simple and inexpensive appliances. The preparation of gases, such as hydrogen, carbonic acid, and oxygen, is very easily accomplished, but we shall here point out principally a series of experiments that are not so much known. We will commence, for example, by describing an interesting experiment which often occurs in a course of chemistry. Ammoniacal gas combined with the elements of water is analogous to a metallic oxide which includes a metallic root, ammonium. This hypothetically composed metal may be in a manner perceived, since it is possible to amalgamate it with mercury by operating in the following manner:—We take a porcelain mortar, in which we pour a quantity of mercury, and then cut some thin strips of sodium, which are thrown into the mercury. By stirring it about with the pestle a loud cracking is produced, accompanied by a flame, which bears evidence to the union of the mercury and the sodium, and the formation of an amalgam of sodium. If this amalgam of sodium is put into a slender glass tube containing a concentrated solution of hydrochlorate of ammonia in water, we see the ammonia expand in an extraordinary manner, and spout out from the end of the tube, which is now too small to contain it, in the form of a metallic substance (fig. 302). In this case, the ammonium, the radical which exists in the ammoniacal salts, becomes amalgamated with the mercury, driving out the sodium with which it had previously been combined; the ammonium thus united with the mercury becomes decomposed in ammoniacal gas and hydrogen, the mercury assuming its ordinary form. Phosphate of ammonia is very valuable from its property of rendering the lightest materials, such as gauze or muslin, incombustible. Dip a piece of muslin in a solution of phosphate of ammonia, and dry it in contact with the air; that done, you will find it is impossible to set fire to the material; it will get charred, but you cannot make it burn. It is to be wished that this useful precaution were oftener taken in the matter of ball-dresses, which have so frequently been the cause of serious accidents. There is no danger whatever with a dress that[Pg 314] has been soaked in phosphate of ammonia, which is very inexpensive, and easily procured.

Fig. 302.—Experiment with ammonium.

For preparing cool drinks in the summer ammoniacal salts are very useful; some nitrate of ammonia mixed with its weight in water, produces a considerable lowering of the temperature, and is very useful for making ice. Volatile alkali, which is so useful an application for stings from insects, is a solution of ammoniacal gas in water, and sal-volatile, which has such a refreshing and reanimating odour, is a carbonate of ammonia. We often see in chemists’ shops large glass jars, the insides of which are covered with beautiful transparent white crystals, which are formed over a red powder placed at the bottom of the vase. These crystals are the result of a combination of cyanogen and iodine. Nothing is easier than the preparation of iodide of cyanogen, a very volatile body, which possesses a strong tendency to assume a definite crystalline form. We pound in a mortar a mixture of 50 grams of cyanide of mercury, and 100 grams of iodine; under the action of the pestle the powder, which was at first a brownish colour, assumes a shade of bright vermilion red. The cyanogen combines with the iodine, and transforms itself into fumes with great rapidity. If the powder is placed at the bottom of a stoppered glass jar, the fumes of the iodide of cyanogen immediately condense, thereby producing beautiful crystals which often attain considerable size (fig. 303). Cyanogen forms with sulphur a remarkable substance, sulpho-cyanogen, the properties of which we cannot describe without exceeding the limits of our present treatise; we shall therefore confine ourselves to pointing out one of its combinations, which is well known at the present day, owing to its singular properties. This is sulpho-cyanide of mercury, with which small combustible cones are made, generally[Pg 315] designated by the pompous title of Pharaoh’s serpents. For making these, some sulpho-cyanide of potassium is poured into a solution of nitrate acid on mercury, which forms a precipitate of sulpho-cyanide of mercury. This is a white, combustible powder, which after passing through a filter, should be transformed into a stiff pulp by means of water containing a solution of gum. The pulp is afterwards mixed with a small quantity of nitrate of potash, and fashioned into cones or cylinders of about an inch and a quarter in length, which should be thoroughly dried. The egg thus obtained can be hatched by the simple application of a lighted match, and gives rise to the phenomenon. The sulpho-cyanide slowly expands, the cylinder increases in length, and changes to a yellowish substance, which dilates and extends to a length of twenty or five-and-twenty inches. It has the appearance of a genuine serpent, which has just started into existence, and stretches out its tortuous coils, endeavouring to escape from its narrow prison (fig. 304). The residue is composed partly of cyanide of mercury and of para-cyanogen; it constitutes a very poisonous substance, which should be immediately thrown away or burned. It can be easily powdered into dust in the fingers. During the decomposition of the sulpho-cyanide of mercury, quantities of sulphurous acid are thrown off, and it is to be regretted that Pharaoh’s serpent should herald his appearance by such a disagreeable, suffocating odour.

Fig. 303.—Iodide of cyanogen.

Fig. 304.—Pharaoh’s serpent.

After these few preliminary experiments, we will endeavour to show the interest afforded by the study of chemistry in relation to the commonest substances of every-day life. We will first consider the nature of a few pinches of salt. We know that kitchen salt, or sea salt, is white or greyish, according to its degree of purity; that it has a peculiar flavour, is soluble[Pg 316] in water, and makes a peculiar crackling when thrown in the fire. But though its principal physical properties may be familiar enough, many people are entirely ignorant of its chemical nature and elementary composition. Kitchen salt contains a metal, combined with a gas possessing a very suffocating odour; the metal is sodium, the gas is chlorine. The scientific name for the substance is chloride of sodium (salt).19 The metal contained in common salt in no way resembles ordinary metals; it is white like silver, but tarnishes immediately in contact with air, and unites with oxygen, thus transforming itself into oxide of sodium. To preserve this singular metal it is necessary to protect it from the action of the atmosphere, and to keep it in a bottle containing oil of naptha. Sodium is soft, and it is possible with a pair of[Pg 317] scissors to cut it like a ball of soft bread that has been kneaded in the hand. It is lighter than water, and when placed in a basin of water floats on the top like a piece of cork; only it is disturbed, and takes the form of a small brilliant sphere; great effervescence is also produced as it floats along, for it reduces the water to a common temperature by its contact. By degrees the small metallic ball disappears from view, after blazing into flame (fig. 305).

Fig. 305.—Combustion of sodium in water.

This remarkable experiment is very easy to carry out, and sodium is now easily procured at any shop where chemicals are sold. The combustion of sodium in water can be explained in a very simple manner. Water, as we know, is composed of hydrogen and oxygen. Sodium, by reason of its great affinity for the latter gas, combines with it, and forms a very soluble oxide; the hydrogen is released and thrown off, as we shall perceive by placing a lighted match in the jar, when the combustible gas ignites.

Oxide of sodium has a great affinity for water; it combines with it, and absorbs it in great quantities. It is a solid, white substance, which burns and cauterizes the skin; it is also alkaline, and brings back the blue colour to litmus paper that has been reddened by acids.

Sodium combines easily also with chlorine. If plunged into a jar containing this gas it is transformed into a substance, which is sea salt. If the chlorine is in excess a part of the gas remains free, for simple substances do not mingle in undetermined ratios; they combine, on the contrary, in very definite proportions, and 35·5 gr. of dry chlorine always unites with the same quantity of soda equal to 23 grams. A gram of kitchen salt is formed, therefore, of 0·606 gr. of chlorine, and 0·394 gr. of sodium. Besides sea salt, there are a number of different salts which may be made the object of curious experiments. We know that caustic soda, or oxide of sodium, is an alkaline product possessing very powerful properties; it burns the skin, and destroys organic substances.

Sulphuric acid is endowed with no less powerful properties; if a little is dropped on the hand it produces great pain and a sense of burning; a piece of wood plunged into this acid is almost immediately carbonized. If we mix forty-nine grams of sulphuric acid and thirty-one grams of caustic soda a very intense reaction is produced, accompanied by a considerable elevation of temperature; after it has cooled we have a substance which can be handled with impunity; the acid and alkali have combined, and their properties have been reciprocally destroyed. They have now originated a salt which is sulphate of soda. This substance exercises no influence on litmus paper, and resembles in no way the substances from which it originated.

There are an infinite number of salts which result in like manner from the combination of an acid with an alkali or base. Some, such as sulphate[Pg 318] of copper, or chromate of potash, are coloured; others, like sulphate of soda, are colourless. The last-mentioned salt, with a number of others, will take a crystalline form; if dissolved in boiling water, and the solution left to stand, we shall perceive a deposit of transparent prisms of very remarkable appearance. This was discovered by Glauber, and was formerly called Glauber’s salts.

Fig. 306.—Preparation of a solution saturated with sulphate of soda.

Sulphate of soda is very soluble in water, and at a temperature of thirty-three (Centigr.) water can dissolve it in the greatest degree. If we pour a layer of oil on a solution saturated with Glauber’s salts, and let it stand, it will not produce crystals; but if we thrust a glass rod through the oil into contact with the solution, crystallization will be instantaneous. This singular phenomenon becomes even more striking when we put the warm concentrated solution into a slender glass tube, A B, which we close after having driven out the air by the bubbling of the liquid (fig. 306). When the tube has been closed, the crystals of sulphate of soda will not form, even with the temperature at zero; nevertheless the salts, being less soluble cold than hot, are found in the fluid in a proportion ten times larger than they would contain under ordinary conditions. If the end of the tube be broken the salt will crystallize immediately. We will describe another experiment, but little known and very remarkable, which exhibits in a striking manner the process of instantaneous crystallizations. Let one hundred and fifty parts of hyposulphite of soda be dissolved in fifteen parts of water, and the solution slowly poured into a test-glass, previously warmed by means of boiling water, until the vessel is about half-full. One hundred parts of acetate of soda is then dissolved in fifteen parts of water, and poured slowly into the first solution, so that they form two layers perfectly distinct from each other. The two solutions are then covered with a little boiling water, which, however is not represented in our illustration. After it has been left to stand[Pg 319] and cool slowly, we have two solutions of hyposulphite of soda and acetate of soda superposed on each other. A thread, at the end of which is fixed a small crystal of hyposulphite of soda, is then lowered into the test-glass; the crystal passes through the solution of acetate without disturbing it, but it has scarcely reached the lower solution of hyposulphite than the salt crystallizes instantaneously. (See the test-glass on the left of fig. 307.) We then lower into the upper solution a crystal of acetate of soda, suspended from another thread. This salt then crystallizes also. (See experiment glass on the right of fig. 307.) This very successful experiment is one of the most remarkable belonging to the subject of instantaneous crystals. The successive appearance of crystals of hyposulphite of soda, which take the form of large, rhomboidal prisms, terminating at the two extremities with an oblique surface, and the crystals of acetate of soda, which have the appearance of rhomboidal, oblique prisms, cannot fail to strike the attention and excite the interest of those who are not initiated into these kinds of experiments.

Fig. 307.—Experiment of instantaneous crystallization.

Another remarkable instantaneous crystallization is that of alum. If we leave standing a solution of this salt it gradually cools, at the same time becoming limpid and clear. When it is perfectly cold, if we plunge into it a small octahedral crystal of alum suspended from a thread, we perceive that crystallization instantly commences on the surface of the small crystal; it rapidly and perceptibly increases in size, until it nearly fills the whole jar.

[Pg 320]

Common Metals and Precious Metals.

How many invalids have swallowed magnesia without suspecting that this powder contains a metal nearly as white as silver, and is malleable, and capable of burning with so intense a light that it rivals even the electric light in brilliancy! If any of our readers desire to prepare magnesium themselves it can be done in the following manner:—Some white magnesia must be obtained from the chemist, and after having been calcined, must be submitted to the influence of hydrochloric acid and hydrochlorate of ammonia. A clear solution will thus be obtained, which by means of evaporation under the influence of heat, furnishes a double chloride, hydrated and crystallised. This chloride, if heated to redness in an earthenware crucible, leaves as a residue a nacreous product, composed of micaceous, white scales, chloride of anhydrous magnesium.

Fig. 308.—Group of alum crystals.

If six hundred grams of this chloride of magnesium are mixed with one hundred grams of chloride of sodium, or kitchen salt, and the same quantity of fluoride of calcium and metallic sodium in small fragments, and the mixture is put into an earthenware crucible made red-hot, and heated for a quarter of an hour under a closed lid, we shall find on pouring out the fluid on to a handful of earth, that we have obtained instead of scoria, forty-five grams of metallic magnesium. The metal thus obtained is impure, and to remove all foreign substances it must be heated in a charcoal tube, through which passes a current of hydrogen.

Magnesium is now produced in great abundance, and is very inexpensive. It is a metal endowed with a great affinity for oxygen, and it is only necessary to thrust it into the flame of a candle to produce combustion; it burns with a brightness that the eye can scarcely tolerate, and is transformed[Pg 321] into a white powder—oxide of magnesium, or magnesia. Combustion is still more active in oxygen, and powder of magnesium placed in a jar filled with this gas produces a perfect shower of fire of very beautiful effect. To give an idea of the lighting power of magnesium, we may add that a wire of this metal, which is 29/100 of a millimetre in diameter, produces by combustion a light equal to that of seventy-four candles.

Fig. 309.—Calcined alum.

The humble earth of the fields—the clay which is used in our potteries, also contains aluminium, that brilliant metal which is as malleable as silver, and unspoilable as gold. When clay is submitted to the influence of sulphuric acid and chloride of potassium, we obtain alum, which is a sulphate of alumina and potash. Alum is a colourless salt, which crystallizes on the surface of water in beautiful octahedrons of striking regularity. Fig. 308 represents a group of alum crystals. This salt is much used in the colouring of fabrics; it is also used for the sizing of papers, and the clarification of tallow. Doctors also use it as an astringent and caustic substance. When alum is submitted to the action of heat in an earthenware crucible, it loses the water of crystallization which it contains, and expands in a singular manner, overflowing from the jar in which it is calcined (fig. 309).

Fig. 310.—Preparation of metallic iron.

Iron, the most important of common metals, rapidly unites with oxygen, and, as we know, when a piece of this metal is exposed to the influence of damp air, it becomes covered with a reddish substance. In the well-known experiment of the formation of rust, the iron gradually oxidises without its temperature rising, but this combination of iron with oxygen is effected much more rapidly under the influence of heat. If, for example, we redden at the fire a nail attached to a wire, and give it a movement of rotation as of a sling, we see flashing out from the metal a thousand bright sparks due to the combination of iron with oxygen, and the formation of an oxide. Particles of iron burn spontaneously in contact with air, and this property for many[Pg 322] centuries has been utilized in striking a tinder-box; that is to say, in separating, by striking a flint, small particles of iron, which ignite under the influence of the heat produced by the friction. We can prepare iron in such atoms that it ignites at an ordinary temperature by simple contact with the air. To bring it to this state of extreme tenuity, we reduce its oxalate by hydrogen. We prepare an apparatus for hydrogen as shown in fig. 310, and the gas produced at A is passed through a desiccative tube, B, and finally reaches a glass receptacle, C, in which some oxalate of iron is placed. The latter salt, under the combined influence of hydrogen and heat, is reduced to metallic iron, which assumes the appearance of a fine black powder. When the experiment is completed the glass vessel is closed, and the iron, thus protected from contact with the air, can be preserved indefinitely; but if it is exposed to the air by breaking off the end of the receptacle (fig. 311), it ignites immediately, producing a shower of fire of very beautiful effect. Iron thus prepared is known under the name of pyrophoric iron. Iron is acted upon in a very powerful manner by most acids. If some nitric acid is poured on iron nails, a stream of red, nitrous vapour is let loose, and the oxidised iron is dissolved in the liquid to the condition of nitrate of iron. This experiment is very easy to perform, and it gives an idea of the energy of certain chemical actions. We have endeavoured to represent its appearance in fig. 312. Fuming nitric acid does not act on iron, and prevents it being attacked by ordinary nitric acid. This property has given rise to a very remarkable experiment on passive iron. It consists in placing some nails in a glass, into which some fuming nitric acid is poured, which produces no result; the fuming acid is then taken out, and is replaced by ordinary nitric acid, which no longer acts on the iron rendered passive by the smoking acid. After[Pg 323] this, if the nails are touched by a piece of iron, which has not undergone the action of nitric acid, they are immediately acted upon, and a giving off of nitrous vapour is manifested with great energy. Lead is a very soft metal, and can even be scratched by the nails. It is also extremely pliable, and so entirely devoid of elasticity that when bent it has no tendency whatever to return to its primitive form. Lead is heavy, and has a density represented by 11·4; that is to say, the weight of a quart of water being one kilogram, that of the same volume of lead is 11·400 k.

Fig. 311.—Pyrophoric iron.

Fig. 313 represents cylindrical bars of the best known metals, all weighing the same, showing their comparative density.

Fig. 312.—Iron and nitric acid.

Lead, like tin, is capable of taking a beautiful crystalline form when placed in solution by a metal that is less oxydisable. The crystallization of lead, represented in fig. 314, is designated by the name of the Tree of Saturn. This is how the experiment is produced: Thirty grams of acetate of lead are dissolved in a quart of water, and the solution is poured into a vase of a spherical shape. A stopper for this vase is made out of a piece of zinc, to which five or six separate brass wires are attached; these are plunged into the fluid, and we see the wires become immediately covered with brilliant crystallized spangles of lead, which continue increasing in size.

The alchemists, who were familiar with this experiment, believed that it consisted in a transformation of copper into lead, while in reality it only[Pg 324] consists in the substitution of one metal for another. The copper is dissolved in the liquid, and is replaced by the lead, but no metamorphosis is brought about. We may vary at will the form of the vase or the arrangement of the wire; thus it is easy to form letters, numbers, or figures, by the crystallization of brilliant spangles.

Fig. 313.—Representation of bars of metal, all of the same weight.

Copper, when it is pure, has a characteristic red colour, which prevents it being confounded with any other metal; it dissolves easily in nitric acid, and with considerable effervescence, giving off vapour very abundantly. This property has been put to good use in engraving with aqua fortis. A copper plate is covered with a layer of varnish, and when it is dry some strokes are made on it by means of a graver; if nitric acid is poured on the plate when thus prepared, the copper is only acted on in the parts that have been exposed by the point of steel. By afterwards removing the varnish, we have an engraved plate, which will serve for printing purposes.

Fig. 314.—Tree of Saturn.

Among experiments that may be attempted with common metals, we may mention that in which salts of tin are employed. Tin has a great tendency to assume a crystalline form, and it will be easy to show this property by an interesting experiment. A concentrated solution of proto-chloride of tin, prepared by dissolving some metallic tin in hydrochloric acid, is placed in a test glass; then a rod of tin is introduced, as shown in fig. 315. Some water is next slowly poured on the rod, so that it gradually trickles down, and prevents the mingling of the proto-chloride of tin. The vessel is then left to stand, and we soon see brilliant crystals starting out from the rod. This crystallization is not effected in the water; it is explained by an electric influence, into the details of which we cannot enter without overstepping our limits; it is known as “Jupiter’s Tree.” It is well known that alchemists, with their strange system of nomenclature, believed there was a certain mysterious relation between the seven metals then known and the seven planets; each metal was dedicated to a planet; tin was called Jupiter; silver,[Pg 325] Luna; gold, Sol; lead, Saturn; iron, Mars; quicksilver, Mercury; and copper, Venus. The crystallization of tin may be recognised also by rubbing a piece of this metal with hydrochloric acid; the fragments thus rubbed off exhibit specimens of branching crystals similar to the hoar-frost which we see in severe winter weather. If we bend a rod of tin in our hands the crystals break, with a peculiar rustling sound.

When speaking of precious metals, we may call to mind that the alchemists considered gold as the king of metals, and the other valuable ones as noble metals. This definition is erroneous, if we look upon the useful as the most precious; for, in that case, iron and copper would be placed in the first rank. If gold were found abundantly on the surface of the soil, and iron was extremely rare, we should seek most eagerly for this useful metal, and should despise the former, with which we can neither make a ploughshare nor any other implement of industry. Nevertheless, the scarcity of gold, its beautiful yellow colour, and its unalterability when in contact with air, combine to place it in the first rank in the list of precious metals. Gold is very heavy; its density is represented by the figure 19·5. It is the most malleable and the most ductile of metals, and can be reduced by beating to such thin sheets that ten thousand can be laid, one over the other, to obtain the thickness of a millimetre. With a grain of gold a thread may be manufactured extending a league in length, and so fine that it resembles a spider’s web. When gold is beaten into thin sheets it is no longer opaque; if it is fastened, by means of a solution of gum, on to a sheet of glass, the light passes right through it, and presents a very perceptible green shade. Gold is sometimes found scattered in sand, in a condition of impalpable dust, and, in certain localities, in irregular lumps of varying size, called nuggets. Gold is the least alterable of the metals, and can be exposed, indefinitely, to the contact of humid atmosphere without oxidizing. It is not acted on by the most powerful acids, and only dissolves in a mixture of nitric acid and hydrochloric acid. We can prove that gold resists the influence of acids by the following operation:—

Some gold-leaf is placed in two small phials, the first containing hydrochloric acid, and the second nitric acid. The two vessels are warmed on the stove, and whatever the duration of the ebullition of the acids, the gold-leaf remains intact, and completely resists their action. If we then empty the contents of one phial into the other, the hydrochloric and nitric acids are mixed, and we see the gold-leaf immediately disappear, easily dissolved by the action of the liquid (aqua regia). Gold also changes when in contact with mercury; this is proved by suspending some gold-leaf above the surface of this liquid (fig. 316); it quickly changes, and unites with the fumes of the mercury, becoming of a greyish colour.

Silver is more easily affected than gold, and though so white when fused, tarnishes rapidly in contact with air. It does not oxidize, but sulphurizes under the influence of hydro-sulphuric emanations. Silver does not combine directly with the oxygen of the atmosphere; but under[Pg 326] certain conditions it can dissolve great quantities of this gas. If it is fused in a small bone cupel, in contact with the air, and left to cool quickly, it expands in a remarkable manner, and gives off oxygen.

