velvet curtain receives a certain amount of vibration either as heat or light, and it absorbs-i.e., retains it. It is not sent through, or it would be transmission; it is not returned, or it would be reflection. The curtain is not able to reflect the force; the force is not able to move the curtain as a whole; but it is able to set in motion the particles of the curtain amongst themselves. Just as, when a cannon-ball is absorbed by an earthwork too weak to reflect it and too thick to be penetrated, the force is spent in moving the particles of earth amongst themselves. This motion of the particles, without the motion of the curtain as a whole, is heat. So that absorption really means that a force too weak for penetration and too strong for reflection is spent in vibration of the particles of the body into which it penetrates.

(8.) Examples of Absorption. In the spring of this year, the Great Eastern, when conveying the Indian Telegraph to Bombay, had a fresh coat of whitewash before entering the harbour. The immediate result of this was a fall of 8° in the temperature of the tank containing the cable. The sun's rays falling on the hull had been absorbed more by the dirty than by the clean surface, and this absorption became in time so great as to be transmission. Another example is the difference between light and dark coloured clothing. A dark coat is warmer than a white one of the same material, but not because it is dark. Its darkness and its warmth are two consequences of the same cause. The heat and light falling on it are absorbed; therefore it is both warm and dark. A light coat reflects, more or less, the heat and light; therefore it is both light and cool. The formation of colour, generally speaking, is an example of absorption. A ray of light falls on a flower, and is partially absorbed, the part reflected giving to our eye the sense of colour. But we must be on our guard against the possible error of considering white light as a reality compounded of various coloured lights. I have already spoken of this on page

118.

Can sound be absorbed? If so, what becomes of it? An omnibus makes more noise on an ordinary road than on a tramway, and can be drawn with more ease on the tramway than on the road. Here the noise is as much the result of force applied as the motion. Part of the force moves the omnibus, part produces sound; motion is transferred from the horse to the vehicle, from the vehicle to the ground, from the ground to the air, from the air to the ear. But if there be no ear to receive the sound there is none. What becomes of it? Clearly it is absorbed, just as heat or light would be under parallel circumstances,

While a carriage passes my window I cover my ears, and consequently do not hear it. The vibrations that, falling on my ear, would have been sound, now fall on my hands and are partially reflected, but chiefly absorbed. Through the open door of my room I hear a piano being played in the next room. I shut the

NATURE OF ABSORPTION.

283

door and can scarcely hear it. The sound falling on the door is partly reflected, partly absorbed, and partly transmitted. That is, a sound-wave falling on the door does three things: (1) Sends back a smaller wave, reflection: (2) sets up a smaller wave on this side, which is audible to me, transmission: (3) sets in motion, amongst themselves, the particles of the door, absorption.

(9.) Nature of Absorption.-A sponge put in water absorbs some of it, and the water so absorbed will remain in the sponge, and require some force to recover it. A piece of leather or of bread will in like manner absorb and retain some of any water into which it may be placed. If I put the sponge, or leather, or bread, under the tap of a cistern, and turn the water on, some of it will be reflected, some absorbed, some transmitted. That is, some will rebound from the surface, some pass through, some will remain within the interstices of the sponge or leather.

Just so force is reflected, transmitted, and absorbed, by almost every substance in nature. Just as the water absorbed can be recovered, so can the absorbed force. Just as the water gradually disappears, leaving the sponge dry, so will the force be gradually

diffused.

In football it often happens that the ball is for a time almost motionless under the influence of a number of conflicting forces, moving only short distances from boy to boy. Just so absorbed force consists in the motion to and fro of the particles of the absorbing body. But this absorbed force is given off little by little. As the motion of the particles amongst themselves reaches one of the outer particles, it is given off to the surrounding air.

So that absorbed force is no more than the parallel to latent heat (p. 86). We might call latent heat absorbed heat, or might call absorbed force latent force. The term absorbed force means a force that is not appreciable to our senses, and has no effect upon anything but the particles of the absorbing body.

(10.) Absorption by gases.-Heat and light pass through air with but very little loss-i.e., the air does not absorb any very perceptible portion of any heat or light passing through it. The same is true of hydrogen, oxygen, and nitrogen. This led to the idea that gases were perfectly diathermanous. But chlorine, hydrochloric, and carbonic acids, and especially aminonia, are found to absorb, by comparison with air, considerable quantities of heat. Thus, considering ammonia to absorb any given quantity of heat in any given time, an equal quantity of hydrochloric acid will absorb, in the same time, only as much, and an equal quantity of carbonic acid only about. Chlorine would absorb about 3, while air, oxygen, nitrogen, and hydrogen, would each absorb less than as much as the ammonia.

