Opening…

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 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.

SCIENTIFIC RECREATIONS.

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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 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.

Every one, of course, may not possess the ambition to make such 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 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:

{ Zoology, referring to Animals } Biology. Natural History { Botany, referring to Plants } { Mineralogy } referring to Minerals, etc. { Geology }

{ Physics. Phenomena without essential change Natural Philosophy { of the Objects. { Chemistry. Phenomena with change of the Objects. { Physiology. Phenomena of animated Objects.

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 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

“Imagination’s utmost stretch In wonder dies away.”

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.

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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.

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 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 aphides[1] (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.

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′.

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 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 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.

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; 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.

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 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.

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, 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.

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 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 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 distinguishable, 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.

We cannot point out too strongly to our readers that the essential condition 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 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 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. Renon[5] 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 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.

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 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”?

FOOTNOTES:

[1] It is well known that ants, by touching the skin of aphides, extract therefrom a secretion of viscous matter, which nourishes them. They will frequently carry off the aphides to their habitations, and keep them there; thus one may say they keep a cow in their stable.

[2] The preservation of insects, and their preparation for collections, necessitates some precaution. Entomologists are in the habit of spreading them out on a small board, and arranging the legs and antennæ by means of large pins. The wings should be dried by placing them on strips of paper, which preserves them. These precautions are indispensable if it is wished that the insects in a collection should retain their distinctive characters. Worms and caterpillars can be raised in pots filled with earth, if carefully covered over with muslin or wire gauze with very fine meshes. The process of hatching may give rise to many interesting observations.

[3] It frequently happens that in a small aquarium, constructed after this fashion, the animals escape. This is avoided by covering the vase with a net.

[4] The infusion of parsley has the advantage of not sensibly obscuring the water.

[5] Detailed accounts in Vol. lxxxiii., pp. 243 and 292 of “La Nature.”

[6] See “La Nature,” 4th year, 1876, 2nd half-year, p. 167. M. A. Guillemin mentions, in connection with the phenomenon of July 12th, 1876, the presence of light masses of cloud of a greyish-blue colour, similar to those perceived in the phenomena just described.

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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 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.

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.

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.

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 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.)

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 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.

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 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 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 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 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 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 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. 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 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.

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, 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.

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 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.

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.

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.

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.

FOOTNOTES:

[7] M. A. G. has written us an interesting letter on the subject of similar experiments, which we here transcribe:—

“When a siphon of seltzer water has been opened some little time, and the equilibrium of tension is nearly established between the escaped gas and the dissolved gas, a vertical stream of bubbles is seen to rise from the bottom of the apparatus, which present a very clear example of the law of ascension of bubbles; that is to say (putting out of the question the expansion of the bubbles in their passage upwards), it is an inverse representation of the law of gravity affecting falling bodies. The bubbles, in fact, detach themselves from their starting point with perfect regularity; and as the interval varies in one file from another, we have before us a multiplied representation of that terrible law which Attwood’s machine made such a bugbear to the commercial world. I believe it is possible, by counting the number of bubbles that detach themselves in a second, in each file, and the number which the whole stream contains at a given instant, to carry the verification further; but I must confess that I have not done so myself.”

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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.

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 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).

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 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.

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 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]

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,— 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 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 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.

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.

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. No body can give unlimited work, therefore the perpetual motion theory is untenable and impossible.

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°.

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 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.

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).

FOOTNOTES:

[8] The experiment we have just described is a very old one. M. V. Sircoulon has told us that it was described at length in the works of Rabelais. The following remarks are in “Pantagruel,” book II., chap. xvii.

“Panuræ then took two glasses of the same size, filled them with water, and put one on one stool, and the other on another, about five feet apart, and placed the staff of a javelin about five-and-a-half feet long across, so that the ends of the staff just touched the brim of the glasses. That done, he took a stout piece of wood, and said to the others: “Gentlemen, this is how we shall conquer our enemies; for in the same way that I shall break this staff between these two glasses, without the glasses being broken or injured, or spilling a single drop of water, so shall we break the head of our Dipsodes, without any injury to ourselves, and without getting wounded. But that you may not think there is magic in it, you, Eusthenes, strike with this stick as hard as you can in the centre.” This Eusthenes did, and the staff broke in two pieces, without a drop of water being spilt.

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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.

The force of air can very soon be shown as acting with considerable 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.

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 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 atmospheric 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.