Fig. 315.—Jupiter’s Tree.

Nitric acid dissolves silver very easily, by causing the formation of abundant fumes. When the solution evaporates, we perceive white crystals forming, which are nitrate of silver. This fused nitrate of silver takes the name of lunar caustic, and is employed in medicine. Nitrate of silver is very poisonous; it possesses the singular property of turning black under the action of the sun’s rays, and is used in many curious operations in photography. It is also employed in the manufacture of dyes for the hair; it is applied to white hair with gall-nut, and under the influence of the light it turns black, and gives the hair a very dark shade. Salts of silver in solution with water have the property of forming a precipitate under the influence of chlorides, such as sea salt. If a few grains of common salt are thrown into a solution of nitrate of silver, it forms an abundant precipitate of chloride of silver, which blackens in the light. This precipitate, insoluble in nitric acid, dissolves very easily in ammonia.

Platinum, which is the last of the precious metals that we have to consider, is a greyish-white colour, and like gold is only affected by a mixture of nitric acid and hydrochloric acid. It is the heaviest of all the[Pg 327] ordinary metals; its density is 21·50. It is very malleable and ductile, and can be beaten into very thin sheets, and into wires as slender as wires of gold. Platinum wires have even been made so fine that the eye can scarcely perceive them; these are known as Wollaston’s invisible wires. Platinum resists the action of the most intense fire, and we can only fuse it by means of a blow-pipe and hydro-oxide gas. Its inalterability and the resistance it opposes to fire render it very valuable for use in the laboratory. Small crucibles are made of it, which are used by chemists to calcine their precipitates in analytical operations, or to bring about reactions under the influence of a high temperature. Platinum may be reduced to very small particles; it then takes the form of a black powder. In this pulverulent condition it absorbs gases with great rapidity, to such an extent that a cubic centimetre can condense seven hundred and fifty times its own volume of hydrogen gas. It also condenses oxygen, and in a number of cases acts as a powerful agent. Platinum is also obtained in porous masses (“spongy platinum”), which produce phenomena of oxidation.

Fig. 316.—Gold-leaf exposed to the fumes of mercury.

A very ingenious little lamp may be constructed which lights of itself without the help of a flame. It contains a bell of glass, which is filled with hydrogen gas, produced by the action exercised by a foundation of zinc on acidulated water. If the knob on the upper part of the apparatus is pressed, the hydrogen escapes, and comes in contact with a piece of spongy platinum, which, acting by oxidation, becomes ignited. The flame produced sets fire to a small oil lamp, which is opposite the jet of gas. This very ingenious lamp is known under the name of Gay-Lussac’s lamp. Platinum can also produce, by mere contact, a great number of chemical reactions. Place in a test glass an explosive mixture formed of two volumes of hydrogen and one volume of oxygen; in this gas plunge a small piece of spongy platinum,[Pg 328] and the combination of the two bodies will be instantly brought about, making a violent explosion. Make a small spiral of platinum red-hot in the flame of a lamp, having suspended it to a card; then plunge it quickly into a glass containing ether, and you will see the metallic spiral remain red for some time, while in the air it would cool immediately. This phenomenon is due to the action of oxidation which the platinum exercises over the fumes of ether. This curious experiment is known under the name of the lamp without a flame. This remarkable oxidizing power of platinum, which has not yet been explained, was formerly designated by the title of catalytic action. But a phrase is not a theory, and it is always preferable to avow one’s ignorance than to simulate an apparent knowledge. Science is powerful enough to be able to express her doubts and uncertainties boldly. In observing nature we find an experience of this, and often meet with facts which may be put to profit, and become useful in application; nevertheless it is often the case that the why and the wherefore will for a long time escape the most penetrating eye and lucid intelligence. It is true the admirable applications of science strike us with the importance of their results, and the wonderful inventions they originate; but if they turn to account the observed facts of nature, what do they teach us as to the first cause of all things, the wherefore of nature?—Almost nothing. We must humbly confess our powerlessness, and say with d’Alembert: “The encyclopædia is very abundant, but what of that if it discourses of what we do not understand?”

Fig. 317.—Discolouration of periwinkles by sulphuric acid.

[Pg 329]

Artificial Colouring of Flowers.

In a course of chemistry, the action exercised by sulphurous acid on coloured vegetable matter is proved by exposing violets to the influence of this gas, which whitens them instantaneously. Sulphurous acid, by its dis-oxidating properties, destroys the colour of many flowers, such as roses, periwinkles, etc. The experiment succeeds very readily by means of the little apparatus which we give in fig. 317. We dissolve in a small vessel some sulphur, which ignites in contact with air, and gives rise, by its combination with oxygen, to sulphurous acid; the capsule is covered with a conical chimney, made out of a thin sheet of copper, and at the opening at the top the flowers that are to be discoloured are placed. The action is very rapid, and a few seconds only are necessary to render roses, periwinkles, and violets absolutely white.

Fig. 318.—Experiment for turning columbines a green colour with ammoniacal ether.

M. Filpol, a distinguished savant, has exhibited to the members of the Scientific Association, Paris, the results which he obtained by subjecting flowers to the influence of a mixture of sulphuric ether and some drops of ammonia; he has shown that, under the influence of this liquid, a great number of violets or roses turn a deep green. We have recently made on this subject a series of experiments which we will here describe, and which may be easily attempted by those of our readers who are interested in the[Pg 330] question. Some common ether is poured into a glass, and to it is added a small quantity of liquid ammonia (about one-tenth of the volume). The flowers with which it is desired to experiment are then plunged into the fluid (fig. 318). A number of flowers, whose natural colour is red or violet, take instantaneously a bright green tint; these are red geranium, violet, periwinkle, lilac, red and pink roses, wall-flower, thyme, small blue campanula, fumeter, myosotis, and heliotrope. Other flowers, whose colours are not of the same shade, take different tints when in contact with ammoniacal ether. The upper petal of the violet sweet-pea becomes dark blue, whilst the lower petal turns a bright green colour. The streaked carnation becomes brown and bright green. White flowers generally turn yellow, such as the white poppy, the variegated snow-dragon, which becomes yellow and dark violet, the white rose, which takes a straw colour, white columbine, camomile, syringa, white daisy, potatoe blossom, white julian, honeysuckle, and white foxglove, which in contact with ammoniacal ether assume more or less deep shades of yellow. White snap-dragon becomes yellow and dark orange. Red geranium turns blue in a very remarkable fashion; with the monkey-flower the ammoniacal ether only affects the red spots, which turn a brownish green; red snap-dragon turns a beautiful brown; valerian takes a shade of grey; and the red corn-poppy assumes a dark violet. Yellow flowers are not changed by ammoniacal ether; buttercups, marigolds, and yellow snap-dragon preserve their natural colour. Leaves of a red colour are instantly turned green when placed in contact with ammoniacal ether. The action of this liquid is so rapid that it is easy to procure green spots by pouring here and there a drop of the solution. In like manner violet flowers, such as periwinkles, can be spotted with white, even without gathering them. We will complete our remarks on this subject with a description of experiments performed by M. Gabba in Italy by means of ammonia acting on flowers. M. Gabba simply used a plate, in which he poured a certain quantity of solution of ammonia. He placed on the plate a funnel turned upside down, in the tube of which he arranged the flowers on which he wished to experiment. He then found that under the influence of the ammonia the blue, violet, and purple flowers became a beautiful green, red flowers black, and white yellow, etc.

The most singular changes of colour are shown by flowers which are composed of different tints, their red streaks turning green, the white yellow, etc. Another curious example is that of red and white fuchsias, which, through the action of ammonia, turn yellow, blue, and green. When flowers have been subjected to these changes of colour, and afterwards plunged into pure water, they preserve their new tint for several hours, after which they gradually return to their natural colour. Another interesting observation, due to M. Gabba, is that asters, which are naturally inodorous, acquire an agreeable aromatic odour under the influence of ammonia. Asters of a violet colour become red when wetted with nitric acid mixed with water. On the other hand, if these same flowers are enclosed in a wooden box, where they[Pg 331] are exposed to the fumes of hydrochloric acid, they become, in six hours’ time, a beautiful red colour, which they preserve when placed in a dry, shady place, after having been properly dried. Hydrochloric acid has the effect of making flowers red that have been rendered green by the action of ammonia, and also alters their appearance very sensibly. We may also mention, in conclusion, that ammonia, combined with ether, acts much more promptly than when employed alone.

Phosphorescence.

Artificial flowers are frequently to be seen prepared in a particular manner, which have the property of becoming phosphorescent in darkness, when they have been exposed to the action of a ray of light, solar or electric. These curious chemical objects are connected with some very interesting phenomena and remarkable experiments but little known at the present time, to which we will now draw the reader’s attention.

The faculty possessed by certain bodies of emitting light when placed in certain conditions, is much more general than is usually supposed.

M. Edmond Becquerel, to whom we owe a remarkable work on this subject, divides the phenomena of phosphorescence into five distinct classes:

1. Phosphorescence through elevation of temperature. Among the substances which exhibit this phenomenon in a high degree we may mention certain diamonds, coloured varieties of fluoride of calcium, some minerals; and sulphur, known under the name of artificial phosphorus, when it has previously been exposed to the action of the light.

2. Phosphorescence through mechanical action. This is to be observed when we rub certain bodies together, or against a hard substance. If we rub together two quartz crystals in the dark, we perceive red sparks; and when pounding chalk or sugar, there is also an emission of sparks.

3. Phosphorescence through electricity. This is manifested by the light accompanying disengagement of electricity, and when gases and rarefied vapours transmit electric discharges.

4. Spontaneous Phosphorescence is observed, as every one knows, in connection with several kinds of living creatures,—glow-worms, noctilucids, etc., and similar phosphorescent effects are produced also with organic substances, animal or vegetable, before putrefaction sets in. It is manifested also at the flowering time of certain plants, etc.

5. Phosphorescence through insolation and the action of light. “It consists,” says M. Edmond Becquerel, “in exposing for some instants to the action of the sun, or to that of rays emanating from a powerful luminous source, certain mineral or organic substances, which immediately become luminous, and shine in the dark with a light, the colour and brilliancy of which depend on their nature and physical character; the light gradually diminishes in intensity during a period varying from some seconds to several hours. When these substances are exposed anew to the action of light, the same effect is[Pg 332] reproduced. The intensity of the light emitted after insolation is always much less than that of the incidental light.” These phenomena appear to have been first observed with precious stones; then, in 1604, in calcined Bologna stone, and later, in a diamond by Boyle, in 1663; in 1675 it was noticed in Baudoin phosphorus (residuum of the calcination of nitrate of lime), and more recently still in connection with other substances which we will mention. The substances most powerfully influenced by the action of light are sulphates of calcium and barium, sulphate of strontium, certain kinds of diamonds, and that variety of fluoride of calcium, which has received the name of chlorophane.

Fig. 319.—Artificial flower coated with phosphorescent powder, exposed to the light of magnesium wire.

Phosphorescent sulphate of calcium is prepared by calcining in an earthenware crucible a mixture of flowers of sulphur and carbonate of lime. But the preparation only succeeds with carbonate of lime of a particular character. That obtained from the calcination of oyster shells produces very good results. Three parts of this substance is mixed with one part of flowers of sulphur, and is made red-hot in a crucible covered in from contact with the air. The substance thus obtained gives, after its insolation, a yellow light in the dark. The shells of oysters, however, are not always[Pg 333] pure, and the result is sometimes not very satisfactory; it is therefore better to make use of some substance whose composition is more to be relied on.

“When we desire to prepare a phosphorescent sulphate with lime, or carbonate of lime,” says M. E. Becquerel, “the most suitable proportions are those which in a hundred parts of the substance are composed of eighty to a hundred of flowers of sulphur in the first case, and forty-eight to a hundred in the second, that is, when we employ the quantity of sulphur which will be necessary for burning with carbonate of lime to produce a monosulphate.20 It is necessary to have regard to the elevation of the temperature in the preparation. By using lime procured from arragonite, and reducing the temperature below five hundred degrees for a sufficient time for the reaction between the sulphur and lime to take place, the excess of sulphur is eliminated, and we have a feebly luminous mass, of a bluish tint; if this mass is raised to a temperature of eight hundred or nine hundred degrees, it will exhibit a very bright light.”

Sulphate of calcium possesses different phosphorescent properties according to the nature of the salt which has served to produce the carbonate of lime employed. If we transform marble into nitrate of lime, by dissolving it in water and nitric acid, and form a precipitate with carbonate of ammonium, and use the carbonate of lime thus obtained in the preparation of sulphate of calcium, we have a product which gives a phosphorescence of a violet-red colour. If the carbonate of lime used is obtained from chloride of calcium precipitated by carbonate of ammonia, the phosphorescence is yellow. If we submit carbonate of lime, prepared with lime water and carbonic acid, to the influence of sulphur, we obtain a sulphur giving a phosphorescent light of very pure violet. Carbonate of lime obtained by forming a precipitate of crystallized chloride of calcium with different alkaline carbonates also gives satisfactory results.

Luminous sulphates of strontium may be obtained, like those of calcium, by the action of sulphur on strontia or the carbonate of this base, by the reduction of sulphates of strontia with charcoal. Blue and green shades are the most common. Sulphates of barium also present very remarkable phenomena of phosphorescence; but to obtain very luminous intensity a higher temperature is needed than with the other substances mentioned, and we have the same result when we reduce native sulphate of baryta with charcoal; that is to say, when the reaction takes place which produces the phosphorus formerly known as phosphorus of Bologna. Preparations obtained from baryta have a phosphorescence varying from orange-red to green.

The preparation of such substances as we have just enumerated afford an easy explanation of the method of manufacturing the luminous flowers which we described at the commencement of this chapter. We obtain some artificial flowers, cover them with some liquid gum, sprinkle with phosphorescent sulphur, and let them dry. The pulverulent matter then adheres to them securely, and it is only necessary to expose the flowers[Pg 334] thus prepared to the light of the sun, or the rays emanating from magnesium wire in a state of combustion (fig. 319), to produce immediate phosphorescent effects. If taken into a dark room (fig. 320) they shine with great brilliancy, and give off very exquisite coloured rays. Phosphorescent sulphates are used also in tracing names or designs on a paper surface, etc., and it can easily be conceived that such experiments may be infinitely varied according to the pleasure of the experimenter.

Fig. 320.—Phosphorescent flower emitting light in a dark room.

But let us ask ourselves if these substances are not capable of being put to more serious uses, and of being classed among useful products. To this we can reply very decidedly in the affirmative. With phosphorescent matter we can obtain luminous faces for clocks placed in dark, obscure spots, and it is not impossible to use it for making sign-boards for shops, or numbers of houses, which can be lit up at night. Professor Norton even goes so far as to propose in the “Journal of the Franklin Institute,” not only coating the walls of rooms with these phosphorescent substances, but also the fronts of houses, when he considers it would be possible to do away entirely with street lights, the house-fronts absorbing sufficient light during the day to remain luminous the whole of the night.

[Pg 335]

Chemistry Applied to Sleight of Hand.

While physics has provided the species of entertainment called “sleight of hand” with a number of interesting effects, chemistry has only offered it very feeble contributions. Robert Houdin formerly made use of electricity to move the hands of his magic clock, and the electric magnet in making an iron box so heavy instantaneously that no one could lift it. Robin has made use of optics to produce the curious spectacle of the decapitated man, spectres, etc. Those persons who are fond of this kind of amusement may, however, borrow from chemistry some original experiments, which can be easily undertaken, and I will conclude this chapter by describing a juggling feat which I have seen recently executed before a numerous audience by a very clever conjuror.

Fig. 321.—Amusing experiment in chemistry.

The operator took a glass that was perfectly transparent, and placed it on a table, announcing that he should cover the glass with a saucer, and then, retiring to some distance, would fill it with the smoke from a cigarette. And this he carried out exactly, standing smoking his cigarette in the background, while the glass, as though by enchantment, slowly filled with the fumes of the smoke. This trick is easily accomplished. It is only necessary to pour previously into the glass two or three drops of hydrochloric acid, and to moisten the bottom of the saucer with a few drops of ammonia. These two liquids are unperceived by the spectators, but as soon as the saucer is placed over the glass, they unite in forming white fumes of hydrochlorate of ammonia, which bear a complete resemblance to the smoke of tobacco.

This experiment excited the greatest astonishment among the spectators present on the occasion, but understanding something of chemistry myself, I easily guessed at the solution of the mystery. The same result is obtained in a course of chemistry in a more simple manner, and without any attempt at trickery, by placing the opening of a bottle of ammonia against the opening of another bottle containing hydrochloric acid.

[Pg 336]

CHAPTER XXVI.

CHEMISTRY AND ALCHEMY—CHEMICAL COMBINATIONS—THE ATMOSPHERIC AIR.

We have in the foregoing pages given some experiments, and considered several of the metals, but there are numerous very interesting subjects still remaining; indeed, the number is so great that we can only pick and choose. All people are desirous to hear something of the atmosphere, of water, and the earth; and as we proceed to speak of crystals and minerals, and so on to geology, we shall learn a good deal respecting our globe—its conformation and constituents. But the atmospheric air must be treated of first. This will lead us to speak of oxygen and nitrogen. Water will serve to introduce hydrogen with a few experiments, and thus we shall have covered a good deal of ground on our way towards various other elements in daily use and appreciation. Now let us begin with a few words concerning Chemistry itself.

At the very outset we are obliged to grope in the dark after the origin of this fascinating science. Shem, or “Chem,” the son of Noah, has been credited with its introduction, and, at any rate, magicians were in Egypt in the time of Moses, and the lawgiver is stated by ancient writers to have gained his knowledge from the Egyptians. But we need not pursue that line of argument. In more modern times the search for the Philosopher’s Stone and the Elixir of Life, which respectively turned everything to gold, and bestowed long life upon the fortunate finder, occupied many people, who in their researches no doubt discovered the germs of the popular science of Chemistry in Alchemy, while the pursuit took a firm hold of the popular imagination for centuries; and even now chemistry is the most favoured science, because of its adaptability to all minds, for it holds plain and simple truths for our every-day experience to confirm, while it leads us step by step into the infinite, pleasing us with experiments as we proceed.

Alchemy was practised by numerous quacks in ancient times and the Middle Ages, but all its professors were not quacks. Astrology and alchemy were associated by the Arabians. Geber was a philosopher who devoted[Pg 337] himself entirely to alchemy, and who lived in the year 730 A.D. He fancied gold would cure all disease, and he did actually discover corrosive sublimate, nitric acid, and nitrate of silver. To give even a list of the noted alchemists and magicians would fill too much space. Raymond Sully, Paracelsus, Friar Bacon, Albertus Magnus, Thomas Aquinas, Flamel, Bernard of Treves, Doctor Dee, with his assistant Kelly, and in later times Jean Delisle, and Joseph Balsamo (Cagliostro), who was one of the most notorious persons in Europe about one hundred years ago (1765-1789), are names taken at random; and with the older philosophers chemistry was an all-absorbing occupation—not for gold, but knowledge.

The revelation was slow. On the temperature of bodies the old arts of healing were based—for chemistry and medicine were allies. The elements, we read, existed on the supposition “that bodies were hot or cold, dry or moist”; and on this distinction for a long time “was based the practice of medicine.” The doctrine of the “three principles” of existence superseded this,—the principles being salt, mercury, and sulphur. Metals had been regarded as living bodies, gases as souls or spirits. The idea remained that the form of the substance gave it its character. Acid was pointed; sweet things were round.

Chemistry, then, has had a great deal to contend against. From the time of the Egyptians and Chinese, who were evidently acquainted with various processes,—dyeing, etc.,—the science filtered through the alchemists to Beecher and Stahl, and then the principle of affinity—a disposition to combine—was promulgated, supplemented in 1674 by Mayow, by the theory of divorce or analysis. He concluded that where union could be effected, separation was equally possible. In 1718 the first “Table of Affinities” was produced. Affinity had been shown to be elective, for Mayow pointed out that fixed salts chose one acid rather than another. Richter and Dalton made great advances. Before them Hales, Black, Priestley, Scheele, Lavoisier, and numerous others penetrated the mysteries of the science whose history has been pleasantly written by more than one author who we have not been able to consult, and have no space to do more than indicate. In later days Faraday, De la Rive, Roscoe, and many others have rendered chemistry much more popular, while they have added to its treasures. The story of the progress of chemistry would fill a large volume, and we have regretfully to put aside the introduction and pass on.

Before proceeding to investigate the elements, a few words concerning the general terms used in chemistry will be beneficial to the reader. If we look at the list of the elements, pp. 308-9, we shall see various terminations. Some are apparently named from places, some from their characteristics. Metals lately discovered by the spectroscope (and recently) end in ium; some end in “ine,” some in “on.” As far as possible in late years a certain system of nomenclature has been adhered to, but the old popular names have not been interfered with.

When elements combine together in certain proportions of each they[Pg 338] receive certain names. The following table will explain the terms used ; for instance, we find that—

The above examples refer to the union in single proportion of each, and are called Binary Compounds. When more than one atom of each element exists in different proportions we have different terms to express such union. If one atom of oxygen be in the compound it is called a “monoxide” or “protoxide”; two atoms of oxygen in combination is termed “dioxide” or “binoxide”; three, “trioxide,” or “tritoxide”; four is the “tetroxide” or “per-oxide,” etc. When more than one atom, but not two atoms is involved, we speak of the sesqui-oxide (one-and-a-half),—“oxide” being interchangeable for “sulphide” or “chloride,” according to the element.