The method of estimating the amount of absorption is a very

delicate one, requiring great care. It consists, essentially, of a tube full of the given gas, through which a given amount of heat is allowed to pass. The effect of the transmitted heat upon the thermopile at the end is taken as a measurement.

SUMMARY.

(1.) Light or heat which falls upon a body and is not reflected is said to be absorbed.

Page 277.

(2.) Colour is frequently due to light being reflected after being partially absorbed.

Page 277.

(3.) Black objects are really invisible, owing to the total absorption of the light falling on them.

Page 278.

(4.) Absorbed light frequently becomes heat.

the absorbing body.

(5.) The amount of absorption depends upon the compactness of

Page 279.

Page 280.

(6.) Whatever increases reflection decreases absorption, and vice

versá.

(7.) Absorbed light becomes heat: absorbed heat.

(8.) Absorption is parallel to latency. (9.) Solids, liquids, and gases, all absorb more light or heat falling on them, but some gases but

amount.

Page 280.

heat remains

Page 281.

Page 283.

or

less of any a very small Page 283.

REFLECTION.

(1.) Introduction.-In order to fully understand how light, heat, sound, &c., can be reflected, we must grasp firmly the idea of what is meant by reflection. To de-flect is to bend away, to turn anything out of its original direction; to re-flect is to do this also, but to do it so that the thing turned out of its course is sent more or less back again towards its starting-place.

A ball thrown against a wall comes back again; the wall prevents its passage farther in its first direction, but the unspent force urges the ball on in the only direction it can move, which is from the wall more or less towards the starting-place. Force cannot be destroyed any more than matter, though it may be changed in its character. A croquet-ball striking against the hoop does not come to rest, for the same force that would have sent it through the hoop now drives it in another direction.

So a ray of light, or of sound, or of heat, falling upon any substance that it cannot penetrate, is bent on one side-i.e., reflected. This reflection is just as real as that of a ball from a wall, though there is the great difference that with the ball there is a transfer of matter, the ball changing its place; while in the case of heat, sound, or light, it is motion, not matter, that is transferred. This may be illustrated thus: Until quite recently, bells in houses have been rung by means of wires and bell-pulls. I want to ring such a bell, and I pull a cord; this pulls a wire, the other end of which is fastened to the bell, which is moved on one side by my pull. A spring pulls the wire and bell-pull back to their original position, and the bell, being suspended loosely by means of a bent spring, is set in vibration. Here the result is effected by means of a transfer of matter to and fro; the wire is moved bodily first in one direction and then back again.

If now, instead of this method, I use the modern one of connecting the bell and my hand by means of an electric wire, I ring the bell, not by moving the wire lengthways as before, but by setting in vibration either the wire itself or some subtle

medium filling its interstices-which, it is not clearly known yet. The wire itself remains stationary, excepting, it may be, the slight tremor of each particle. In the one case I cause a transfer of matter-i. e., I move the wire bodily; in the other, only a transfer of motion-i.e., the wire remains stationary, but a tremor runs along it, each part being set in consecutive vibration.

The fact to be grasped now is that this tremor, this agitation of the consecutive particles of a body (or of some ether between them), can be conveyed, bent round, turned to and fro, sent hither and thither, just as really as an actual substance can be moved from place to place-i.e., motion can be transferred in exactly the same way, and subject to precisely the same laws, as matter.

This transfer of motion may be well illustrated by transferring heat, light, sound, from one place to another. This may be done by using reflectors-i. e., substances which do not transmit or absorb much of the vibration falling on them, and which consequently reflect the greater part of it.

(2.) Parabolic Reflection.—I take two parabolic reflectors, and place them opposite each other at a distance of some yards. The reflectors are of metal, silvered inside and brightly polished, that being the best substance for reflecting purposes. Their form is parabolic, because this form has the property of reflecting in parallel straight lines, which enables me to reflect light, &c., to any greater or less distance without convergence or divergence.

Having arranged my reflectors so that they are exactly opposite each other, I place in the focus of one any source of light, heat, or sound. I may put my reflectors on two chairs leaning against the backs, one at either end of the room (or on the floor), and place a spirit-lamp on one chair in the focus of the reflector. Where this point is may be easily ascertained by moving the hand about in front of the reflector, and noticing where it receives most light. This point is the focus. I find the focus of the second reflector in the same way. Fig. 166 shows the reflectors A and B with foci a and b.

I put a spirit-lamp or a candle in one focus. The light and