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 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.

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 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.

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

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 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.

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 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.

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).

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 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).

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 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).

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.

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 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.

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 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.

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.

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.

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.

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.

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 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.

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

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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 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.

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 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.

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.

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).

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 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.

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 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.

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 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.

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 surfaces[9] in the liquids in the tubes, according as the tube is wetted with the liquid or not. For instance, water, as we 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.

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 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.

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 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.

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.

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.

[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.]

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 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.

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.

[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 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.

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

+----------+----------++----------+----------++------------+----------+ |Substance.| Specific ||Substance.| Specific ||Substance. | Specific | | | Gravity. || | Gravity. || | Gravity. | +----------+----------++----------+----------++------------+----------+ | Platinum | 21·5 || Iron | 7·79 || Water | 1·000 | | Gold | 19·5 || Tin | 7·29 || Sea Water | 1·026 | | Mercury | 13·59 || Granite | 2·62 || Rain Water | 1·001 | | Lead | 11·45 || Oak Wood | 0·77 || Ice | ·916 | | Silver | 10·50 || Cork | 0·24 || Ether | 0·723 | | Copper | 8·96 || Milk | 1·032 || Alcohol | 0·793 | +----------+----------++----------+----------++------------+----------+

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 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.

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.

FOOTNOTES:

[9] The curved surface of a column of liquid is termed a “meniscus,” from the Greek word meniskos, meaning “a little lens.”

Spotted a typo or a problem in this chapter? Tell us

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 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, 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.”

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.

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 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 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 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 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 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 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.

[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:—

TABLE I.

1° Fahr. = 0·55° Cent., or 0·44° Réaumur. 1° Cent. = ·80° Réaumur, or 1·80° Fahr. 1° Réaumur = 1·25° Cent., or 2·25° Fahr.

TABLE II.

|---------------------------+-------+-------+---------| | | Fahr. | Cent. | Réaumur.| |---------------------------+-------+-------+---------| | Boiling point | 212 | 100 | 80 | | | 194 | 90 | 72 | | | 176 | 80 | 64 | | | 158 | 70 | 56 | | | 140 | 60 | 48 | | | 122 | 50 | 40 | | | 104 | 40 | 32 | | | 86 | 30 | 24 | | | 68 | 20 | 16 | | | 50 | 10 | 8 | | Freezing point of water | 32 | 0 | 0 | | | 14 | -10 | -8 | | | -4 | -20 | -16 | | Freezing point of Mercury | -40 | -40 | -32 | |---------------------------+-------+-------+---------|

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 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 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 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:—

+--------------------+------------------+-------------+---------------+ | Place. |Height above level| Barometer | Boiling point | | | of the sea—Feet. | mean height.|of water, Fahr.| +--------------------+------------------+-------------+---------------+ | Quito | 9,541 | 20·75 | 194·2 | | Mexico | 7,471 | 22·52 | 198·1 | | St. Gothard | 6,808 | 23·07 | 199·2 | | Garonne (Pyrenees) | 4,738 | 24·96 | 203·0 | | Geneva | 1,221 | 28·54 | 209·5 | | Paris (1st floor) | 213 | 29·69 | 211·5 | | Sea level | 0 | 30·00 | 212·0 | +--------------------+------------------+-------------+---------------+

[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 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.

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 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.

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). 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.

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.

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.

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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 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 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.

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 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.

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.

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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 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 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.

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.

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 geometrical 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.

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.

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.

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 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.

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 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.

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 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 intend to study this question fully, for there is no doubt that electricity is becoming more and more adapted to our daily life.

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.

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:—

s. d. Wax 7 2½ Sperm Oil 1 10 Paraffin 3 10 Spermaceti 6 8 Coal Gas 0 4½ Paraffin Oil 0 6 Tallow 2 8 Cannel Gas 0 3 Rock Oil 0 7⅔

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.

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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.

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 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.

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’Alméïda[10] 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.

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 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:—

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 auspices[11]. 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:—

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.

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, 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.

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 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.

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.

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 throughout 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.

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.

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.

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 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 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.

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.

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 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 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.

Phenomena of successive contrast develop what are called accidental images. If we fix our eyes for a considerable time on a coloured object, 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.

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 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 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.

FOOTNOTES:

[10] Traité de Physique, Paris 1874.

[11] Traité d’optique Physiologique. French translation by MM. Javal and Klein.

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