There are other distinctions adopted when metals form two series of combinations, such as ous and ic, which apply, as will be seen, to acids. Sulphuric and sulphurous acids, nitric and nitrous acid are familiar examples. In these cases we shall find that in the acids ending in “ous” oxygen is present in less quantity than in the acids ending in ic. The symbolic form will prove this directly, the number of atoms of oxygen being written below,

Sulphurous Acid = H₂ SO₃
Nitrous Acid = HNO₂.
Sulphuric Acid = H₂SO₄.
Nitric Acid = HNO₃.

Whenever a stronger compound of oxygen is discovered than that denominated by ic, chemists adopt the plan of dubbing it the per (ὑπέρ over), as per-chloric acid, which possesses four atoms of oxygen (HClO₄), chloric acid being HClO₃. The opposite Greek term, ὑπὸ (hupo, below), is used for an acid with less than two atoms of oxygen, and in books is written “hypo”-chlorous (for instance). Care has been taken to distinguish between the higher and lower; for “hyper” is used in English to denote excess, as hyper-critical; and hypo might to a reader unacquainted with the derivation convey just the opposite meaning to what is intended.

While speaking of these terminations we may show how these distinctive endings are carried out. We shall find, if we pursue the subject, that when wehave a salt of any acid ending in ic the salt terminates in “ate.” Similarly the salts of acids ending in ous, end in “ite.” To continue the same example we have—

Sulphurous Acid, which forms salts called Sulphites.
Sulphuric Acid, which forms salts called Sulphates.

Besides these are sulphides, which are results of the unions or com[Pg 339]pounds of elementary bodies. Sulphites are more complicated unions of the compounds. Sulphates are the salts formed by the union of sulphuric acid with bases. Sulphides or sulphurets are compounds in which sulphur forms the electro-negative element, and sulphites are salts formed by the union of sulphurous acids with bases, or by their action upon them.

Fig. 322.—Combinations of elements.

The symbolical nomenclature of the chemist is worse than Greek to the uninitiated. We frequently see in so-called popular chemical books a number of hieroglyphics and combinations of letters with figures very difficult to decipher, much less to interpret. These symbols take the place of the names of the chemical compounds. Thus water is made up of oxygen and hydrogen in certain proportions; that is, two of hydrogen to one of oxygen. The symbolic reading is simple, H₂O, = the oxide of hydrogen. Potassium again mingles with oxygen. Potassium is K in our list; KO is oxide of potassium (potash). Let us look into this a little closer.

The union of one particle of a simple body with a particle of another simple body can be easily understood; but, as we have seen, it is possible to have substances consisting of four or five different particles, though the greater number of chemical combinations consist of two or three dissimilar ones. In the diagram (fig. 322) we have some possible combinations.

In these combinations we may have one particle of a in combination with one, two, three, four, or five of b, and many particles of a can unite with various molecules of b. Suppose we have oxygen and sulphur compounds as follows:—

Thus there are three different compounds of these two elements—SO, SO₂, SO₃ (without water).

Fig. 323.-(1) Hydrosulphurous Acid. (2) Sulphurous Acid. (3) Sulphuric Acid.

A compound body may combine with another compound body, and this makes a complicated compound. Suppose we have a mixture of sulphuric acid and potash. We have a sulphate of potassium (K₂SO₄) and combinations of these combinations may likewise be formed. We must read these symbols by the light of the combining weights given in the table, and then we shall find the weight of oxygen or other elements in combination. Thus when we see a certain symbol (Hg.S for instance), we understand that they form a compound including so many parts of mercury and so many of sulphur, which is known as vermilion. Hg.O is oxide of mercury, and by reference to the table of Atomic Weights, we find mercury is Hg., and its combining weight is[Pg 340] 200; while oxygen is O, and its weight is 16. Thus we see at once how much of each element is contained in oxide of mercury, and this proportion never varies; there must be 200 of one and 16 of the other, by weight, to produce the oxide. So if the oxygen has to be separated from it, the sum of 216 parts must be taken to procure the 16 parts of oxygen. When we see, as above, O₂ or O₃, we know that the weight must be calculated twice or three times, O being 16; O₂ is therefore 32 parts by weight. So when we have found what the compounds consist of, we can write them symbolically with ease.

Composition of the Atmospheric Air.

We have already communicated a variety of facts concerning the air. We have seen that it possesses pressure and weight. We call the gaseous envelope of the earth the atmosphere, and we are justified in concluding that other planets possess an atmosphere also, though of a different nature to ours. We have seen how easy it is to weigh the air, but we may repeat the experiment. (See illustration, fig. 45, page 50.) We shall find that a perfectly empty glass globe will balance the weights in the scale-pan; admit the air, and the glass globe will sink. So air possesses weight. We have mentioned the Magdeburg hemispheres, the barometer, the air-pump, and the height and the pressure of the atmosphere have been indicated. The density of the atmosphere decreases as we ascend; for the first seven miles the density diminishes one-fourth that of the air at the sea-level, and so on for every succeeding seven.

In consequence of the equal, if enormous, pressure exercised in every direction, we do not perceive the inconvenience, but if the air were removed from inside of a drum, the parchment would quickly collapse. We feel the air when we move rapidly. We breathe the air, and that statement brings us to consider the composition of the atmosphere, which, chemically speaking, may vary a little (as compared with the whole mass) in consequence of changes which are continually taking place, but to all intents and purposes the air is composed as follows, in 100 parts:

with minute quantities of other ingredients, such as ammonia, iodine, carbonetted hydrogen, hydrochloric acid, sulphuretted hydrogen, nitric acid, carbonic oxide, and dust particles, as visible in the sunbeams, added.

The true composition of the atmosphere was not known till Lavoisier demonstrated that it consisted of two gases, one of which was the vital fluid, or oxygen, discovered by Priestley. To the other gas Lavoisier gave the name of Azote,—an enemy of life,—because it caused death if inhaled alone. The carbonic acid in the air varies very much, and in close, heated, and crowded rooms increases to a large quantity, which causes lassitude and headache.

[Pg 341]

We can easily prove the existence of carbonic acid gas as exhaled from the lungs. Suppose we take a glass and fill it partly with clear lime-water; breathe through a glass tube into the water in the glass, and very quickly you will perceive that the lime-water is becoming cloudy and turbid. This cloudiness is due to the presence of chalk, which has been produced by the action of the carbonic acid gas in the lime-water. This is a well known and always interesting experiment, because it leads up to the vital question of our existence, and the functions of breathing and living.

A popular writer once wrote a book entitled, “Is Life Worth Living?” and a witty commentator replied to the implied question by saying, “It depends upon the liver.” This was felt to be true by many people who suffer, but the scientific man will go farther, and tell you it depends upon the air you breathe, and on the carbonic acid you can raise to create heat,—animal heat,—which is so essential to our well-being. We are always burning; a furnace is within us, never ceasing to burn without visible combustion. We are generating heat by means of the blood. We know that we inhale air into the lungs, and probably are aware that the air so received parts with the oxygen to renew the blood. The nitrogen dilutes the oxygen, for if we inhaled a less-mixed air we should either be burnt up or become lunatics, as light-headed as when inhaling “laughing-gas.” This beautifully graduated mixture is taken into our bodies, the oxygen renews the blood and gives it its bright red colour; the carbon which exists in all our bodies is cold and dead when not so vivified by oxygen. The carbonic acid given off produces heat, and our bodies are warm. But when the action ceases we become cold, we die away, and cease to live. Man’s life exemplifies a taper burning; the carbon waste is consumed as the wax is, and when the candle burns away—it dies! It is a beautiful study, full of suggestiveness to all who care to study the great facts of Nature, which works by the same means in all matter. We will refer to plants presently, after having proved by experiment the existence of nitrogen in the air.

Rutherford experimented very cruelly upon a bird, which he placed beneath a glass shade, and there let it remain in the carbonic acid exhaled from its lungs, till the oxygen being at length all consumed by the bird, it died. When the atmosphere had been chemically purified by a solution of caustic potash, another bird was introduced, but though it lived for some time, it did not exist so long as the first. Again the air was deprived of the carbonic acid, and a third bird was introduced. The experiment was thus repeated, till at length a bird was placed beneath the receiver, and it perished at once. This is at once a cruel and clumsy method of making an experiment, which can be more pleasantly and satisfactorily practised by burning some substance in the air beneath the glass. Phosphorus, having a great affinity for oxygen, is usually chosen. The experiment can be performed as follows with a taper, but the phosphorus is a better exponent.

Let us take a shallow basin with some water in it, a cork or small plate floating upon the water, and in the plate a piece of phosphorus.[Pg 342] We must be careful how we handle phosphorus, for it has a habit, well known, but sometimes forgotten by amateur chemists, of suddenly taking fire. Light this piece of phosphorus,—a small piece will do if the jar be of average “shade” size,—and place the glass over it, as in the illustration (fig. 325). The smoke will quickly spread in the jar, and the entry of air being prevented, because the jar is resting under water, phosphoric acid will be formed, and the oxygen thereby consumed. The water, meanwhile, will rise in the jar, the pressure of the air being removed. The burning phosphorus will soon go out, and when the glass is cool, you will be able to ascertain what is inside the jar. Put a lighted taper underneath, and it will go out. The taper would not go out before the phosphorus was burnt in the glass, and so now we perceive we have azote in the receptacle—that is, nitrogen. The other, the constituent of our atmosphere, carbonic acid, as we have seen, is very injurious to the life of animals, and as every animal breathes it out into the air, what becomes of it? Where does all this enormous volume of carbonic acid, the quantities of this poison which are daily and nightly exhaled, where do they all go to? We may be sure nature has provided for the safe disposal of it all. Not only because we live and move about still,—and of course that is a proof,—but because nature always has a compensating law. Remember nothing is wasted; not even the refuse, poisonous air we get rid of from our lungs. Where does it go?

Fig. 324.—Rutherford’s experiment.

It goes to nourish the plants and trees and vegetables that we delight to look upon and to eat the fruit of. Thus the vegetable world forms a link between the animals and the minerals. Vegetables obtain food, so to[Pg 343] speak, and nourishment from water, ammonia, and carbonic acid, all compound bodies, but inorganic.

Water consists of oxygen and hydrogen, carbonic acid of carbon and oxygen, and ammonia of hydrogen and nitrogen. Water and ammonia are present in the air; so are oxygen and nitrogen. Water falls in the form of rain, dew, etc. So in the atmosphere around us we find nearly every necessary for plant-life; and in the ground, which supplies some metallic oxides for their use, we find the remainder. From the air, then, the plant derives its life.

Fig. 325—Drawing the oxygen from air by combustion.

The vegetable kingdom in turn gives all animals their food. This you will see at a glance is true. Certainly animals live on animals. Man and wilder animals live on the beasts of the field in a measure, but those beasts derive their nourishment from vegetables—the vegetable kingdom. So we live on the vegetable kingdom, and it separates the carbonic acid from the air, and absorbs it. What we do not want it takes. What we want it gives. Vegetables give out oxygen, and we consume it gladly. We throw away carbonic acid, and the plants take it greedily; and thus the atmosphere is retained pure for our use. We can, if desirable, prove that plants absorb carbonic acid and give out oxygen by placing leaves of a plant in water, holding the acid in solution, and let the sun shine upon them. Before long we shall find that the carbonic acid has disappeared, and that oxygen has come into the water.

Carbonic acid is sufficiently heavy to be poured from one vessel to another; and if we have obtained some in a glass, we can extinguish a taper by pouring the invisible gas on to the lighted taper, when it will be immediately extinguished.

[Pg 344]

From the foregoing observations it will be perceived how very desirable it is that ventilation should be attended to. People close up windows and doors and fireplaces, and go to bed and sleep. In the morning they complain of headache and lassitude; they wonder what is the matter, and why the children are not well. Simply because they have been rebreathing the carbonic acid. Go into a closed railway carriage which is nearly filled (and it is astonishing to us how people can be so foolish as to close every outlet), and you will recoil in disgust. These travellers shut the ventilators and windows “because of the cold.” A very small aperture will ventilate a railway carriage; but a close carriage is sickening and enervating, as these kind of travellers find out by the time they reach their journey’s end. Air was given us to breathe at night as well as by day; and though from man’s acts or omissions there may be circumstances in which “night” air may affect the health, we maintain that air is no more injurious naturally than “day” air. Colder it may be, but any air at night is “night” air, in or out of doors at night; and we are certain that night air in itself never hurt any healthy person. It is not nature’s plan to destroy, but to save. If a person delicate in constitution gets hot, and comes out into a colder atmosphere, and defy nature in that way, he (or she) must take the consequences. But air and ventilation (not draught) are necessaries of health, and to say they injure is to accuse nature falsely. There are many impurities in the air in cities, and in country places sometimes, but such impurities are owing to man’s acts and omissions. With average sanitary arrangements and appliances in a neighbourhood no one need be afraid to breathe fresh air night or day; and while many invalids have, we believe, been retarded in recovery from being kept in a close room, hundreds will be benefited by plenty of fresh air. We should not so insist upon these plain and simple truths were there not so many individuals who think it beneficial to close up every avenue by which air can enter, and who then feel ill and out of spirits, blaming everything but their own short-sightedness for the effect of their own acts. An inch or two of a window may be open at night in a room, as the chimney register should be always fully up in bedrooms. When there are fires the draught supplies fresh air to the room with sufficient rapidity. But many seaside journeys might be avoided if fresh air were insisted on at home.

There is another and an important constituent of the atmosphere called Ozone, which is very superior oxygen, or oxygen in what is termed the “Allotropic” state, and is distantly related to electricity, inasmuch as it can be produced by an electrical discharge. This partly accounts for the freshness in the air after a thunderstorm, for we are all conscious that the storm has “cleared the air.” The fresh, crisp ozone in the atmosphere is evident. Ozone differs from oxygen in possessing taste and smell, and it is heavier by one-half than the oxygen gas. There is a good deal of ozone in the sea breeze, and we can, though not infallibly, detect its presence by test-paper prepared with iodide of potassium, which, when ozone is present, will turn[Pg 345] blue. We have still something to learn about ozone, which may be considered as “condensed oxygen.”

Fig. 326.   Development of gas by combustion.   Fig. 327.

We have frequently mentioned “combustion,” and as under ordinary circumstances such effects cannot take place without atmospheric air, we will consider it. Combustion is chemical action accompanied by light and heat. Chemical union is always attended by the development of heat, not always by light, because the union varies in intensity and quickness. But when a candle is burning we can study all the interesting phenomena of combustion. We have already spoken of Heat and Light, so we need only refer the readers to those subjects in the former parts of this volume. Heat is referable to chemical action, and varies according to the energy of union. Heat is always present, remember, in a greater or less degree; and when visible combustion takes place we see light. Invisible combustion goes on in our bodies, and we feel heat; when we get cold we feed the fire by eating, or blow it by exercise and air in our lungs.

Fig. 328.—Gas evolved from flame.

We shall speak, however, of combustion now as it affects us in daily life; our fires, our candles, gas, etc., and under these ordinary circumstances hydrogen and carbon are present. (We shall hear more about carbon presently.) These unite with the oxygen to form water and carbonic acid; the water being visible as we first put the cold shade upon the lighted lamp, and the carbonic acid renders the air impure.

[Pg 346]

In the case of a common candle, or lamp, combustion takes place in the same way. The wick is the intermediary. The oil mounts in the lamp wick, where it is converted into a gas by heat; it then “takes fire,” and gives us light and heat. The candle-flame is just the same with one exception: the burning material is solid, not liquid, though the difference is only apparent, for the wax is melted and goes up as gas. The burning part of the wick has a centre where there is no combustion, and contains carbon. We can prove this by placing a bent tube, as in the illustration (fig. 326), one end in the unburning part of the flame. We shall soon see a dark vapour come over into the receiver. This is combustible, for if we raise the tube without the glass we can light the gas (fig. 327). If we insert the end of the tube into the brilliant portion of the flame we shall perceive a black vapour, which will extinguish the combustion, for it is a mixture of carbonic acid gas and aqueous vapour, in which (fig. 328) particles of carbon are floating.

Fig. 329.—Davy’s safety lamp.

Fig. 330.—Davy lamp (section).

When we proceed to light our lamps to read or to write by, we find some difficulty in making the wick burn at first. We present to it a lighted taper, and it has no immediate effect. Here we have oil and cotton, two[Pg 347] things which would speedily set a warehouse in flames from top to bottom, but we cannot even ignite them, try all we can. Why?—Because we must first obtain a gas, oil will not burn liquid; it must be heated to a gaseous point before it will burn, as all combustion depends upon that,—so flames mount high in air. Now in a candle-flame, as will be seen in the diagram (fig. 331.), there are three portions,—the inner dark core, which consists of unburnt gas; the outer flame, which gives light; and the outside rim of perfect combustion non-luminous. In the centre, A, there is no heat. If we place a piece of gauze wire over the flame at a little distance the flame will not penetrate it. It will remain underneath, because the wire, being of metal, quickly absorbs the heat, and consequently there is no flame. This idea led to the invention of the “safety” lamp by Sir Humphrey Davy, which, although it is not infallible, is the only lamp in general use in mines (figs. 329, 330).

Fig. 331.—Construction of a candle flame.

Mines must have light, but there is a gas in mines, a “marsh” gas, which becomes very explosive when it mixes with oxygen. Of course the gas will be harmless till it meets oxygen, but, in its efforts to meet, it explodes the moment the union takes place; instead of burning slowly like a candle it goes off all at once. This gas, called “fire damp,” is carburetted hydrogen, and when it explodes it develops into carbonic acid gas, which suffocates the miners.

Fig. 332.—Pouring carbonic acid on a lighted taper.

[Pg 348]

CHAPTER XXVII.
NON-METALLIC ELEMENTS.

OXYGEN—SYMBOL O; ATOMIC WEIGHT 16.

Oxygen is certainly the most abundant element in nature. It exists all around us, and the animal and vegetable worlds are dependent upon it. It constitutes in combination about one-half of the crust of the earth, and composes eight-ninths of its weight of water. It is a gas without taste or colour. Oxygen was discovered by Priestley and Scheele, in 1774, independently of each other.

Fig. 333.—Oxygen from oxide of mercury.

Oxygen can be procured from the oxides of the metals, particularly from gold, silver, and platinum. The noble metals are reducible from their oxides by heat, and this fact assists us at once. If we heat chlorate of potash, mixed with binoxide of manganese, in a retort in a furnace, the gas will be given off. There are many other ways of obtaining oxygen, and we illustrate two (figs. 333, 335).

The red oxide of mercury will very readily evolve oxygen, and if we heat a small quantity of the compound in a retort as per illustration (fig. 333) we shall get the gas. In a basin of water we place a tube test-glass, and the gas from the retort will pass over and collect in the test tube, driving out the water.

Fig. 334.—Showing retort placed in furnace.

The other method mentioned above,—viz., by heating chlorate of potash, etc., in a furnace, is shown in the following illustration. Oxygen, as we have said, is a colourless and inodorous gas, and for a long time it could not be obtained in any other form; but lately both oxygen and hydrogen have[Pg 349] been liquified under tremendous pressure at a very low temperature. Oxygen causes any red-hot substance plunged into it to burn brightly; a match will readily inflame if a spark be remaining, while phosphorus is exceedingly brilliant, and these appearances, with many others equally striking, are caused by the affinity for those substances possessed by the gas. Combustion is merely oxidation, just as the process of rusting is, only in the latter case the action is so slow that no sensible heat is produced. But when an aggregate of slowly oxidising masses are heaped together, heat is generated, and at length bursts into flame. This phenomenon is called “spontaneous combustion.” Cases have been known in which the gases developed in the human body by the abuse of alcoholic drinks have ended fatally; in like manner the body being completely charred. (Combustion must not be confounded with ignition, as in the electric light.) Oxygen then, we see, is a great supporter of combustion, though not a combustible itself as coal is. When the chemical union of oxygen with another substance is very rapid an explosion takes place.

Fig. 335.—The generation of oxygen from oxide of manganese and potash.

Oxidation occurs in various ways. Besides those already mentioned, all verdigris produced on copper, all decays of whatever kind, disintegration, and respiration, are the effects of oxygen. The following experiment for the extraction of oxygen directly from the air was made by M. Boussingault, who passed the gas upon a substance at a certain temperature, and released it at a higher. The illustration on page 351 will show the way in which the experiment was performed.

[Pg 350]

Boussingault permitted a thin stream of water to flow into a large empty flask, and by this water the air was gradually driven out into a flask containing chloride of calcium and sulphuric acid, which effectually dried it. This dry air then passed into a large tube inside the reverberatory furnace, in which tube were pieces of caustic baryta. Heated to a dull redness this absorbs oxygen, and when the heat is increased to a bright red the superabundant gas is given off. Thus the oxygen was permitted to pass from the furnace-tube into the receiving glass, and so pure oxygen was obtained from the air which had been in the glass bottle at first (fig. 338).

Fig. 336.—Phosphorus burning in oxygen.

HYDROGEN—SYMBOL H; ATOMIC WEIGHT 1.

Hydrogen is abundant in nature, but never free. United with oxygen it forms water, hence its name, “water-former.” It is to Parcelcus that its discovery is due, for he found that oil of vitriol in contact with iron disengaged a gas which was a constituent of water. This gas was subsequently found to be inflammable, but it is to Cavendish that the real explanation of hydrogen is owing. He explained his views in 1766.

Hydrogen is obtained in the manner illustrated in the cut, by means of a furnace, as in fig. 339, or by the bottle method, as per fig. 340. The first method is less convenient than the second. A gun-barrel or fire-proof tube is passed through the furnace, and filled with iron nails or filings; a delivery tube is at the farther end, and a flask of water boiling at the other. The oxygen combines with the iron in the tube, and the hydrogen passes over. The second method is easily arranged. A flask, as in the cut, is provided, and in it some zinc shavings are put. Diluted sulphuric acid is then poured upon the metal. Sulphate of zinc is formed in the flask, and the hydrogen passes off.

Hydrogen being the lightest of all known bodies, its weight is put as 1, and thus we are relatively with it enabled to write down the weights of all the other elements. Hydrogen is fourteen-and-a-half times lighter than atmospheric air, and would do admirably for the inflation of balloons were it not so expensive to procure in such large quantities as would be necessary. Ordinary coal gas, however, contains a great deal of hydrogen, and answers the same purpose.

Fig. 337.—Magnesium wire burning in oxygen.

[Pg 351]

A very pretty experiment may be made with a bladder full of hydrogen gas. If a tube be fitted to the bladder already provided with a stop-cock, and a basin of ordinary soap-suds be at hand, by dipping the end of the tube in the solution and gently expressing the gas, bubbles will be formed which are of exceeding lightness (fig. 341). They can also be fired with a taper.

Another experiment may be made with hydrogen as follows:—If we permit the gas to escape from the flask, and light it, as in the illustration, and put a glass over it, we shall obtain a musical note, higher or lower, according to the length, breadth, and thickness of the open glass-tube (fig. 342). If a number of different tubes be employed, we can obtain a musical instrument—a gas harmonium.

Fig. 338.—Extraction of oxygen from air.

Hydrogen burns with a blue flame, and is very inflammable. Even water sprinkled upon a fire will increase its fierceness, because the hydrogen burns with great heat, and the oxygen is liberated. Being very light, H can be transferred from one vessel to another if both be held upside down. Some mixtures of H and O are very explosive. The oxyhydrogen blow-pipe is used with a mixture of O and H, which is forcibly blown through a tube and then ignited. The flame thus produced has a most intense heating-power.

A very easy method of producing hydrogen is to put a piece of sodium[Pg 352] into an inverted cylinder full of water, standing in a basin of water. The sodium liberates the hydrogen by removing the oxygen from the liquid.

WATER—SYMBOL H₂O; ATOMIC WEIGHT 18.

At page 59 of this volume we said something about water, and remarked (as we have since perceived by experiment) that “water is composed of oxygen and hydrogen in proportions, by weight, of eight of the former to one of the latter gas; in volume, hydrogen is two to one”; and we saw that “volume and weight were very different things.” This we will do well to bear in mind, and that, to quote Professor Roscoe, “Water is always made up of sixteen parts of oxygen to two parts of hydrogen by weight”; sixteen and two being eighteen, the combining weight of water is eighteen.

Fig. 339.—Preparation of hydrogen with furnace.

Fig. 340.—Apparatus for generating hydrogen by flask.

We can prove by the Eudiometer that hydrogen when burnt with[Pg 353] oxygen forms water; and here we must remark that water is not a mere mechanical mixture of gases, as air is. Water is the product of chemical combination, and as we have before said, is really an oxide of hydrogen, and therefore combustion, or electricity, must be called to our assistance before we can form water, which is the result of an explosion, the mixture meeting with an ignited body—the aqueous vapour being expanded by heat.

The ancients supposed water to be a simple body, but Lavoisier and Cavendish demonstrated its true character. Pure water, at ordinary temperatures, is devoid of taste and smell, and is a transparent, nearly colourless, liquid. When viewed in masses it is blue, as visible in a marked degree in the Rhone and Rhine, at Geneva, and Bâle respectively. Its specific gravity is 1, and it is taken as the standard for Sp. Gravity, as hydrogen is taken as the standard for Atomic Weight. The uses of water and the very important part it plays in the arrangements of nature as a mechanical agent, geology can attest, and meteorology confirm. It composes the greater portions of animals and plants; without water the world would be a desert—a dead planet.

Fig. 341.—Blowing bubbles with hydrogen gas.

We sometimes speak of “pure” spring water, but such a fluid absolutely pure can scarcely be obtained; and though we can filter water there will always remain some foreign substance or substances in solution. It is well known that the action of water wears away and rounds off hard rocks, and this power of disintegration is supplemented by its strength as a solvent, which is very great. Rain-water is purest in the country as it falls from the clouds. In smoky towns it becomes sooty and dirty. It is owing to the solvent properties of water, therefore, that we have such difficulty in obtaining a pure supply. There is hard water and soft water. The former[Pg 354] is derived from the calcareous formations, and contains lime, like the Kent water. This can be ascertained by noticing the incrustations of the vessels wherein the water is boiled. But water rising from hard rocks, such as granite, can do little to disintegrate them at the moment, and therefore the water rises purer. Springs from a great depth are warm, and are known as “thermal springs”; and when they come in contact with carbonic acid and some salts in their passage to the surface, they are known as “mineral waters.” These waters hold in solution salts of lime and magnesia, or carbonates of soda with those of lime and magnesia; salts of iron, and compounds of iodine and bromine are found in the natural mineral waters also, as well as sulphurous impregnations, instances of which will occur to every reader.

Fig. 342.—Experiment with hydrogen.

Fig. 343.—The composition of water.

We mentioned the Eudiometer just now, and we give an illustration of it. This instrument is used to ascertain the proportions in which the elements of water are composed by synthesis, or a putting together of the constituents of a body to make it up. This is distinguished from analysis, which means separating the compound body into its elements, as we do when we pass the electric current through water.

The Eudiometer consists of a stout glass tube sealed hermetically at one end; two platinum wires are pushed in through the glass just before the end is sealed. The tube is now filled with mercury, and inverted in a bowl of the same metal. Hydrogen, and then oxygen, are admitted through the mercury in the recognised proportions of two to one. By the time the mercury is somewhat more than half displaced, the tube should be held[Pg 355] upon a sheet of india-rubber at the bottom of the vessel to keep the metal in the tube, for when the necessary explosion takes place the mercury might also be driven out. A spark from the electrophorus or from a Leyden jar may now be passed through the gases in the tube. The explosion occurs, and water is formed inside. If the mercury be again admitted it will rise nearly to the very top of the tube, driving the bubble up. Thus we find we have formed water from the two gases.

The decomposition of water is easily affected by electricity, and if a little sulphuric acid be added to the water, the experiment will be thereby facilitated. Two wires from a battery should be inserted through a glass filled with the water, and into two test tubes also filled. The wires terminate in platinum strips, and are fastened at the other end to the positive and negative poles of the galvanic battery. The gases will collect in the test tubes, and will be found in proper proportions when the current passes.

Fig. 344.—The Eudiometer.

Fig. 345.—Decomposition of water.

So much for water in its liquid state. The solid condition of water (ice) is equally interesting. When we apply heat to water, we get a vapour called “steam”; when we cool water to 32° Fahr., we get a solid mass which weighs just the same as the liquid we have congealed, or the steam we have raised from an equal amount of water. But water expands while in the process of solidification, just as it does when it becomes gaseous, and as we have remarked before, our water-pipes bear full testimony to this scientific fact. When ice forms it has a tendency to crystallize, and some of these ice crystals are, as we see, very beautiful. Snow is only water in a nearly solid form, and the crystals are extremely elegant, appearing more like flowers than congealed water, in tiny six-pointed ice crystals. Many philosophers of late years have written concerning these tiny crystals, which, in common with all crystals, have their own certain form, from which they[Pg 356] never depart. Snowflakes are regular six-sided prisms grouped around a centre forming angles of 60° and 120°. There are a number of forms, as will be seen from the accompanying illustrations, and at least ninety-six varieties have been observed. One snowflake, apparently so like all other flakes that fall, can thus be viewed with much interest, and yet, while so very various, snowflakes never get away from their proper hexagonal structure. It has been remarked that snowflakes falling at the same time have generally the same form.

Of the latent heat of ice, etc., we have already spoken in our article upon Heat, and therefore it will be sufficient to state that the latent heat of water is 79 thermal units, because when passing from the liquid to the solid state a certain amount of water absorbs sufficient heat to raise an equal quantity of the liquid 79°. This can be proved by taking a measured quantity (say a pint) of water at 79° and adding ice of the same weight to the water. The mixture will be found to be at zero. Therefore the ice has absorbed or rendered latent 79° of heat which the water possessed. If we melt ice until only a trace of it is left, we shall still find the water as cold as the ice was; all the latent heat is employed in melting the ice. So it will take as much heat to bring a pound of ice at zero to a pound of water at zero, as it would to raise 79 pounds of water 1°. The same law applies to steam.

Fig. 346.—Snow crystals.

Water can be distilled in small quantities by an apparatus, as figured in the illustration, and by these means we get rid of all impurities which are[Pg 357] inseparable from the liquid otherwise. When it is desirable to distil large quantities of water a larger apparatus is used, called an “Alembic.” The principle is simply to convert the liquid by heat into vapour, then cool it, by condensation, in another vessel.

Fig. 347.—Distilling water.

The evaporation of water, with its effects upon our globe, belong more to the study of Meteorology.

Fig. 348.—Distillation.

Rain-water is the purest, as we have said, because it goes through the process of distillation by nature. The sun takes it up, by evaporation, into the air, where it is condensed, and falls as rain-water. Water containing carbonate of lime will petrify or harden, as in stalactite caverns. The carbonic acid escapes from the dripping water, the carbonate in solution is deposited as a stalactite, and finally forms pillars in the cave. Sea-water[Pg 358] contains many salts; its composition is as follows, according to Dr. Schwertzer, of Brighton:—

Fig. 349.—Stalactite Cavern.

There is much more oxygen in water than in air, as can be ascertained by analysis of these compounds. This great proportion in favour of water enables fish to breathe by passing the water through the gills. Marine animals (not fishes), like the whale,—which is a warm-blooded creature, and therefore not suited to exist without air,—are obliged to come to the surface to breathe. The density of salt water is much greater than that of fresh water, and therefore swimming and flotation is easier in the sea than in a river. We shall have more to say of water by-and-by.

[Pg 359]

NITROGEN—SYMBOL N; ATOMIC WEIGHT 14.

We have already made some reference to this gas when speaking of the atmosphere and its constituents, of which nitrogen is the principal. From its life-destroying properties it is called “azote” by French chemists, and when we wish to obtain a supply of nitrogen all we have to do is to take away the oxygen from the air by burning phosphorus on water under a glass. Nitrogen is not found frequently in solid portions of the globe. It is abundant in animals. It is without colour or smell, and can be breathed in air without danger. It is heavy and sluggish; but if we put a taper into a jar of nitrogen it will go out, and animals die in the gas for want of oxygen, as nitrogen alone cannot support life.

Fig. 350.—Obtaining nitrogen.

The affinity of nitrogen for other substances is not great, but it gives rise to five compounds, which are as below, in the order they are combined with oxygen:—

These compounds are usually taken as representative examples of combining weight, and as explanatory of the symbolic nomenclature of chemistry, as they advance in such regular proportions of oxygen with nitrogen. The combining weight of nitrogen is 14, and when two parts combine with five of oxygen it makes nitric acid, and we put it down as N₂O₅; on adding water, HNO₃, as we can see by eliminating the constituents and putting in the proportions. Actually it is H₂N₂O₆, or, by division, HNO₃.

Nitrogen plays a very important part in nature, particularly in the vegetable kingdom. Nitric acid has been known for centuries. Geber, the alchemist, was acquainted with a substance called “nitric,” which he found would yield a dissolvent under certain circumstances. He called it “dissolving fluid.” At the end of the twelfth century Albert Magnus investigated the properties of this acid, and in 1235 Raymond Lully prepared nitre with clay, and gave the liquid the name of “aqua-fortis.” But till 1849 nitric acid was only known as a hydrate,—that is, in combination with water,—but now we have the anhydrous acid.

[Pg 360]

Fig. 351.—Apparatus for obtaining nitrogen by using metal to absorb the oxygen of the air.

Oxygen and nitrogen combine under the influence of electricity, as shown by Cavendish, who passed a current through an atmospheric mixture of oxygen and nitrogen, in a tube terminating in a solution of potash, lime, and soda. Every time the spark passed, the volume of gas diminished, and nitric acid was formed, as it is in thunderstorms, when it does not remain free, but unites with ammonia, and forms a highly useful salt, which promotes vegetable growth. Here is another instance of the usefulness of thunderstorms, and of the grand provisions of nature for our benefit. Nitric acid is obtained by distilling nitre with sulphuric acid. The liquid is, when pure, colourless, and is a powerful oxidizer. It dissolves most metals, and destroys vegetable and animal substances. By an addition of a little sulphuric acid the water is taken from the nitric acid, and a very powerful form of it is the result. The acid is of great use in medicine, and as an application to bites of rabid animals or serpents. It converts cotton waste into “gun-cotton” by a very simple process of steeping, washing, and pressing. From the hydraulic press it comes in discs like “quoits,” which will burn harmlessly and smoulder away, but if detonated they explode with great violence. As a rule, when damp, it is not dangerous, but it can be fired even when wet. It will explode at a less temperature than gunpowder, and, moreover,[Pg 361] yields no smoke, nor does it foul a gun. Gun-cotton, when dissolved in ether, gives us collodion for photographic purposes.

Fig. 352.—Nitric acid obtained from nitre and sulphuric acid.

In speaking farther of the compounds of nitrogen with oxygen, we will limit ourselves to the monoxide, or laughing gas. This is now used as an anæsthetic in dentistry, etc., and is quite successful, as a rule. People afflicted with heart disease should not use it without advice, however. When inhaled into the lungs it makes the subject very hilarious, and the effect is rather noisy. It is obtained from the nitrate of ammonia, which, on the application of heat, decomposes into nitrous oxide and vapour. Warm water should be used for the trough. The gas is a powerful supporter of combustion.

Fig. 353.—Cavendish’s experiment.

Binoxide of nitrogen is of importance in the manufacture of sulphuric acid.

[Pg 362]

Fig. 354.—Experiment to obtain nitric acid.

Nitrogen combines with hydrogen, forming various compounds. These are the “amines,” also ammonia, and ammonium. Ammonia possesses the properties of a base. Its name is derived from Jupiter Ammon, near whose temple it was prepared, from camels’ dung. But bodies containing nitrogen give off ammonia in course of distilling, and hartshorn is the term applied to horn-cuttings, which yield ammonia, which is a colourless gas of strong odour and taste now obtained from gas-works.

Fig. 355.—Apparatus for obtaining laughing-gas.

Fig. 356.—Inhaling laughing gas.

Fig. 357.—Generation of ammonia.

To obtain ammonia heat equal parts of chloride of ammonia (sal ammoniac) and quick-lime powdered (see fig. 357). The gas must be collected over mercury, because it is very soluble in water. Ammonia is useful to restore tipsy people and fainting ladies. A solution of ammonia is used for cauteries. Ammoniacal gas is remarkable for its solubility in water. To prepare the solution the gas is forced through a series of flasks. The tubes carrying the gas should be continued to the bottoms of the flasks, else the solution, being lighter than water, the upper portion alone would be saturated. The tubes carrying away the solution are raised[Pg 363] a little, so that the renewal is continually proceeding. The gas liquifies under a pressure of six atmospheres, at a temperature of 10° Cent. This experiment can be artificially performed by heating chloride of silver saturated with ammonia, and the silver will part with the gas at a temperature of 40° C. The gas will then condense in a liquid form in the tube. The experiment may be facilitated by placing the other extremity of the tube in snow and salt, and by the liquid we can obtain intense cold. This experiment has been made use of by M. Carré in his refrigerator (which was described in the Physics’ section), by which he freezes water. We may, however, just refer to the process. Whenever the condition of a body is changed from that of liquid to a gas, the temperature is greatly lowered, because the heat becomes “latent.” The latest freezing machine consists of an apparatus as shown in the illustrations herewith (figs. 359 and 360). The machine is of wrought iron, and contains, when ready for action, a saturated[Pg 364] solution of ammonia at zero. This is in communication with another and an air-tight vessel, of which the centre is hollow. The first process is to heat the solution, and the gas escapes into the second “vase,” which is surrounded by cold water, and quite unable to escape. A tremendous pressure is soon obtained, and this, added to the cold water, before long liquifies the ammonia, and when the temperature indicates 130° the hot vessel is suddenly cooled by being put into the water. The gas is thus suddenly converted into a liquid, the water in the second hollow vase is taken out, and the[Pg 365] bottle to be frozen is put into the cavity. The cold is so great, in consequence of the transformation of the liquid ammonia into a gas, that it freezes the water in any vessel put into the receiver. The ammonia can be reconverted into liquid and back again, so no loss is occasioned by the process, which is rapid and simple. This is how great blocks of ice are produced in water-bottles.

Fig. 358.—Liquefaction of ammonia.

The one important point upon which care is necessary is the raising of the temperature. If it be elevated beyond 130° C., the pressure will be too great, and an explosion will occur.

Fig. 359.—Carré’s refrigerator (first action).

The abundant formation of ammonia from decaying animal matter is evident to everyone, and depends upon the presence of moisture to a great extent. Chloride of ammonia is called sal-ammoniac, and the carbonate of ammonia crystallizes from the alkaline liquid produced by the distillation of certain animal matter. The compounds of ammonia are easily recognized by a certain sharp taste. They are highly valuable remedial agents, acting particularly upon the cutaneous system, and when taken internally, produce the effect of powerful sudorifics. Their volatility, and the facility with which they are expelled from other substances, render them of great importance in chemistry, and peculiarly fit them for the purposes of many chemical analyses. The ammonia compounds display a remarkable analogy to the corresponding combinations of potash and soda. The compounds of ammonia are highly important in their relation to the vegetable kingdom. It may be assumed that all the nitrogen of plants is derived from the ammonia which they absorb from the soil, and from the surrounding atmosphere.

Fig. 360.—Carré’s refrigerator (second action).

The similarity of ammonia to the metallic oxides has led to the conjecture that all its combinations contain a compound metallic body, which has received the name ammonium (NH₄); but no one has yet succeeded in its preparation, although by peculiar processes it may be obtained in the form of an amalgam.

Ammonias, in which one or more atoms of hydrogen are replaced by basic radicals, are termed Amides, or Amines.

[Pg 366]

CHAPTER XXVIII.
NON-METALLIC ELEMENTS (continued).

CHLORINE—BROMINE—IODINE—FLUORINE—CARBON—SULPHUR—PHOSPHORUS—SILICON—BORON—TELLURIUM—ARSENIC.

Chlorine (Cl.) is usually found with sodium in the mineral kingdom, and this chloride of sodium is our common salt. Chlorine can be obtained by heating hydrochloric acid with binoxide of manganese. (Atomic weight 35·5.)

Fig. 361.—Generation of chlorine.

Chlorine possesses a greenish-yellow colour, hence its name “Chloros,” green. It should be handled carefully, for it is highly injurious and suffocating. It possesses a great affinity for other substances, and attacks the metals. For hydrogen it has a great affection, and when hydrogen is combined with any other substances chlorine immediately attacks them, and in time destroys them. But even this destructive and apparently objectionable quality makes chlorine very valuable; for if we carry the idea to its conclu[Pg 367]sion, we shall find that it also destroys offensive and putrid matter, and purifies the atmosphere very much. Most colouring matters include hydrogen, and therefore they are destroyed by chlorine, which is a great “bleacher” as well as a purifier. If we dip any vegetable dyes into a jar of chlorine, they will become white if the dyed substances are damp.

Hydrochloric acid is known as muriatic acid and spirits of salt. It is obtained when salt is treated with sulphuric acid and the gas comes off into water. Equal parts of the acid and the salt are put into a flask as in the cut (fig. 362), and diluted with water. The mixture is then heated. The gas is condensed in the bottles half-full of water. The result gives sulphate of soda and hydrochloric acid. This acid is procured in soda manufactories, and with nitric acid is called “aqua regia,” a solvent for gold. When chlorine and hydrogen are mixed in equal proportions they explode in sunlight. In the dark or by candle-light they are harmless. Dry chlorine gas can be obtained by interposing a glass filled with some chloride of calcium. The gas being heavier than air (about 2½ times), displaces it in the flask, and when it is filled another can be placed in position. This mode causes a little waste of gas, which should not be breathed.

Fig. 362.—Production of hydrochloric acid.

Chlorine possesses a great affinity for certain bodies. If the gas be thrown upon phosphorus, the latter will burn brilliantly. Arsenic, tin, and antimony when powdered and poured from a shoot into a vase of chlorine will burst into brilliant sparks, and other metals will glow when introduced to this gas. Chlorine forms many unstable combinations with oxygen. Its combination with hydrogen has already been referred to.

Bromine is a rare element. (Symbol Br. Atomic weight 80.)

It is deep brownish red, very volatile, and of a peculiar odour. Bromine unites with the elementary bodies, and forms some oxygen compounds. It resembles chlorine in its properties, and is used in medicine and in photography. It is found in saline springs and in salt water, combined with soda and magnesium. The presence of bromine may easily be[Pg 368] detected in the strong smell of seaweed. Its combinations with metals are termed bromides. It is a powerful poison.

Iodine is another relative of chlorine. It is found in seaweed, which by burning is reduced to kelp. When iodine is heated a beautiful violet vapour comes off, and this characteristic has given it its name (“iodes,” violet). Iodine was discovered by Courtois, of Paris, and in 1813, Gay Lussac made it a special study. It is solid at ordinary temperatures, and assumes crystallized forms in plates of metallic lustre. It is an excellent remedy in “goitre” and such affections. (Symbol I. Atomic weight 127.)

Fluorine is very difficult to prepare. Fluor spar is a compound of fluorine and calcium. This element is gaseous, and combines so rapidly that it is very difficult to obtain in a free state. Etching on glass is accomplished by means of hydrofluoric acid, for fluorine has a great affinity for silicic acid, which is contained in glass. The glass is covered with wax, and the design is traced with a needle. The acid attacks the glass and leaves the wax, so the design is eaten in. (Symbol F. Atomic weight 19.)

Fig. 363.—Apparatus for obtaining dry chlorine gas.

Chlorine, fluorine, bromine, and iodine are termed “Halogens” (producers of salts). They appear, as we have seen, in a gaseous, liquid, and solid form respectively.

Carbon is the most, or one of the most, largely diffused elements in nature, and claims more than a passing notice at our hands, though even that must be brief. We may put down carbon next to oxygen as the most important element in the world. The forms assumed by carbon are very variable, and pervade nature in all its phases. We have carbon in crystals,[Pg 369] in the animal and vegetable kingdoms, and amongst the chief minerals a solid, odourless, tasteless, infusible, and almost insoluble body. In various combinations carbon meets us at every turn; united with oxygen it forms carbonic acid, which we exhale for the plants to imbibe. We have it in coal, with hydrogen and oxygen. We have it building up animal tissues, and it is never absent in two out of the three great divisions of nature—the plants and the animals (Symbol C; Atomic W. 12).

Fig. 364.—Facets of a brilliant.

Fig. 365.—Facets of a rose diamond.

We have carbon in three different and well-known conditions; as the diamond, as graphite, or black-lead, and as charcoal. The properties of the diamond are well known, and we shall, when we get to Crystallography, learn the forms of diamond or crystals of carbon. At present we give an illustration or two, reserving all explanation for the present. Diamond cutting is a matter of some difficulty, and it requires skill to cut in the proper direction. Diamonds are found in India, Brazil, and at the Cape of Good Hope, in alluvial soil. The identity of diamond and charcoal was discovered accidentally. An experiment to fuse a few small diamonds[Pg 370] resulted in their disappearance, and when the residue was examined it was found that the diamonds had been burned, that they had combined with oxygen and formed carbonic acid, just as when coal burns. The diamond is the hardest of all substances, the most valuable of gems, and the purest condition in which carbon appears.

Graphite (Plumbago) is termed “black-lead,” and is the next purest form of carbon. It crystallizes and belongs to the primitive formations. In Cumberland it is dug up and used to make pencils; the operations can be seen at Keswick. It has other uses of a domestic character.

Charcoal is the third form of carbon, and as it possesses no definite form, is said to be amorphous. Charcoal is prepared in air-tight ovens, so that no oxygen can enter and burn the wood thus treated. Coke is the result of the same process applied to coal. The gas manufactories are the chief depôts for this article, and it is used in locomotive engines. The various smokeless coals and prepared fuels, however, are frequently substituted.

Fig. 366.—Coke ovens.

Coke ovens were formerly much resorted to by the railway companies, who found the ordinary coal too smoky for locomotive purposes, and apt to give rise to complaints by passengers and residents near the line.

The origin of wood charcoal we have seen. All vegetable substances contain carbon. When we burn wood, in the absence of air as far as possible, oxygen and hydrogen are expelled. The wood is piled in layers as in the illustration (fig. 368), covered over with turf and mould, with occasional apertures for air. This mass is ignited, the oxygen and hydrogen are driven[Pg 371] off, and carbon remains. (Animal charcoal is obtained from calcining bones). Wood charcoal attracts vapours, and water, if impure, can be purified by charcoal, and any impure or tainted animal matter can be rendered inoffensive by reason of charcoal absorbing the gases, while the process of decay goes on just the same. Housekeepers should therefore not always decide that meat is good because it is not offensive to the olfactory nerves. Charcoal will remove the aroma, but the meat may be nevertheless bad. The use of charcoal in filters is acknowledged universally, and as a constituent of gunpowder it is important.

Fig. 367.—Charcoal burning.

Carbon is not easily affected by the atmospheric air, or in the earth; so in many instances wood is charred before being driven into the ground; and casks for water are prepared so. Soot is carbon in a pulverised condition, and Indian ink is manufactured with its assistance.

Fig. 368.—Wood piles of charcoal burners.

The preparation of wood charcoal gives occupation to men who are frequently wild and untutored, but the results of their labour are very beneficial. Care should be taken not to sleep in a room with a charcoal stove burning, unless there is ample vent for the carbonic acid gas, for it will cause suffocation. Lampblack is obtained by holding a plate over the flame of some resinous substance, which deposits the black upon it. There is a special apparatus for this purpose.

Fig. 369.—Seltzer-water manufactory.

Carbon combines with oxygen to make carbonic acid gas, as we have already mentioned, and in other proportions to form a more deadly compound [Pg 372]than the other. The former is the dioxide (CO₂), the latter the monoxide, or carbonic oxide (CO). The dioxide is the more important, being held in the atmosphere, and combined with lime in chalk. All sparkling beverages contain carbonic acid, to which their effervescence is due. The soda and other mineral waters owe their sparkle to this gas. Soda-water consists of a weak solution of carbonate of soda and the acid. There is a vessel holding chalk and water, and another containing some sulphuric acid. When the sulphuric acid is permitted to unite with the chalk and water, carbonic acid is liberated. A boy turning a wheel forces the gas into the water in the bottles, or the water and carbonate of soda is drawn off thus impregnated into bottles and corked down, in the manner so familiar to all. The bottles are made of the shape depicted, so that the[Pg 373] bubble of air shall be at the top when the bottle lies down. If it be not kept so, the air will eventually escape, no matter how tightly the cork be put in. The ordinary “soda-water” contains scarcely any soda. It is merely water, chalk, and carbonic acid. The “Gazogene” is made useful for small quantities of soda-water, and is arranged in the following manner. The appearance of it is familiar to all. It consists of a double vessel, into the upper part of which a solution of any kind—wine and water, or even plain water—is put, to be saturated with carbonic acid, or “aerated,” and into the lower one some carbonate of soda and tartaric acid. A tube leads from this lower to the top of the upper vessel, which screws on and off. By shaking the apparatus when thus charged and screwed together, some of the liquid descends through the tube into the lower vessel and moistens the soda and acid, which therefore act on each other, and cause carbonic acid to be disengaged; this, rising up through the tube (which is perforated with small holes at the upper part), disperses itself through the liquid in small bubbles, and causes sufficient pressure to enable the liquid to absorb it, which therefore effervesces when drawn off by the tap.

Fig. 370.—Gazogene.

Carbonic acid can be liquified, and then it is colourless. In a solid form it resembles snow, and if pressed with the fingers it will blister them. Being very heavy the gas can be poured into a vase, and if there be a light in the receptacle the flame will be immediately extinguished.

Fig. 371.—Soda-water apparatus.

That even the gas introduced into seltzer-water is capable of destroying life, the following experiment will prove. Let us place a bird within a[Pg 374] glass case as in the illustration (fig. 373), and connect the glass with a bottle of seltzer-water or a siphon. As soon as the liquid enters, the carbonic acid will ascend, and this, if continued for a long time, would suffocate the bird, which soon begins to develop an appearance of restlessness.

Fig. 372.—Pouring out the carbonic acid gas.

We have already remarked upon the important part taken by this gas in nature, so we need only mention its existence in pits and caves. There are many places in which the vapour is so strong as to render the localities uninhabitable. In the Middle Ages the vapours were attributed to the presence of evil spirits, who were supposed to extinguish miners’ lamps, and suffocate people who ventured into the caves. In the Grotto Del Cane there is still an example, and certain caves of Montrouge are often filled with the gas. A lighted taper held in the hand will, by its extinction, give the necessary warning. Oxygen and carbon are condensed in carbonic acid, for the gas contains a volume of oxygen equal to its own. If we fill a glass globe, as per illustration (fig. 374), with pure oxygen, and in the globe insert two carbon points, through which we pass a current of electricity, we shall find, after the experiment, that if the stop-cock be opened, there is no escape of gas, and yet the mercury does not rise in the tube, so the oxygen absorbed has been replaced by an equal volume of carbonic acid.

Fig. 373.—Experiment with carbonic acid.

The other combination of carbon with oxygen is the carbonic oxide (CO), and when a small quantity of oxygen is burnt with it it gives a blue flame, as on the top of the fire in our ordinary grates. This gas is present[Pg 375] in lime kilns, and is a very deadly one. We must now pass rapidly through the compounds of carbon with hydrogen, merely referring to coal for a moment as we go on.

Coal, of which we shall learn more in Mineralogy and Geology, is a combination, mechanical or otherwise, and is the result of the decomposition of vegetable matter in remote ages,—the so-called “forests,” which were more like the jungles than the woods of the present day. Moss and fern played prominent parts in this great transformation, as we can see in the Irish peat-bogs, where the first steps to the coal measures are taken.

Fig. 374.—Experiment showing that carbonic acid contains oxygen and carbon.

The compounds of carbon with hydrogen are important. There is the “light” carburetted hydrogen (CH₄), which is usually known as fire-damp in coal mines. It is highly inflammable and dangerous. The safety-lamp invented by Davy is a great protection against it, for as the gas enters it is cooled by the wire, and burns within harmlessly. The explosion warns the miner. “Heavy” carburetted hydrogen possesses double the quantity of carbon (C₂H₄). It is also explosive when mixed with oxygen.

Fig. 375.—Temperature reduced by contact with wire.

The most useful compound is coal-gas, and though its principal function appears to be in some manner superseded by electricity, “gas” is still too important to be put aside. It can easily be obtained by putting small fragments of coal in the bowl of a tobacco-pipe, closing the bowl with clay, and putting it in[Pg 376] the fire. Before long the gas will issue from the stem of the pipe, and may either be lighted or collected in a bladder. For the use of the “million,” however, gas is prepared upon a very large scale, and is divided into three processes—its “formation,” “purification,” and its “collection” for distribution to consumers. The first process is carried on by means of retorts shown in the illustration (fig. 376). The first portion of the next figure is a section of a furnace, the other part shows two furnaces from the front. The following is the mode employed. The coal is put into retorts fitted to the furnace, so that they are surrounded by the flames, and terminating in a horizontal tube called the hydraulic main, E, which is in its turn connected with a pit or opening for the reception of the tar and ammoniacal liquor, etc., which condenses from the gas. It then passes up and down a series of tubes in water, called a “condenser,” and in this are reservoirs or receptacles for any tar and ammonia that remain. But sulphur is still present, so the gas is carried to the purifying apparatus (D in fig. 378), which consists of a large cylindrical vessel air-tight, with an inverted funnel, nearly filled with a mixture of lime and water. The gas bubbles in, and the sulphur unites with the lime, while the gas rises to the top (trays of lime are used when the gas enters from the bottom). The Gasometer, a large vessel closed at the top and open below, dips into a large trough of circular shape. The gasometer is balanced by weights and chains,[Pg 377] and may be raised (See fig. 379). When quite empty the top rests upon the ground, and when the gas enters it is raised to the top of the frame which supports it. We have now our Gasometer full. When the time comes to fill the pipes for lighting purposes, some of the weights are removed, the Gasometer falls down slowly, and forces the gas through the tubes into the main supply to be distributed. About four cubic feet of gas is obtained from every pound of coal. When gas and air become mixed, the mixture is very explosive. In a house where an escape of gas is detected let the windows be opened at the top, and no light introduced for several minutes.

Fig. 376.—Retorts.

Fig. 377.—Section. Front view.

Fig. 378.—Condenser. Purifier. Gasometer.

Fig. 379.—Gasometer.

It has been calculated that one ton of good coal produces the following:—

Fig. 380.—Gasometer.

We can thus estimate the profits of our gas companies at leisure. The analysis of gas made by Professor Bunsen is as under, in 100 parts.

Gas, therefore, is very injurious, for it rapidly vitiates the atmosphere[Pg 378] it burns in, and is very trying to the eyes, as well as destructive to gilt ornaments.

Tar is familiar to all readers, and though unpleasant to handle or to smell, it produces the beautiful aniline dyes. Tar pills are very efficacious for some blood disorders, and will remove pimples, etc., from the face, and cure “boils” effectually. If a dose of five be taken first, in a day or two four, and so on, no second remedy need be applied. We have known cases finally cured, and no recurrence of boils ever ensued after this simple remedy.

Fig. 381.—Tar manufactory

Tar is one of the results left in the distillation both of wood and coal: in places where wood is plentiful and tar in request, it is produced by burning the wood for that purpose; and in some of the pits in which charcoal is produced, an arrangement is made to collect the tar also. Coal-tar and wood-tar are different in some respects, and are both distilled to procure the napthas which bear their respective names. From wood-tar creosote is also extracted, and it is this substance which gives the peculiar tarry flavours to provisions, such as ham, bacon, or herrings, cured or preserved by being smoked over wood fires. Tar is used as a sort of paint for covering wood-work and cordage when much exposed to wet, which it resists better than anything else at the same price; but the tar chiefly used for these purposes is that produced by burning fir or deal wood and condensing the tar in a pit below the stack of wood; it is called Stockholm tar, as it comes chiefly from that place.

Carbon only combines with nitrogen under peculiar circumstances. This indirect combination is termed cyanogen (CN). It was discovered by Gay-Lussac, and is used for the production of Prussian blue. Hydrocyanide of potassium (Prussic acid) is prepared by heating cyanide of[Pg 379] potassium with sulphuric acid. It is a deadly poison, and found in peach-stones. Free cyanogen is a gas. The bisulphide of carbon is a colourless, transparent liquid. It will easily dissolve sulphur and phosphorus and several resins. When phosphorus is dissolved in it, it makes a very dangerous “fire,” and one difficult to extinguish. We must now leave carbon and its combinations, and come to sulphur.

Fig. 382.—Sulphur furnace.

Sulphur is found in a native state in Sicily and many other localities which are volcanic. It is a yellow, solid body, and as it is never perfectly free from earthy matter, it must be purified before it can be used. It possesses neither taste nor smell, and is insoluble in water. Sulphur is purified in a retort, C D, which communicates with a brick chamber, A. The retort is placed over a furnace, K, and the vapour passes into the chimney through the tube, D, where it condenses into fine powder called “flowers of sulphur” (brimstone). A valve permits the heated air to pass off, while no exterior air can pass in, for explosions would take place were the heated vapour to meet the atmospheric air. The danger is avoided by putting an air reservoir outside the chimney which is heated by the furnace. The sulphur is drawn out through the aperture, r, when deposited on the floor of the chamber. The sulphur is cast into cylinders and sold. Sulphur is soluble in bisulphide of carbon, and is used as a medical agent.

The compounds of sulphurs with oxygen form an interesting series. There are two anhydrous oxides (anhydrides),—viz., sulphurous and sulphuric [Pg 380]anhydride (SO₂ and SO₃). There are two notable acids formed by the combination with water, sulphurous and sulphuric, and some others, which, as in the case of nitrogen, form a series of multiple proportions, the oxygen being present in an increasing regularity of progression, as follows:—

The last four are termed “polythionic,” because the proportions of sulphur vary with constant proportions of the other constituents.

Fig. 383.—Liquefaction of sulphuric acid.

The sulphurous anhydride mentioned above is produced when we burn sulphur in the air, or in oxygen; it may be obtained in other ways. It is a colourless gas, and when subjected to pressure may be liquified, and crystallized at very low temperature. It was formerly called sulphuric acid. It is a powerful “reducing agent,” and a good antiseptic. It dissolves in water, and forms the H₂SO₃, now known as sulphurous acid.

Fig. 384.—Retorts and receivers for acid.

Sulphuric acid is a most dangerous agent in wicked or inexperienced hands, and amateurs should be very careful when dealing with it. It takes the water from the moist air, and from vegetable and animal substances. It carbonizes and destroys all animal tissues. Its discovery is due to Basil Valentine, in 1440. He distilled sulphate of iron, or green vitriol, and the result was “oil of vitriol.” It is still manufactured in this way in the Hartz district, and the acid passes by retorts into receivers. The earthen retorts, A, are arranged[Pg 381] in the furnace as in the illustration, and the receivers, B, containing a little sulphuric acid, are firmly fixed to them. The oily brown product fumes in the air, and is called “fuming sulphuric acid,” or Nordhausen acid. Sulphuric acid is very much used in chemical manufactures, and the prices of many necessaries, such as soap, soda, calico, stearin, paper, etc., are in close relationship with the cost and production of sulphur, which also plays an important part in the making of gunpowder. The manufacture of the acid is carried on in platinum stills.

Fig. 385.—Experiment to show the existence of gases in solution.

Sulphuretted hydrogen, or the hydric sulphide (H₂S), is a colourless and horribly-smelling gas, and arises from putrefying vegetable and animal matter which contains sulphur. The odour of rotten eggs is due to this gas, which is very dangerous when breathed in a pure state in drains, etc. It can be made by treating a sulphide with sulphuric acid. It is capable of precipitating the metals when in solution, and so by its aid we can discover the metallic ingredient if it be present. The gas is soluble in water, and makes its presence known in certain sulphur springs. The colour imparted to egg-spoons and fish-knives and forks sometimes is due to the presence of metallic sulphides. The solution is called hydro-sulphuric acid.

Phosphorus occurs in very small quantities, though in the form of phosphates we are acquainted with it pretty generally, and as such it is absorbed by plants, and is useful in agricultural operations. In our organization—in the brain, the nerves, flesh, and particularly in bones—phosphorus is present, and likewise in all animals. Nevertheless it is highly poisonous. It is usually obtained from the calcined bones of mammalia by obtaining phosphoric acid by means of acting upon the bone-ash with sulphuric acid.[Pg 382] Phosphorus when pure is colourless, nearly transparent, soft, and easily cut. It has a strong affinity for oxygen. It evolves white vapour in atmospheric air, and is luminous; to this element is attributable the luminosity of bones of decaying animal matter. It should be kept in water, and handled—or indeed not handled—but grasped with a proper instrument.

Phosphorus is much used in the manufacture of lucifer matches, and we are all aware of the ghastly appearance and ghostly presentment it gives when rubbed upon the face and hands in the dark. In the ripples of the waves and under the counter of ships at sea, the phosphorescence of the ocean is very marked. In Calais harbour we have frequently noticed it of a very brilliant appearance as the mail steamer slowly came to her moorings. This appearance is due to the presence of phosphorus in the tiny animalculæ of the sea. It is also observable in the female glow-worm, and the “fire-fly.” Phosphorus was discovered by Brandt in 1669.

Fig. 386.—Manufacture of sulphuric acid.

It forms two compounds with oxygen-phosphorous acid, H₂PO₄, and phosphoric acid, H₃PO₄. The compound with hydrogen is well marked as phosphuretted hydrogen, and is a product of animal and vegetable decomposition. It may frequently be observed in stagnant pools, for when emitted it becomes luminous by contact with atmospheric air. There is a very pretty but not altogether safe experiment to be performed when phosphuretted hydrogen has been prepared in the following manner. Heat small pieces of phosphorus with milk of lime or a solution of caustic potash; or make a paste of quick-lime and phosphorus, and put into the flask with some quick-lime powdered. Fix a tube to the neck, and let the other end be inserted in a basin of water. (See illustration, fig. 388.) Apply heat; the phosphuretted hydrogen will be given off, and will emerge from the water in the basin in luminous rings of a very beautiful appearance. The greatest care should be taken in the performance of this very simple experi[Pg 383]ment. No water must on any account come in contact with the mixture in the flask. If even a drop or two find its way in through the bent tube a tremendous explosion will result, and then the fire generated will surely prove disastrous. The experiment can be performed in a cheaper and less dangerous fashion by dropping phosphate of lime into the basin. We strongly recommend the latter course to the student unless he has had some practice in the handling of these inflammable substances, and learnt caution by experience. The effect when the experiment is properly performed is very good, the smoke rising in a succession of coloured rings.

Fig. 387.—(Phosphuretted hydrogen and marsh gas) Will-o’-the-Wisp.

Silicon is not found in a free state in nature, but, combined with oxygen, as Silica it constitutes the major portion of our earth, and even occurs in wheat stalks and bones of animals. As flint or quartz (see Mineralogy) it is very plentiful, and in its purest form is known as rock crystal, and approaches the form of carbon known as diamond. When separated from oxygen, silicon is a powder of greyish-brown appearance, and when heated in an atmosphere of oxygen forms silicic “acid” again, which, however, is not acid to the taste, and is also termed “silica,” or “silex.” It is fused with great difficulty, but enters into the manufacture of glass in the form of sand. The chemical composition of glass is mixed silicate of potassium or sodium, with silicates of calcium, lead, etc. Ordinary window-glass is a mixture of silicates of sodium and calcium; crown glass contains calcium and silicate of potassium. Crystal glass is a mixture of the same silicate and lead. Flint glass is of a heavier composition. Glass can be coloured by copper to a gold tinge, blue by cobalt, green by chromium, etc.[Pg 384] Glass made on a large scale is composed of the following materials, according to the kind of glass that is required.

Flint glass (“crystal”) is very heavy and moderately soft, very white and bright. It is essentially a table-glass, and was used in the construction of the Crystal Palace. Its composition is—pure white sea-sand, 52 parts, potash 14 parts, oxide of lead, 34 parts = 100.

The ingredients to be made into glass (of whatever kind it may be) are thoroughly mixed together and thrown from time to time into large crucibles placed in a circle, A A (fig. 389), in a furnace resting on buttresses, B B, and heated to whiteness by means of a fire in the centre, C, blown by a blowing machine, the tube of which is seen at D. This furnace is shown in prospective in fig. 390. The ingredients melt and sink down into a clear fluid, throwing up a scum, which is removed. This clear glass in the fused state is kept at a white heat till all air-bubbles have disappeared; the heat is then lowered to a bright redness, when the glass assumes a consistence and ductility suitable to the purposes of the “blower.”

Fig. 388.—Experiment with phosphuretted hydrogen.

Glass blowing requires great care and dexterity, and is done by twirling a hollow rod of iron on one end of which is a globe of melted glass, the workman blowing into the other end all the time. By reheating and twirling a sheet of glass is produced. Plate glass is formed by pouring the molten glass upon a table with raised edges. When cold it is ground with emery powder, and then polished by machinery.

Fig. 389.—Crucibles.

Many glass articles are cast, or “struck-up,” by compression in moulds, and are made to resemble cut-glass, but they are much inferior in appearance.[Pg 385] The best are first blown, and afterwards cut and polished. Of whatever kind of glass the article may be, it is so brittle that the slightest blow would break it, a bad quality which is got rid of by a process called “annealing,” that is, placing it while quite hot on the floor of an oven, which is allowed to cool very gradually. This slow cooling takes off the brittleness, consequently articles of glass well annealed are very much tougher than others, and will scarcely break in boiling water.

Fig. 390.—Plate-glass casting—bringing out the pot.

The kind generally used for ornamental cutting is flint-glass. Decanters and wine-glasses are therefore made of it; it is very bright, white, and easily cut. The cutting is performed by means of wheels of different sizes and materials, turned by a treadle, as in a common lathe, or by steam power; some wheels are made of fine sandstone, some of iron, others of tin or copper; the edges of some are square, or round, or sharp. They are used with sand and water, or emery and water, stone wheels with water only.

Fig. 391.—Glass furnace. (See also fig. 390 for detail.)

Fig. 392.—Glass-cutting.

In a soluble form silicic acid is found in springs, and thus enters into the composition of most plants and grasses, while the shells and scales of “infusoria” consist of silica. As silicate of alumina,—i.e., clay,—it plays a very important rôle in our porcelain and pottery works.

Boron is found in volcanic districts, in lakes as boracic acid, in combination with oxygen. It is a brownish-green, insoluble powder, in a free state, but as boracic acid it is white. It is used to colour fireworks with the beautiful green tints we see. Soda and boracic acid combine to make[Pg 386] borax (or biborate of soda). Another and inferior quality of this combination is tinkal, found in Thibet. Borax is much used in art and manufactures, and in glazing porcelain. (Symbol B, Atomic Weight 11).

Selenium is a very rare element. It was found by Berzilius in a sulphuric-acid factory. It is not found in a free state in nature. It closely resembles sulphur in its properties. Its union with hydrogen produces a gas, seleniuretted hydrogen, which is even more offensive than sulphuretted hydrogen. (Symbol Se, Atomic Weight 79).

Tellurium is also a rare substance generally found in combination with gold and silver. It is like bismuth, and is lustrous in appearance. Telluretted hydrogen is horrible as a gas. Tellurium, like selenium, sulphur, and oxygen, combines with two atoms of hydrogen. (Symbol Te, Atomic Weight 129).

Fig. 393.—Casting plate-glass.

Arsenic, like tellurium, possesses many attributes of a metal, and on the other hand has some resemblance to phosphorus. Arsenic is sometimes found free, but usually combined with metals, and is reduced from the ores by roasting; and uniting with oxygen in the air, is known as “white arsenic.” The brilliant greens on papers, etc., contain arsenic, and are poisonous on that account. Arsenic and hydrogen unite (as do sulphur and hydrogen, etc.), and produce a foetid gas of a most deadly quality. This element also unites with sulphur. If poured into a glass containing chlorine it will sparkle and scintillate as in the illustration (fig. 395). (Symbol As, Atomic Weight 75).

Before closing this division, and passing on to a brief review of the Metals, we would call attention to a few facts connected with the metalloids we have been considering. Some, we have seen, unite with hydrogen[Pg 387] only, as chlorine; some with two atoms of hydrogen, as oxygen, sulphur, etc., and some with three, as nitrogen and phosphorus; some again with four, as carbon and silicon. It has been impossible in the pages we have been able to devote to the Metalloids to do more than mention each briefly and incompletely, but the student will find sufficient, we trust, to interest him, and to induce him to search farther, while the general reader will have gathered some few facts to add to his store of interesting knowledge. We now pass on to the Metals.

Fig. 394—The manufacture of porcelain in China.

Fig. 395.—Experiment showing affinity between arsenic and chlorine.

[Pg 388]

CHAPTER XXIX.
THE METALS.

WHAT METALS ARE—CHARACTERISTICS AND GENERAL PROPERTIES OF METALS—CLASSIFICATION—SPECIFIC GRAVITY—DESCRIPTIONS.

We have learnt that the elements are divided into metalloids and metals, but the line of demarcation is very faint. It is very difficult to define what a metal is, though we can say what it is not. It is indeed impossible to give any absolute definition of a metal, except as “an element which does not unite with hydrogen, or with another metal to form a chemical compound.” This definition has been lately given by Mr. Spencer, and we may accept it as the nearest affirmative definition of a metal, though obviously not quite accurate.

Fig. 396.—Laminater.

A metal is usually supposed to be solid, heavy, opaque, ductile, malleable, and tenacious; to possess good conducting powers for heat and electricity, and to exhibit a certain shiny appearance known as “metallic lustre.” These are all the conditions, but they are by no means necessary, for very few metals possess them all, and many non-metallic elements possess several. The “alkali” metals are lighter than water; mercury is a fluid. The opacity of a mass is only in relation to its thickness, for Faraday beat out metals into plates so thin that they became transparent. All metals are not malleable, nor are they ductile. Tin and lead, for example, have very little ductility or tenacity, while bismuth and antimony have none at all. Carbon is a much better conductor of electricity than many metals in which such power is extremely varied. Lustre, again, though possessed by metals, is a characteristic of some non-metals. So we see that while we can easily say what is not a metal, we can scarcely define an actual metal, nor depend upon unvarying properties to guide us in our determination.

[Pg 389]

The affinity of metals for oxygen is in an inverse ratio to their specific gravity, as can be ascertained by experiment, when the heaviest metal will be the least ready to oxidise. Metals differ in other respects, and thus classification and division become easier. The fusibility of metals is of a very wide range, rising from a temperature below zero to the highest heat obtainable in the blow-pipe, and even then in the case of osmium there is a difficulty. While there can be no question that certain elements, iron, copper, gold, silver, etc., are metals proper, there are many which border upon the line of demarcation very closely, and as in the case of arsenic even occupy the debatable land.

Specific Gravity is the relation which the weight of substance bears to the weight of an equal volume of water, as already pointed out in Physics. The specific gravities of the metals vary very much, as will be seen from the table following—water being, as usual, taken as 1:—

Some metals are therefore lighter and some heavier than water.

The table underneath gives the approximate fusing points of some of the metals (Centigrade Scale)—

There are some metals which, instead of fusing,—that is, passing from the solid to the liquid state,—go away in vapour. These are volatile metals. Mercury, potassium, and sodium, can be thus distilled. Some do not expand with heat, but contract (like ice), antimony and bismuth, for instance, while air pressure has a considerable effect upon the fusing point. Some vaporise at once without liquefying; others, such as iron, become soft before melting.

[Pg 390]

Alloys are combinations of metals which are used for many purposes, and become harder in union. Amalgams are alloys in which mercury is one constituent. Some of the most useful alloys are under-stated:—

Metals combine with chlorine, and produce chlorides,
Metals combine with sulphu, and produce sulphides,
Metals combine with oxygen, and produce oxides, and so on.

The metals may be classed as follows in divisions:—

We cannot attempt an elaborate description of all the metals, but we will endeavour to give a few particulars concerning the important ones, leaving many parts for Mineralogy to supplement and enlarge upon. We shall therefore mention only the most useful of the metals in this place. We will commence with Potassium.

Metals of the Alkalies.

Potassium has a bright, almost silvery, appearance, and is so greatly attracted by oxygen that it cannot be kept anywhere if that element be present—not even in water, for combustion will immediately ensue on water; and in air it is rapidly tarnished. It burns with a beautiful violet colour, and[Pg 391] a very pretty experiment may easily be performed by throwing a piece upon a basin of water. The fragment combines with the oxygen of the water, the hydrogen is evolved, and burns, and the potassium vapour gives the gas its purple or violet colour. The metal can be procured by pulverizing carbonate of potassium and charcoal, and heating them in an iron retort. The vapour condenses into globules in the receiver, which is surrounded by ice in a wire basket. It must be collected and kept in naphtha, or it would be oxidised. Potassium was first obtained by Sir Humphrey Davy in 1807. Potash is the oxide of potassium, and comes from the “ashes” of wood.

Fig. 397.—Preparation of potassium.

The compounds of potassium are numerous, and exist in nature, and by burning plants we can obtain potash (“pearlash”). Nitrate of potassium, or nitre (saltpetre), (KNO₃), is a very important salt. It is found in the East Indies. It is a constituent of gunpowder, which consists of seventy-five parts of nitre, fifteen of charcoal, and ten of sulphur. The hydrated oxide of potassium, or “caustic potash” (obtained from the carbonate), is much used in soap manufactories. It is called “caustic” from its property of cauterizing the tissues. Iodide, bromide, and cyanide of potassium, are used in medicine and photography.

Fig. 398.—Machine for cutting soap in bars.

Soap is made by combining soda (for hard soap), or potash (for soft soap), with oil or tallow. Yellow soap has turpentine, and occasionally palm oil, added. Oils and fats combine with metallic oxides, and oxide of lead with olive oil and resin forms the adhesive plaister with which we are all familiar when the mixture is spread upon linen. Fats boiled with potash or soda make soaps; the glycerine is sometimes set free and purified as we have it. Sometimes it is retained for glycerine soap. Fancy soap is only common soap coloured. White and brown Windsor are the same soap—in the latter case browned to imitate age! Soap is quite soluble in[Pg 392] spirits, but in ordinary water it is not so greatly soluble, and produces a lather, owing to the lime in the water being present in more or less quantity, to make the water more or less “hard.”

Fig. 399.—Soap-boiling house.

Sodium is not unlike potassium, not only in appearance, but in its attributes; it can be obtained from the carbonate, as potassium is obtained from its carbonate. Soda is the oxide of sodium, but the most common and useful compound of sodium is the chloride, or common salt, which is found in mines in England, Poland, and elsewhere. Salt may also be obtained by the evaporation of sea water. Rock salt is got at Salzburg, and the German salt mines and works produce a large quantity. The Carbonate of Soda is manufactured from the chloride of sodium, although it can be procured from the salsoda plants by burning. The chloride of sodium is converted into sulphate, and then ignited with carbonate of lime and charcoal. The soluble carbonate is extracted in warm water, and sold in crystals as soda, or (anhydrous) “soda ash.” The large quantity of hydrochloric acid produced in the first part of the process is used in the process of making chloride of lime. A few years back, soda was got from Hungary and various other countries where it exists as a natural efflorescence on the shores of some lakes, also by burning sea-weeds, especially the common bladder wrack (Fucus vesiculosus), the ashes of which were melted into masses, and came to market in various states of purity. The bi-carbonate of soda is obtained by passing carbonic acid gas over the carbonate crystals. Soda does not attract moisture from the air. It is used in washing, in glass manufactories, in dyeing, soap-making, etc.

[Pg 393]

Sulphate of Soda is “Glauber’s Salt”; it is also employed in glass-making. Mixed with sulphuric acid and water, it forms a freezing mixture. Glass, as we have seen, is made with silicic acid (sand), soda, potassa, oxide of lead, and lime, and is an artificial silicate of soda.

Fig. 400.—Mottled soap-frames.

Lithium is the lightest of metals, and forms the link between alkaline and the alkaline earth metals. The salts are found in many places in solution. The chloride when decomposed by electricity yields the metal.

Cæsium and Rubidium require no detailed notice from us. They were first found in the solar spectrum, and resemble potassium.

Ammonium is only a conjectural metal. Ammonia, of which we have already treated, is so like a metallic oxide that chemists have come to the conclusion that its compounds contain a metallic body, which they have named hypothetically Ammonium. It is usually classed amongst the alkaline metals. The salts of ammonia are important, and have already been mentioned. Muriate (chloride) of ammonia, or sal-ammoniac, is analogous to chloride of sodium and chloride of potassium. It is decomposed by heating it with slaked lime, and then gaseous ammonia is given off.

The Metals of the Alkaline Earths.

Fig. 401.—Soda furnace.

Barium is the first of the four metals we have to notice in this group, and will not detain us long, for it is little known in a free condition. Its most important compound is heavy spar (sulphate of baryta), which, when powdered, is employed as a white paint. The oxide of barium, BaO, is termed baryta.

Nitrate of Baryta is used for “green fire,” which is made as follows:—Sulphur, twenty parts; chlorate of potassium, thirty-three parts; and nitrate of baryta, eighty parts (by weight).

Calcium forms a considerable quantity of our earth’s crust. It is the metal of lime, which is the oxide of calcium. In a metallic state it possesses no great interest, but its combinations are very important to us. Lime is, of course, familiar to all. It is obtained by evolving the carbonic acid from carbonate of lime (CaO).

The properties of this lime are its white appearance, and it develops[Pg 394] a considerable amount of heat when mixed with water, combining to make hydrate of lime, or “slaked lime.” This soon crumbles into powder, and as a mortar attracts the carbonic acid from the air, by which means it assumes the carbonate and very solid form, which renders it valuable for cement and mortar, which, when mixed with sand, hardens. Caustic lime is used in whitewashing, etc.

Carbonate of Lime (CaCO₃) occurs in nature in various forms, as limestone, chalk, marble, etc. Calc-spar (arragonite) is colourless, and occurs as crystals. Marble is white (sometimes coloured by metallic oxides), hard, and granular. Chalk is soft and pulverizing. It occurs in mountainous masses, and in the tiniest shells, for carbonate of lime is the main component of the shells of the crustacea, of corals, and of the shell of the egg; it enters likewise into the composition of bones, and hence we must regard it as one of the necessary constituents of the food of animals. It is an almost invariable constituent of the waters we meet with in Nature, containing, as they always do, a portion of carbonic acid, which has the power of dissolving carbonate of lime. But when gently warmed, the volatile gas is expelled, and the carbonate of lime deposited in the form of white incrustations upon the bottom of the vessel, which are particularly observed on the bottoms of tea-kettles, and if the water contains a large quantity of calcareous matter, even our water-bottles and drinking-glasses become covered with a thin film of carbonate of lime. These depositions may readily be removed by pouring into the vessels a little dilute hydrochloric acid, or some strong vinegar, which in a short time dissolves the carbonate of lime.

Sulphate of Lime (CaSO₄) is found in considerable masses, and is commonly known under the name of Gypsum. It occurs either crystallized or granulated, and is of dazzling whiteness; in the latter form it is termed Alabaster, which is so soft as to admit of being cut with a chisel, and is admirably adapted for various kinds of works of art. Gypsum contains water of crystallization, which is expelled at a gentle heat. But when ignited, ground, and mixed into a paste with water, it acquires the property of entering into chemical combination with it, and forming the original hydrate, which in a short time becomes perfectly solid. Thus it offers to the artist a highly valuable material for preparing the well-known plaster of Paris figures, and by its use the noblest statues of ancient and modern art have now been placed within the reach of all. Gypsum, moreover, has received a valuable application as manure. In water it is slightly soluble, and imparts to it a disagreeable and somewhat bitterish, earthy taste. It is called “selenite” when transparent.

Phosphate of Lime constitutes the principal mass of the bones of animals, and is extensively employed in the preparation of phosphorus; in the form of ground bones it is likewise used as a manure. It appears to belong to those mineral constituents which are essential to the nutrition of animals. It is found in corn and cereals, and used in making bread; so we derive the phosphorus which is so useful to our system.

[Pg 395]

Chloride of Lime is a white powder smelling of chlorine, and is produced by passing the gas over the hydrate of lime spread on trays for the purpose. It is the well-known “bleaching powder.” It is also used as a disinfectant. The Fluoride of Calcium is Derbyshire spar, or “Blue John.” Fluor spar is generally of a purple hue. We may add that hard water can be softened by adding a little powdered lime to it.

Magnesium sometimes finds a place with the other metals, for it bears a resemblance to zinc. Magnesium may be prepared by heating its chloride with sodium. Salt is formed, and the metal is procured. It burns very brightly, and forms an oxide of magnesia (MgO). Magnesium appears in the formation of mountains occasionally. It is ductile and malleable, and may be easily melted.

Carbonate of Magnesia, combining with carbonate of lime, form the Dolomite Hills. When pure, the carbonate is a light powder, and when the carbonic acid is taken from it by burning it is called Calcined Magnesia.

The Sulphate of Magnesia occurs in sea-water, and in saline springs such as Epsom. It is called “Epsom Salts.” Magnesium wire burns brightly, and may be used as an illuminating agent for final scenes in private theatricals. Magnesite will be mentioned among Minerals.

Strontium is a rare metal, and is particularly useful in the composition of “red-fire.” There are the carbonate and sulphate of strontium; the latter is known as Celestine. The red fire above referred to can be made as follows, in a dry mixture. Ten parts nitrate of strontia, 1½ parts chlorate of potassium, 3½ parts of sulphur, 1 part sulphide of antimony, and ½ part charcoal. Mix well without moisture, enclose in touch paper, and burn. A gorgeous crimson fire will result.

Metals of the Earths.

Aluminium (Aluminum) is like gold in appearance when in alloy with copper, and can be procured from its chloride by decomposition with electricity. It occurs largely in nature in composition with clays and slates. Its oxide, alumina (Al₂O₃), composes a number of minerals, and accordingly forms a great mass of the earth. Alumina is present in various forms (see Minerals) in the earth, all of which will be mentioned under Crystallography and Mineralogy. The other nine metals in this class do not call for special notice.

Heavy Metals

Iron, which is the most valuable of all our metals, may fitly head our list. So many useful articles are made of it, that without consideration any one can name twenty. The arts of peace and the glories of war are all produced with the assistance of iron, and its occurrence with coal has[Pg 396] rendered us the greatest service, and placed us at the head of nations. It occurs native in meteoric stones.

Iron is obtained from certain ores in England and Sweden, and these contain oxygen and iron (see Mineralogy). We have thus to drive away the former to obtain the latter. This is done by putting the ores in small pieces into a blast furnace (fig. 402) mixed with limestone and coal. The process of severing the metal from its ores is termed smelting, the air supplied to the furnace being warmed, and termed the “hot blast.” The “cold blast” is sometimes used. The ores when dug from the mine are generally stamped into powder, then “roasted,”—that is, made hot, and kept so for some time to drive off water, sulphur, or arsenic, which would prevent the “fluxes” acting properly. The fluxes are substances which will mix with, melt, and separate the matters to be got rid of, the chief being charcoal, coke, and limestone. The ore is then mixed with the flux, and the whole raised to a great heat; as the metal is separated it melts, runs to the bottom of the “smelting furnace,” and is drawn off into moulds made of sand; it is thus cast into short thick bars called “pigs,” so we hear of pig-iron, and pig-lead. Iron is smelted from “ironstone,” which is mixed with coke and limestone. The heat required to smelt iron is so very great, that a steam-engine is now generally employed to blow the furnace. (Before the invention of the steam-engine, water-mills were used for the same purpose.) The smelting is conducted in what is called a blast furnace. When the metal has all been “reduced,” or melted, and run down to the bottom of the furnace, a hole is made, out of which it runs into the moulds; this is called “tapping the furnace.”

Fig. 402.—Blast furnace.

Smelting is often confounded with melting, as the names are somewhat alike, but the processes are entirely different; in melting, the metal is simply liquefied, in smelting, the metal has to be produced from ores which often have no appearance of containing any, as in the case of ironstone, which looks like brown clay.

The cone of the furnace, A, is lined with fire-bricks, i i, which is encased by a lining, l l; outside are more fire-bricks, and then masonry, m n;[Pg 397] C is the throat of the furnace; D the chimney. The lower part, B, is called the boshes. As soon as the ore in the furnace has become ignited the carbon and oxygen unite and form carbonic acid, which escapes, and the metal fuses at last and runs away. The coal and ore are continually added year after year. The glassy scum called “slag “ protects the molten iron from oxidation.

Fig. 403.—General foundry, Woolwich Arsenal.

The metal drawn from the blast furnace is “pig iron,” or “cast” iron, and contains carbon. This kind of iron is used for casting operations, and runs into sand-moulds. It contracts very little when cooling. It is hard and brittle.

Fig. 404.—Wire rollers.

Fig. 405.—Cutting edges.

Bar Iron is the almost pure metal. It is remarkably tenacious, and may be drawn into very fine wire or rolled. But it is not hard enough for tools. It is difficult to fuse, and must be welded by hammering at a red heat. Wire-drawing is performed by taking the metal as a bar, and passing it between rollers (fig. 404), which flattens it, and then between a new set, which form cutting edges on the rolled plate (fig. 405), the projections of one set fitting into the hollows of the other closely as in the illustration. The strips of metal come out at the aperture seen at A in the next illustration. These rods are drawn through a series of diminishing holes in a steel plate, occasionally being heated to keep it soft and ductile. When the wire has got to a certain fineness it is attached to a cylinder and drawn away, at the same[Pg 398] time being wound round the cylinder over a small fire. Some metals can be drawn much finer than others. Gold wire can be obtained of a “thickness” (or thinness) of only the 5,000th part of an inch, 550 feet weighing one grain! But platinum has exceeded this marvellous thinness, and wire the 30,000th part of an inch has been produced. Ductility and malleability are not always found in the same metal in proportion. The sizes of wires are gauged by the instrument shown in the margin. The farther the wire will go into the groove the smaller its “size.”

Fig. 406.—Rollers.

Fig. 407.—Wire size.

Steel contains a certain amount of carbon, generally about 1 to 2 per cent. Cast steel is prepared from cast iron. Steel from bar-iron has carbon added, and is termed bar-steel. The process is called “cementation,” and is carried on by packing the bars of iron in brick-work boxes, with a mixture of salt and soot, or with charcoal, which is termed “cement.” Steel is really a carbide of iron, and Mr. Bessemer founded his process of making steel by blowing out the excess of carbon from the iron, so that the proper amount—1·5 per cent.—should remain.

Fig. 408.—Coarse wire-drawing.

A brief summary of the Bessemer process may be interesting. If a bar of steel as soft as iron be made red-hot and plunged into cold water, it will become very hard. If it be then gently heated it will become less hard, and is then fitted for surgical instruments. The various shades of steel are[Pg 399] carefully watched,—the change of colour being due to the varying thickness of the oxide; for we know that when light falls upon very thin films of a substance,—soap-bubbles, for instance,—the light reflected from the under and upper surfaces interfere, and cause colour, which varies with the thickness of the film. These colours in steel correspond to different temperatures, and the “temper” of the steel depends upon the temperature it has reached. The following table extracted from Haydn’s “Dictionary of Science” gives the “temper,” the colour, and the uses of the various kinds of steel.

Fig. 409.—Fine wire-drawing.

The Bessemer process transfers the metal into a vessel in which there are tubes, through which air is forced, which produces a much greater heat than a bellows does. Thus in the process the carbon of the iron acts as fuel to maintain the fusion, and at the same time by the bubbling of the carbonic acid mixes the molten iron thoroughly.

During the bubbling up of the whole mass of iron, and the extreme elevation of temperature caused by the union of the carbon of the impure iron with the oxygen of the air, the oxide of iron is formed, and as fast as it forms fuses into a sort of glass; this unites with the earthy matters of the “impure” iron, and floats on the upper part as a flux, thus ridding the “cast iron” of all its impurities, with no other fuel than that contained in the[Pg 400] metal itself, and in the air used. When the flame issuing from the “converter” contracts and changes its colour, then the time is known to have arrived when the iron is “ de-carbonized.” The amount of carbon necessary is artificially added, ebullition takes place, a flame of carbonic oxide comes out, and the metal is then run into ingots.

The compounds of iron which are soluble in water have a peculiar taste called chalybeate (like ink). Many mineral springs are so flavoured, and taste, as the immortal Samuel Weller put it, “like warm flat-irons.” Iron is frequently used as a medicine to renew the blood globules.

Protoxide of Iron is known only in combination.

Sesqui-Oxide of Iron is “red ironstone.” Powdered it is called English rouge, a pigment not altogether foreign to our use. In a pure state it is a remedy for arsenical poisoning, and is really the “rust” upon iron.

Fig. 410.—Bessemer’s process.

Bisulphide of Iron is iron pyrites, and is crystalline.

Chloride of Iron is dissolved from iron with hydrochloric acid. It is used in medicine.

Cyanide of Iron makes, with cyanide of potassium, the well-known prussiate of potash (ferro-cyanide of potassium), which, when heated, precipitates Prussian blue (cyanogen and iron).

The Sulphate of the Protoxide is known as copperas, or green vitriol, and is applied to the preparation of Prussian blue.

Manganese is found extensively, but not in any large quantities, in one place; iron ore contains it. It is very hard to fuse, and is easily oxidised. The binoxide is used to obtain oxygen, and when treated with[Pg 401] potassium and diluted, it becomes the permanganate of potassium, and is used as “Condy’s fluid.” It readily oxides organic matters, and is thus a disinfectant. It crystallizes in long, deep, red needles, which are dissolved in water. It is a standard laboratory test. There are other compounds, but in these pages we need not detail them.

Cobalt and Nickel occur together. They are hard, brittle, and fusible. The salts of cobalt produce beautiful colours, and the chloride yields an “invisible” or sympathetic ink. The oxide of cobalt forms a blue pigment for staining glass which is called “smalt.” Nickel is chiefly used in the preparation of German silver and electro-plating. The salts of nickel are green. Nickel is difficult to melt, and always is one of the constituents of meteoric iron, which falls from the sky in aërolites. It is magnetic like cobalt, and is extracted from the ore called kupfer-nickel. A small United-States coin is termed a “nickel.”

Fig. 411.—Native copper.

Copper is the next metal we have to notice. It has been known for centuries. It is encountered native in many places. The Cornish copper ore is the copper pyrites. The fumes of the smelting works are very injurious, containing, as they do, arsenic and sulphur. The ground near the mines is usually bare of vegetation in consequence of the “smoke.” Sheet copper is worked into many domestic utensils, and the alloy with zinc, termed Brass, is both useful and ornamental. Red brass is beaten into thin leaves, and is by some supposed to be “gold leaf”; it is used in decorative work. Bronze is also an alloy of copper, as are gun-metal, bell-metal, etc.

Next to silver, copper is the best conductor of electricity we have. It is very hard and tough, yet elastic, and possesses malleability and ductility in a high degree. It forms two oxides, and there are several sulphides; the principal of the latter are found native, and worked as ores. The sulphate of copper is termed blue vitriol, and is used in calico-printing, and from it all the (copper) pigments are derived. It is also used in solution by agriculturists to protect wheat from insects. When copper or its alloys are exposed to air and water, a carbonate of copper forms, which is termed verdigris. All copper salts are poisonous; white of eggs is an excellent remedy in such cases of poisoning.

Lead is obtained from galena, a sulphide of lead. It is a soft and easily-worked metal. When freshly cut it has quite a bright appearance,[Pg 402] which is quickly tarnished. Silver is often present in lead ore, and is extracted by Pattison’s process, which consists in the adaptation of the knowledge that lead containing silver becomes solid, after melting, at a lower temperature than lead does when pure. Pure lead therefore solidifies sooner.

One great use of lead is for our domestic water-pipes, which remind us in winter of their presence so disagreeably. Shot is made from lead, and bullets are cast from the same metal. Shot-making is very simple, and before the days of breech-loading guns and cartridges, no doubt many readers have cast bullets in the kitchen and run them into the mould over a basin of water or a box of sand. For sporting purposes lead is mixed with arsenic, and when it is melted it is poured through a sort of sieve (as in the cut) at the top of a high tower. (See figs. 413 and 414). The latter illustration gives the section of the shot tower; A is the furnace, B is the tank for melting the lead, and the metal is permitted by the workman at C to run through the sieve in fine streams. As the lead falls it congeals into drops, which are received in water below to cool them. They are, of course, not all round, and must be sorted. This operation is performed by placing them on a board tilted up, and under which are two boxes. The round shot rush over the first holes and drop into the second box, but the uneven ones are caught lagging, and drop into box No. 1. They are accordingly sent to the furnace again.

Fig. 412.—Shot tower.

The next process is to sort the good shot for size. This is done by sieves—one having holes a little larger than the size of shot required. This sieve passes through it all of the right size and smaller, and keeps the bigger ones. Those that have passed this examination are then put into another sieve, which has holes in it a little smaller than the size of shot wanted. This sieve retains the right shot, and lets the smaller sizes pass, and so on. The shot are sized and numbered, glazed by rolling them in a barrel with graphite, and then they are ready for use. Bullets are made by machinery by the thousand, and made up into cartridges with great speed.

Fig. 413.—Sieve for making shot.

The compounds of lead are also poisonous, and produce “colic,” to[Pg 403] which painters are subject. Red lead, or minium, is a compound of the protoxide and the binoxide, and may be found native. The former oxide is litharge; white lead, or the carbonate of lead, is a paint, and is easily obtained by passing a stream of carbonic acid into a solution of acetate of lead. It is used as a basis of many paints.

Fig. 414.—Section of shot tower.

Tin is another well-known metal. It is mentioned by Moses. It possesses a silver-like lustre, and is not liable to be oxidised. The only really important ore is called Tinstone, from which the oxygen is separated, and the metal remains. Cornwall has extensive tin mines. Tin is malleable and ductile, and can be beaten into foil or “silver leaf,” or drawn into wire. It prevents oxidation of iron if the latter be covered with it, and for tinning copper vessels for culinary purposes. The Romans found tin in Cornwall, and the term “Stanneries” was applied to the courts of justice among the tin miners in Edward the First’s time. We have already mentioned the alloys of tin. The oxides of tin, “Stannous” and “Stannic,” are useful to dyers. The latter is the tinstone (SnO₂). Sulphide of tin is called “Mosaic gold,” and is much used for decorative purposes.

Fig. 415.—Preparing lead for bullets.

Zinc is procured from calamine, or carbonate of zinc, and blende, or sulphide of zinc. It has for some years been used for many purposes for which lead was once employed, as it is cheap and light. Zinc is a hard metal of a greyish colour, not easily bent, and rather brittle; but when made nearly red-hot, it can be rolled out into sheets or beaten into form by the hammer. Zinc is about six-and-three-quarter times heavier than water. Like many other metals, it is volatile (when heated to a certain extent it passes off into vapour), and the probable reason that it was not known or used of old is that it was lost in the attempt to smelt its ores. Zinc is now obtained by a sort of distil[Pg 404]lation; the ores are mixed with the flux in a large earthen crucible or pot.

We have already noticed the alloy of zinc with copper (brass), and the use of zinc to galvanize iron by covering the latter with a coating of zinc in a bath is somewhat analogous to electro-plating. The metal is largely used as the positive element in galvanic batteries, and for the production of hydrogen in the laboratory. Zinc forms one oxide (ZnO), used for zinc-white. The sulphate of zinc is white vitriol, and the chloride of zinc is an “antiseptic.” Certain preparations of the metal are used in medicine as “ointments” or “washes,” and are of use in inflammation of the eyelids.

Chromium. This “metallic element” is almost unknown in the metallic state. But although little known, the beautiful colours of its compounds make it a very interesting study. The very name leads one to expect something different to the other metals—chroma, colour. The metal is procured from what is known as chrome-ironstone, a combination of protoxide of iron and sesqui-oxide of chromium (FeOCr₂O₃). By ignition with potassium we get chromic acid and chromate of potassium, a yellow salt which is used to make the other compounds of chromium. The metal is by no means easy to fuse.

Sesqui-Oxide of Chromium is a fine green powder employed in painting porcelain.

Chromate of Lead is termed “chrome yellow,” and in its varieties is employed as a paint.

Chromate of Mercury is a beautiful vermilion. There are numerous other combinations which need not be mentioned here.

Fig. 416.—Type-casting.

Antimony was discovered by Basil Valentine. The Latin term is Stibium, hence its symbol, Sb. It is very crystalline, and of a peculiar bluish-white tint. It will take fire at a certain high temperature, and can be used for the manufacture of “Bengal Lights,” with nitre and sulphur in the proportions of antimony “one,” the others two and three respectively.

The compounds of antimony are used in medicine, and are dangerous when taken without advice. They act as emetics if taken in large quantities. Our “tartar emetic” is well known.

Antimony, in alloy with lead and a little tin, form the type metal to[Pg 405] which we are indebted for our printing. Type-casting is done by hand, and requires much dexterity. A ladle is dipped into the molten metal, and the mould jerked in to fill it properly, and then the type is removed and the mould shut ready for another type; and a skilful workman can perform these operations five hundred times in an hour,—rather more than eight times a minute,—producing a type each time; this has afterwards to be finished off by others. The metal of which type is made consists of lead and antimony; the antimony hardens it and makes it take a sharper impression. The letters are first cut in steel, and from these “dies” the moulds are made in brass, by stamping, and in these the types are cast.

Stereotype consists of plates of metal taken, by casting, from a forme of type set up for the purpose: an impression was formerly carried on by plaster-of-Paris moulds, but lately what is termed the papier-maché process is adopted. The paper used is now made in England, and the prepared sheet is placed upon the type and beaten upon it. Paste is then filled in where there are blanks, and another and thicker sheet of the prepared paper is placed over all, dried, and pressed. When this is properly done the paper is hardened, and preserves an impression of the type set up. The paper mould is then put into an iron box, and molten metal run in. In a very short time a “stereotype” plate is prepared from the paper, which can be used again if necessary. The metal plate is put on the machine.

There are several compounds of antimony, which, though valuable to chemists, would not be very interesting to the majority of readers. We will therefore at once pass to the Noble Metals.

The Noble Metals.

There are nine metals which rank under the above denomination:—Mercury, Silver, Gold, Platinum, Palladium, Rhodium, Ruthenium, Osmium, Iridium. We will confine ourselves chiefly to the first four on the list.

Mercury, or Quicksilver, is the first of the metals which remain unaltered by exposure to atmospheric air, and thus are supposed to earn their title of nobility. Mercury is familiar to us in our barometers, etc., and is fluid in ordinary temperatures, though one of the heaviest metals we possess. It is principally obtained from native cinnabar, or sulphide of mercury (vermilion), and the process of extraction is very easy. Mercury was known to the ancients, and is sometimes found native. In the mines the evil effects of the contact with mercury are apparent.

This metal forms two oxides,—the black (mercurous) oxide, or suboxide (Hg₂O), and the red (mercuric) oxide, or red precipitate. The chlorides are two,—the subchloride, or calomel, and the perchloride, or corrosive sublimate. The sulphides correspond with the oxides; the mercuric sulphide has been mentioned. Its crimson colour is apparent in nature, but the Chinese prepare it in a particularly beautiful form. Many[Pg 406] amalgams are made with mercury, which is useful in various ways that will at once occur to the reader.

Silver is the whitest and most beautiful of metals, and its use for our plate and ornaments is general. It is malleable and ductile, and the best conductor of electricity and heat that we have. It is not unfrequently met with in its native state, but more generally it is found in combination with gold and mercury, or in lead, copper, and antimony ores. The mines of Peru and Mexico, with other Western States of America, are celebrated—Nevada, Colorado, and Utah in particular. The story of the silver mine would be as interesting as any narrative ever printed. The slavery and the death-roll would equal in horror and in its length the terrible records of war or pestilence. We have no opportunity here to follow it, or its kindred metals with which it unites, on the sentimental side; but were the story of silver production written in full, it would be most instructive.

Fig. 417.—Native silver.

Silver is found with lead (galena), which is then smelted. The lead is volatilized, and the silver remains. It is also extracted by the following process, wherein the silver and golden ore is crushed and washed, and quicksilver, salt, and sulphate of copper added, while heat is applied to the mass. From tank to tank the slime flows, and deposits the metals, which are put into retorts and heated. The mercury flies off; the silver and gold remain in bars.

In some countries, as in Saxony and South America, recourse is had to another process, that of amalgamation, which depends on the easy solubility of silver and other metals in mercury. The ore, after being reduced to a fine powder, is mixed with common salt, and roasted at a low red heat, whereby any sulphide of silver the ore may contain is converted into chloride. The mixture is then placed, with some water and iron filings, in a barrel which revolves round its axis, and the whole agitated for some time, during which process the chloride of silver becomes reduced to the metallic state. A portion of mercury is then introduced, and the agitation continued. The mercury combines with the silver, and the amalgam is then separated by washing. It is afterwards pressed in woollen bags to free it from the greater part of the mercury, and then heated, when the last trace of mercury volatilizes and leaves the silver behind.

Nitrate of Silver is obtained when metallic silver is dissolved in nitric acid. It is known popularly as lunar caustic, and forms the base of “marking inks.” Chloride of silver is altered by light, but the iodide of silver is even more rapidly acted on, and is employed in photography. Fulminating silver is oxide of silver digested in ammonia. It is very dan[Pg 407]gerous in inexperienced hands. It is also prepared by dissolving silver in nitric acid, and adding alcohol. It cools in crystals. Fulminating mercury is prepared in the same way.

Gold is the most valuable of all metals,—the “king of metals,” as it was termed by the ancients. It is always found “native,” frequently with silver and copper. Quartz is the rock wherein it occurs. From the disintegration of these rocks the gold sands of rivers are formed, and separated from the sands by “washing.” In Australia and California “nuggets” are picked up of considerable size.

It is a rather soft metal, and, being likewise costly, is never used in an absolutely pure state. Coins and jewellery are all alloyed with copper and silver to give them the requisite hardness and durability. Gold is extremely ductile, and very malleable. One grain of gold may be drawn into a wire five hundred feet in length, and the metal may be beaten into almost transparent leaves 1/200000 of an inch in thickness!

Fig. 418.—Native gold.

Aqua-regia, a mixture of hydrochloric and nitric acids, is used to dissolve gold, which is solved only by selenic acid, though the free chlorine will dissolve it. Faraday made many experiments as to the relation of gold to light. (See “Phil. Trans.,” 1857, p. 145.) The various uses of gold are so well known that we need not occupy time and space in recording them. Gilding can be accomplished by immersing the articles in a hot solution of chloride of gold and bicarbonate of potash mixed; but the electro process is that now in use, by which the gold precipitates on the article to be plated.

We have already described the process of electro-plating in the case of silvered articles, and we need only mention that electro-gilding is performed very much in the same way. But gilding is also performed in other ways; one of which, the so-called water gilding, is managed as follows. Gilding with the gold-leaf is merely a mechanical operation, but water-gilding is effected by chemistry.

Water-gilding is a process (in which, however, no water is used) for covering the surface of metal with a thin coating of gold; the best metal for water-gilding is either brass, or a mixture of brass and copper. A mixture of gold and mercury, in the proportion of one part of gold to eight of mercury, is made hot over a fire till they have united; it is then put into a bag of chamois-leather, and the superfluous mercury pressed out. What remains is called an “amalgam”; it is soft, and of a greasy nature, so that it can be smeared over any surface with the fingers. The articles to be gilt[Pg 408] are made perfectly clean on the surface, and a liquid, made by dissolving mercury in nitric acid (aqua-fortis), is passed over them with a brush made of fine brass wire, called a “scratch-brush.” The mercury immediately adheres to the surface of the metal, making it look like silver; when this is done, a little of the amalgam is rubbed on, and the article evenly covered with it. It is then heated in a charcoal fire till all the mercury evaporates, and the brass is left with a coating of gold, which is very dull, but may be burnished with a steel burnisher and made bright if necessary. In former times articles were inlaid with thin plates of gold, which were placed in hollows made with a graver, and melted in, a little borax being applied between.

When a solution of “chloride of gold” is mixed with ether, the ether takes the gold away from the solution, and may be poured off the top charged with it. This solution, if applied to polished steel by means of a camel-hair pencil, rapidly evaporates, leaving a film of gold adhering to the steel, which, when burnished with any hard substance, has a very elegant appearance. In this way any ornamental design in gold may be produced, but it is not very durable. The gilt ornaments, scrolls, and mottoes on sword-blades, are sometimes done in this way.

Platinum is the heaviest of all metals, gold being next. Platinum is practically infusible, and quite indifferent to reagents. It is therefore very useful in certain manufactories, and in the laboratory. It can be dissolved by aqua-regia. The stills for sulphuric acid are made of platinum, and the metal is used for Russian coinage, but must be very difficult to work on account of its infusible property.

Fig. 419.—Döbereiner’s lamp.

In the finely-divided state it forms a gray and very porous mass, which is known as spongy platinum, and possesses the remarkable property of condensing gases within its pores. Hence, when a jet of hydrogen is directed upon a piece of spongy platinum, the heat caused by its condensation suffices to inflame the gas. This singular power has been applied to the construction of a very beautiful apparatus, known as Döbereiner’s lamp, which consists of a glass jar, a, covered by a brass lid, e, which is furnished with a suitable stop-cock, c, and in connection with a small bell jar, f, in which is suspended, by means of a wire, a cylinder of metallic zinc, z. When required for use, the outer jar is two-thirds filled with a mixture of one part sulphuric acid and four parts water, and the stop-cock opened to allow the escape of atmospheric air, the spongy platinum contained in the small brass cylinder, d, being covered by a piece of paper. The stop-cock is then closed, and the bell jar, f, allowed to fill with hydrogen, and after it has been filled and emptied several times, the paper is removed from the platinum and the cock is again opened, when the gas, which escapes first, makes the metal red-hot and finally inflames. This property of platinum is also used in the “Davy” lamp.

[Pg 409]

The remaining metals do not call for detailed notice.

In conclusion, we may refer to the following statement, which in general terms gives the properties of the metals, their oxides and sulphides for ordinary readers.

General Classification of the Metals.

The metals admit of being really distinguished by the following table, in which they are presented in several groups, according to their peculiar properties, and each distinguished by a particular name:—

[Pg 410]

CHAPTER XXX.
ORGANIC CHEMISTRY.

RADICALS—ACIDS—BASES—NEUTRALS.

In the introduction to these brief chapters upon Chemistry, we said that the science was divided into two sections, the first section consisting of the simple combinations, and the other of compound combinations. The latter being met with chiefly in animal and vegetable matter, as distinguished from dead or inert matter, was termed Organic. This distinction will be seen below.

We have already placed before our readers the elements and their simple combinations, and have incidentally mentioned the decomposition by electricity and by light. In the section upon Electricity the positive and negative poles are explained. Oxygen appears always at the positive pole, potassium at the negative. The other simple bodies vary. Chlorine, in combination with oxygen, is evolved at the negative pole, but when with hydrogen at the positive pole. In the series below each element behaves electro-negatively to those following it, and electro-positively to those above it; and the farther they are apart the stronger their opposite affinities are.

Electrical Relation of the Elements.

• Oxygen.
• Sulphur.
• Nitrogen.
• Chlorine.
• Bromine.
• Iodine.
• Fluorine.
• Phosphorus.
• Arsenic.
• Carbon.
• Chromium.
• Boron.
• Antimony.
• Silicium.
• Gold.
• Platinum.
• Mercury.
• Silver.
• Copper.
• Bismuth.
• Lead.
• Cobalt.
• Nickel.
• Iron.
• Zinc.
• Hydrogen.
• Manganese.
• Aluminium.
• Magnesium.
• Calcium.
• Strontium.
• Barium.
• Sodium.
• Potassium.

[Pg 411]

The importance of these facts to science is unmistakable, and, indeed, many attempts have been made to explain, from the electrical condition of the elements, the nature of chemical affinity, and of chemical phenomena in general.

Electrotyping is another instance of decomposition by means of electricity, and respecting decomposition by light we know how powerful the action of the sun’s rays are upon plants, and for the evolution of oxygen. The daguerreotype and photographic processes are also instances which we have commented upon. So we can pass directly to the consideration of the compound groups.

In nearly every complex organic compound we have a relatively simple one of great stability, which is termed the radical, which forms, with other bodies, a compound radical.22 In these complex groups we find certain elements generally,—viz., carbon, hydrogen, nitrogen, sulphur, and phosphorus. Some compounds may consist of two of these, but the majority contain three (hydrogen, oxygen, and carbon). Many have four (carbon, oxygen, hydrogen, and nitrogen), and some more than four, including phosphorus and sulphur. Others, again, may contain chlorine and its relatives, arsenic, etc., in addition. Now we will all admit that in any case in which carbon is present in composition with other simple bodies forming an organic body, and if that body be ignited in the air, it burns and leaves (generally) a black mass. This is a sure test of the presence of carbon, and forms an organic compound. Similarly in decomposition nitrogen and sulphur in combination inform us they are present by the odour they give off. We need not go farther into this question of radicals and compound radicals than to state that a compound radical plays the part of an element in combination. We find in alcohol and ether a certain combination termed Ethyl. This “compound radical” occurs in same proportions in ether, chloride of ethyl, iodide of ethyl, etc., as C₂H₅; so it really acts as a simple body or element, though it is a compound of carbon and hydrogen. A simple radical is easily understood; it is an element, like potassium, for instance. We may now pass to the organic combinations classified into Acids, Bases, and Indifferent, or Neutral, Bodies.

I. Acids.

There are several well-known organic acids, which we find in fruits and in plants. They are volatile and non-volatile; acids are sometimes known as “Salts of Hydrogen.” We have a number of acids whose names are familiar to us,—viz., acetic, tartaric, citric, malic, oxalic, tannic, formic, lactic, etc.

Acetic acid (HC₂H₄O₂) is a very important one, and is easily found when vegetable juices, which ferment, are exposed to the air, or when wood and other vegetable matter is subjected to the process of “dry distillation.[Pg 412]” Vinegar contains acetic acid, which is distilled from wood, as we shall see presently. Vinegar is made abroad by merely permitting wine to get sour; hence the term Vin-aigre. In England vinegar is made from “wort,” of malt which is fermented for a few days, and then put into casks, the bung-holes of which are left open for several weeks, until the contents have become quite sour. The liquid is then cleared by isinglass. The vinegar of commerce contains about 6 per cent. of pure acetic acid, and some spirit, some colouring matter, and, of course, water. Wood vinegar (pyroligneous acid) is used for pickles. The ordinary vinegar when distilled is called white vinegar, and it may also be obtained from fruits, such as gooseberries or raspberries.

Fig. 420.—Vinegar ground.

Fig. 421.—Boiler or copper.

Acetic acid, or “wood vinegar,” is prepared as follows:—There are some large iron cylinders set in brickwork over furnaces, and these cylinders have each a tube leading to a main pipe in which the liquid is received for condensation. The cylinders, which contain about seven or eight hundredweight, are filled with logs of wood, either oak, beech, birch, or ash, the door is closely fastened, and the joints smeared with clay; the fires are now lighted and kept up all day, till the cylinders are red-hot; at night they are allowed to cool. In the morning, the charcoal, into which the wood is now converted, is withdrawn, and a fresh charge supplied; it is then found that about thirty or forty gallons of liquid has condensed in the main tube from each cylinder, the remainder being charcoal and gases which pass off; the liquid is acid, brown, and very offensive, and contains acetic acid,[Pg 413] tar, and several other ingredients, among which may be named creosote; it is from this source all the creosote, for the cure of toothache, is obtained. To purify this liquid it is first distilled, and this separates much of the tar; it is then mixed with lime, evaporated to dryness, and heated to expel the remaining tar and other impurities; it is next mixed with sulphate of soda and water, and the whole stirred together; the soda, now in union with the acetic acid, is washed out from the lime and strained quite clear; it is afterwards evaporated till it crystallises, and vitriol (sulphuric acid) then added; finally the acetic acid is distilled over, and the sulphuric acid left in union with the soda, forming sulphate of soda, to be used in a similar process for the next batch of acid. The acetic acid is now quite colourless, transparent, and very sour, possessing a fragrant smell. This is not pure acetic acid, but contains a considerable quantity of water. The acetic acid of commerce, mixed with seven times its bulk of water, forms an acid of about the strength of malt vinegar, perfectly wholesome, and agreeable as a condiment.

Fig. 422.—Vinegar-cooling process.

Pure acetic acid may be made by mixing dry acetate of potash with oil of vitriol in a retort, and distilling the acetic acid into a very cold receiver; this, when flavoured with various volatile oils, forms the aromatic vinegar sold by druggists. It is a very strong acid, and if applied to the skin will quickly blister it.

Fig. 423.—Tan-yard and pits.

Acetate of lead, or sugar of lead, is obtained by dissolving oxide of[Pg 414] lead in vinegar. A solution of this salt makes the goulard water so familiar to all. Acetate of lead is highly poisonous.

Acetate of copper is verdigris, and poisonous. Other acetates are used in medicine.

We may pass quickly over some other acids. They are as follows:—

Tartaric Acid (C₄H₆O₆) is contained in grape juice, and crystallizes in tabular form. The purified powdered salt is cream of Tartar.

Citric Acid (C₆H₈O₇) is found native in citrons and lemons, as well as in currants and other fruits. It is an excellent anti-scorbutic.

Malic Acid (C₄H₆O₅) is found chiefly in apples, as its name denotes (malum, an apple). It is prepared from mountain-ash berries.

Oxalic Acid (C₂H₂O₄). If we heat sugar with nitric acid we shall procure this acid. It is found in sorrel plants.

Tannic Acid (C₂₇H₂₂O₁₇). It is assumed that all vegetables with an astringent taste contain this acid. Tannin is known for its astringent qualities. The name given to this acid is derived from the fact that it possesses a property of forming an insoluble compound with water, known as leather. Tanning is the term employed. Tannin is found in many vegetable substances, but oak bark is usually employed, being the cheapest. The “pelts,” hides, or skins, have first to be freed from all fat or hair by scraping, and afterwards soaking them in lime and water. Then they are placed in the tan-pit between layers of the bark, water is pumped in, and the hides remain for weeks, occasionally being moved from pit to pit, or relaid, so as to give all an equal proportion of pressure, etc. The longer the leather is tanned—it may be a year—the better it wears.

Skins for gloves and binding are tanned with “sumach,” or alum and salt. Sometimes the leather is split by machinery for fine working. Parchment is prepared from the skins of asses, sheep, goats, and calves, which are cleaned, and rubbed smooth with pumice stone.

Tannic acid, with oxide of iron, produces Ink, for the gall-nut contains a quantity of the acid. All the black inks in use generally are composed of green vitriol (sulphate of iron) in union with some astringent vegetable matter; the best is the gall-nut, although, for cheapness, logwood and oak bark have each been used. An excellent black ink may be made by putting into a gallon stone bottle twelve ounces of bruised galls, six ounces of green vitriol, and six of common gum, and filling up the bottle with rain water; this should be kept three or four weeks before using, shaking the bottle from time to time.

Blue ink has lately been much used; it is made by dissolving newly-formed Prussian blue in a solution of oxalic acid. To make it, dissolve[Pg 415] some yellow prussiate of potash in water in one vessel, and some sulphate of iron in another, adding a few drops of nitric acid to the sulphate of iron; now mix the two liquids, and a magnificent blue colour will appear, in the form of a light sediment; this is to be put upon a paper filter, and well washed by pouring over it warm water, and allowing it to run through; a warm solution of oxalic acid should now be mixed with it, and the Prussian blue will dissolve into a bright blue ink.

Fig. 424.—Unhairing the hide.

Red ink is made by boiling chips or raspings of Brazil wood in vinegar, and adding a little alum and gum; it keeps well, and is of a good colour. A red ink of more beautiful appearance, but not so durable, may be made by dissolving a few grains of carmine in two or three teaspoonfuls of spirit of hartshorn.

Marking ink is made by dissolving nitrate of silver in water, and then adding some solution of ammonia, a little gum water, and some Indian ink to colour it. Printers’ ink is made by grinding drying oil with lamp-black.

The powdered gall-nut is an excellent test for iron in water. It will turn violet if any iron be present.

Fig. 425.—Drying rooms for hides.

Formic Acid (CH₂O₂) is the caustic means of defence employed by ants, hence the term formic. It can be artificially prepared by distilling a mixture of sugar, binoxide of manganese, and sulphuric acid. On the skin it will raise blisters.

Lactic Acid (C₃H₆O₃) is present in vegetable and animal substances. Sour whey contains it, and the presence of the acid in the whey accounts for its power of removing from table-linen stains. When what is called “lactic fermentation” occurs, milk is said to be “turned.”

II.

Bases.

The definition of a base is not easy. We have described bases as substances which, combining with acids, form salts, but the definition of a base is as unsatisfactory as that of acid or salt. All vegetable bases contain nitrogen, are usually very bitter, possess no smell or colour, and are insoluble in water. They are usually strong poisons, but very useful in medicine.

[Pg 416]

The most important are the following bases:—

Quinine is contained in the cinchona (yellow) bark. One hundred parts of the bark have been calculated to yield three of quinine.

Morphine is the poisonous base of opium, which is the juice of the poppy, and is prepared chiefly in India and China.

Nicotine is the active principle of tobacco, and varies in quantity in different tobaccos. Havannah tobacco possesses the least. It is a powerful poison, very oily, volatile, and inflammable.

Conia is prepared from the hemlock. It is fluid and volatile. It is also a deadly poison, and paralyses the spine directly.

Fig. 426.—Hemlock.

Strychnine is found in poisonous trees, particularly in the nux-vomica seeds of Coromandel. It produces lock-jaw and paralysis. There is no antidote for strychnine; emetics are the only remedy.

The above are chiefly remarkable for their uses in medicine, and in consequence of their highly poisonous character are best left alone by unpractised hands.

A German chemist, named Serturner, was the first to extract the active principle from Opium. The question of opium importation has lately been attracting much attention, and the opinions concerning its use are divided. Probably in moderation, and when used by ordinary people (not demoralized creatures), it does little harm.

Fig. 427.—The Poppy.

Opium is the juice of the “common” poppy, and derives its name from the Greek opos, juice. The plant is cultivated in India, Persia, and Turkey. After the poppy has flowered the natives go round, and with a sharp instrument wound, or puncture, every poppy head. This is done very early in the morning, and under the influence of the sun during the day the juice oozes out. Next morning the drops are scraped off. The juice is then placed in pots, dried, and sent for export. The “construction” of opium is very complicated, for it contains a number of ingredients, the most important being morphia, narcotine, meconic acid, and codeia. It is to the first named constituent that the somnolent effect of opium is due.

[Pg 417]

III.

Indifferent Substances.

There are a great number of so-called “indifferent” substances to which we cannot be indifferent. Such bodies as these have neither acid nor basic properties, and stand no comparison with salts. They are of great importance, forming, as they do, the principal nutriment of animals. Some contain nitrogen, some do not; they may therefore be divided into nitrogenous and non-nitrogenous substances; the former for solid portions of the body, the latter for warmth.

We will take the latter first, and speak of some of them—such as starch, gum, sugar, etc.

Starch is found in the roots of grain, in the potato, dahlia, artichoke, etc., and by crushing the parts of the plant, and washing them, the starch can be collected as a sediment. In cold water and in spirits of wine starch is insoluble. The various kinds of starch usually take their names from the plants whence they come. Arrowroot is obtained from the West Indian plant Maranta Arundinacea. Cassava and tapioca are from the manioc; sago, from the sago palm; wheat starch, and potato starch are other examples.

Fig. 428.—Plantation of sugar-canes.

If starch be baked in an oven at a temperature of about 300° it becomes, to a great extent, soluble in cold water, forming what is called “British gum”; this is largely used for calico printing and other purposes; if boiled in water under great pressure, so that the temperature can be raised to the same degree, it is also changed into an adhesive sort of gum, “mucilage”; this is the substance made use of by the government officials to spread over the backs of postage and receipt stamps to make them adhere. The starch of grain, during germination, or growth, contains diastase, which converts the starch into gum and sugar; the same effect can be produced by heating starch with diluted sulphuric acid.

Gum found in plants is chiefly procured from the Mimosa trees, from which it flows in drops, and is called Gum Arabic. There are other so-called “gums,” but this is the one generally referred to.

[Pg 418]

Sugar exists in fruits, roots, and in the stalks of plants, in the juice of the cane, maple, and beet-root particularly. The canes are crushed, the juice is clarified with lime to prevent fermentation, and the liquid is evaporated. It is then granulated and cleared from the molasses. Sugar, when heated, becomes dark, and is called “caramel.” It is used for colouring brandy, and gives much difficulty to the sugar refiners.

Fig. 429.—Refining vacuum pan.

Fig. 430.—Sugar moulds.

Fig. 431.—Turning the loaves.

Sugar refining is conducted as follows. The raw (brown) sugar is mixed into a paste with water, and allowed to drain. The sugar thus becomes white. It is then dissolved in water, with animal charcoal and bullocks’ blood. The liquid is boiled, and put into a dark cistern with holes at the bottom, and cotton fibres being fastened in the holes, are hung into another dark cistern, into which the liquid runs pure and white. It is then pumped into a copper vessel,—vacuum pan,—and condensed to the proper consistence. Subsequently it is poured into conical moulds, and pure syrup poured upon the crystal shapes. The caramel is then removed through a hole at the end. The moulds or loaves are then dried, and if not even or elegant they are turned in a lathe. Finally they are packed up as “loaf sugar.” Sugar undergoes no decomposition, and is the cause of non-decomposition in other substances. For this reason it is employed in “preserving” fruit, etc. Sugar is obtained from beet by crushing and rasping the roots, as the cane is treated.

[Pg 419]

Spirit of Wine, or Alcohol, is not a natural product. It is found by the decomposition of grape-sugar by fermentation. There is a series of alcohols which exhibit a regular gradation, founded, so to speak, upon one, two, or three molecules of water. They are called respectively alcohols, glycols, and glycerins. Thus we have—

• Alcohols.
• Methylic alcohol.
• Ethylic   “
• Prophylic   “
• Amylic   “
• Glycols.
• Enthelein glycol.
• Prophylene   “
• Butylene   “
• Amylene   “
• Glycerins.
• (Ordinary Glycerine is the only one known.)

The cetyl and melissylic alcohols are contained in spermaceti and bees-wax respectively. The usual alcohol is the Vinic, a transparent, colourless liquid, which is the spirituous principle of wine, spirits, and beer, and when sugar is fermented the alcohol and carbonic acid remain.

Spirits of wine has a very powerful affinity for water, and thus the use of stimulants in great quantity is to be deprecated, for alcohol absorbs the water from the mucous membranes of the stomach and the mouth, making them dry and hard. The state of “intoxication,” unfortunately so familiar, is the effect produced by alcohol upon the nerves. We append a list of the beverages which are most in use, and the percentage of alcohol in each according to Professor Hart:—

Fig. 432.—Hydrometer.

Spirit of wine is contained in many mixtures, and for the purpose of ascertaining how much alcohol may be in wine, or any other liquid, a hydrometer is used (fig. 432). This instrument consists of a glass tube with a bulb at the end. It is put into water, and the place the water “cuts” is marked by a line on the stem, and called zero 0°. Spirit of wine has less specific gravity than water, so in absolute alcohol the instrument will sink lower than in water, and will descend to a point which is marked 100. In any mixture of alcohol and water, of course the hydrometer will rise or sink between the extreme points accordingly as the mixture may contain less alcohol or more. So a scale can be furnished. The instrument, as described, was invented by MM. Gay-Lussac and Tralles, and called the “percentage” hydrometer. There are many other instruments marked in a more or less arbitrary manner. We append a comparative table of a few hydrometers. (See page 420.)

Ether, or sulphuric ether, is a mixture of spirits of wine with sulphuric acid, and distilled. It loses water, and the product is ether, which is volatile, and transparent, with a peculiarly penetrating odour. It will not mix with water, and if inhaled will produce a similar effect to chloroform.

[Pg 420]

Comparative Table of Hydrometers.

Chloroform is transparent, and will sink in water. Diluted alcohol with hypo-chloride of lime, will produce it. When inhaled, chloroform produces a pleasing insensibility to pain, and is useful in surgery.

A certain compound of alcohol with mercury dissolved in nitric acid will cause decomposition, and white crystals will eventuate. These compound crystals are termed fulminating mercury.

We must now pass rapidly over the few remaining subjects we have to notice, such as fats and soaps, wax, oils, etc.

Fats are of the greatest use to man, particularly in cold climates, for upon them depends the heat of the body. Fatty acid, if liquid, is known as oleic acid; if solid, stearic acid. Soaps are compounds of fatty acids. Many “fats” are consumed as food, others as fuel or for lighting purposes, in the shape of oils. Such oils are not primarily useful for burning. Petroleum and other mineral oils are found in enormous quantities in America.

There are what we term fixed oils, and essential or volatile oils. A list is annexed as given by “Hadyn’s Dictionary of Science”:—

Fixed Oils.

• Drying.
• Linseed oil.
• Poppy oil.
• Sunflower oil.
• Walnut oil.
• Tobacco-seed oil.
• Cress-seed oil.
• Non-Drying.
• Almond oil.
• Castor oil.
• Colza oil.
• Oil of mustard.
• Rape-seed oil.
• Olive oil, etc.

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Essential Oils.

• Oil of anise.
• Oil of bergamot.
• Oil of carraway.
• Oil of cassia.
• Oil of cedar.
• Oil of cloves.
• Oil of lavender.
• Oil of lemon.
• Oil of mint.
• Oil of myrrh.
• Oil of nutmeg.
• Oil of peppermint.
• Oil of rose.
• Oil of turpentine.

Vegetable oils are obtained by crushing seeds; animal oils come from the whale and seal tribe. Paraffin oil comes from coal. Linseed is a very drying oil, and on it depends the drying power of paint. We know olive oil will not dry on exposure to the air. Oiled silk is made with linseed oil. When oil is drying in the air considerable heat is evolved, and if oiled substances be left near others likely to catch fire, spontaneous combustion may ensue. Oil of turpentine is found in the pine and fir trees, and many of the oils above mentioned are used by perfumers, etc., the rose oil, or attar of roses, being an Eastern compound.

Fig. 433.—Crushing mill.

Allied to the volatile oils are the RESINS, which are non-conductors of electricity. They are vegetable products. They are soluble in alcohol, in the volatile oils, or in ether, and these solutions are called varnishes; the solvent evaporates and leaves the coating. Turpentine, copal, mastic, shellac, caoutchouc, and gutta-percha are all resinous bodies. Amber is a mineral resin, which was by the ancients supposed to be the “tears of birds” dropped upon the seashore. Moore refers to this in his poetic “Farewell to Araby’s Daughter”—

Amber is not soluble either in water or alcohol; it is, however, soluble in sulphuric acid. It takes a good polish, and when rubbed is very electrical. It is composed of water, an acid, some oil, and an inflammable gas, which goes off when the amber is distilled.

The well-known camphor is got from a tree called the “Laurus Camphora”; it is a white, waxy substance, and can be obtained by oxidizing certain volatile oils. It is generally produced from the Laurus Camphora in a “still.” The behaviour of a piece of camphor in pure water is curious, but its motions can be at once arrested by touching the water or dropping oil on the surface. This phenomenon is due to the surface tension of the liquid, which diminishes when it is in contact with the vapour of the substance.

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Nitrogenous Substances.

There are certain albuminous compounds which we must mention here. These are albumen, fibrine, and caseine. Albumen is the white of egg; fibrine is, when solid, our flesh and muscular fibre, while caseine is the substance of cheese. These are very important compounds, and the albuminous bodies are of the very highest importance as food, for the solid portion of blood, brain, and flesh consist, in a great measure, of them. Albumen, fibrine, and caseine contain carbon, hydrogen, nitrogen, and oxygen, with sulphur and phosphorus.

Albumen. The most familiar and the almost pure form of albumen is in the white of eggs, which is albuminate of sodium. It also exists in the serum of the blood, and therefore it is largely found in the animal kingdom. It can also be extracted from seed or other vegetable substances, but it is essentially the same. Albumen is very useful as an antidote to metallic poisons. It forms about 7 per cent. of human blood. It is soluble up to about 140° Fah.; it then solidifies, and is precipitated in a white mass. Albumen is used in the purification of sugar, etc.

Fibrine is found in a liquid condition in blood. The vegetable fibrine (gluten) is prepared by kneading wheat flour in a bag till the washings are no longer whitened. Like albumen it is found both in a solid and liquid state.

Caseine is seen in the skin which forms upon milk when heated, and forms about 3 per cent. of milk, where it exists in a soluble state, owing to the presence of alkali; but caseine, like albumen, is only soluble in alkaline solutions. As we have said, it is the principal constituent of cheeses. Caseine is precipitated by the lactic acid of milk, which is produced by keeping the milk too warm. Caseine, or curds, as they are called, are thus precipitated. The milk is said to be “sour,” or turned.

Milk, the food of the young of all mammalia, is composed chiefly of water, a peculiar kind of sugar, butter, and caseine. It is this sugar in milk which causes the lactic acid mentioned above. The actual constituents of milk are as follows:—