Part 11

This much, however, is certain: that heat can be caused by motion. Every boy knows that a metal button may be made quite hot by rubbing it. A skilful smith will hammer a piece of iron red hot. The axles of wheels become red hot by rubbing against their bearings, if they are not properly lubricated; and even two pieces of ice may be melted by the heat evolved when they are rubbed together. And there are abundant other reasons, as you will find when you study physics, for the belief that the sensation we call heat, and all the phenomena which we ascribe to heat, are the effects of the rapid motion of matter.

However, a quiescent body may be made hot without exhibiting the least appearance of motion. The surface of the water in a tumbler at 100° is just as unruffled as that of the same water at 32°. What, then, is meant by saying that heat is a kind of motion, and that the greater the heat in any body the greater the amount of motion in that body?

The answer to this question is that the motion which causes the phenomena of heat, is not a visible motion of the whole mass of the hot body, but a motion of the individual =particles= of which it is composed. And each particle moves, not straight forward, but backwards and forwards in the same space, so that its motion may be roughly compared to that of a pendulum, or to that of the balance-wheel of a watch. It is in fact a sort of =vibratory= movement; each vibration taking place through a very short distance and with extreme rapidity. The sensation of heat is caused by the vibratory movements of the particles of matter, just as sound is so caused. The prongs of a tuning-fork which has been struck, certainly vibrate, for you can see them do so if the note is low. If you now put your ear at one end of a long piece of timber and the handle of the vibrating tuning-fork is placed upon the other end, the vibratory motion of the tuning-fork will be communicated to the particles of the wood and will be loudly heard. All the time the sound is heard the particles of the wood are vibrating. Nevertheless, the wood as a whole does not move, but its particles swing backwards and forwards through such a minute space that their motion is imperceptible.

But what are these =particles= of matter which by their vibration give rise to the phenomena of heat?

46. =The Structure of Water.=

We have seen that pure water is perfectly clear and transparent. The naked eye can discern no difference between one part and another. In other words, it has no visible texture or =structure=. It does not follow that it really possesses none, however, for there are many things which seem to be the same throughout, or =homogeneous=, which yet show structure if they are examined with a magnifying glass. Thus the surface of a sheet of fine white paper looks perfectly even and smooth to the eye; but a magnifying glass of no great power will show the minute woody fibres of which it is made up; while, under a powerful microscope, the paper looks like a coarse matting.

But if we put a small drop of water on a slide, such as is used for microscopic objects, and cover it over with a thin glass so as to spread it out into a film, perhaps not more than 1/10000th of an inch thick, it may be examined with the very highest magnifying powers we can command, and yet it looks as completely homogeneous and shows as little evidence of being made up of separate parts as before. However, this is still no proof that the water is not made up of little parts, or particles, distinctly separated from one another. It may merely mean that the particles are so extremely small that they cannot be distinguished even by microscopes which magnify four or five thousand diameters.

It is certain that solid bodies may be divided into particles so minute that the best microscopes show no trace of them. Common gum-mastic cannot be dissolved by water, but it readily dissolves in strong spirit or alcohol, and mastic varnish is an alcoholic solution of gum-mastic. If you add water to mastic varnish, the alcohol takes away the water and the mastic falls out, or =precipitates=, as a curdy solid composed of very visible whitish particles. But if a drop of the varnish is added to a good deal, say half a pint, of water and well stirred at the same time, the mastic, though it is still precipitated as a solid, is in a state of extremely minute division. No separate solid particles of mastic are visible to the naked eye, but the water assumes a faint milky tinge.

This milkiness arises from the presence of solid particles of mastic diffused through the water; and yet, if the experiment has been properly managed, a drop of the fluid may be spread out as before and examined with the highest magnifying powers, and nothing can be seen of such particles. So far as vision goes it might be a drop of pure water. Now our best microscopes are able to show us anything solid which has a diameter of 1/100000th of an inch, quite distinctly; and probably solid opaque particles of much smaller size would make themselves apparent as a turbidity or cloudiness. The particles of mastic must be therefore so much smaller than this that they remain invisible. Hence it follows that if water were made up of separate particles, or droplets, 1/1000000th of an inch in diameter, and thus had the structure of a mass of very fine shot, no microscope that has yet been constructed would enable us to see even a trace of that structure. We could not obtain any direct evidence of it.

47. =Suppositions or Hypotheses; their Uses and their Value.=

When our means of observation of any natural fact fail to carry us beyond a certain point, it is perfectly legitimate, and often extremely useful, to make a supposition as to what we should see, if we could carry direct observation a step further. A supposition of this kind is what is called a =hypothesis=, and the value of any hypothesis depends upon the extent to which reasoning upon the assumption that it is true, enables us to explain or account for the phenomena with which it is concerned.

Thus, if a person is standing close behind you, and you suddenly feel a blow on your back, you have no direct evidence of the cause of the blow; and if you two were alone, you could not possibly obtain any; but you immediately suppose that this person has struck you. Now that is a hypothesis, and it is a legitimate hypothesis, first, because it explains the fact; and, secondly, because no other explanation is probable; probable meaning in accordance with the ordinary course of nature. If your companion declared that you fancied you felt a blow, or that some invisible spirit struck you, you would probably decline to accept his explanation of the fact. You would say that both the hypotheses by which he professed to explain the phenomenon were extremely improbable; or in other words, that in the ordinary course of nature fancies of this kind do not occur, nor spirits strike blows. In fact, his hypotheses would be illegitimate, and yours would be legitimate; and, in all probability, you would act upon your own. In daily life, nine-tenths of our actions are based upon suppositions or hypotheses, and our success or failure in practical affairs depends upon the legitimacy of these hypotheses. You believe a man on the hypothesis that he is always truthful; you give him pecuniary credit on the hypothesis that he is solvent.

Thus, everybody invents, and, indeed, is compelled to invent, hypotheses in order to account for phenomena of the cause of which he has no direct evidence; and they are just as legitimate and necessary in science as in common life. Only the scientific reasoner must be careful to remember that which is sometimes forgotten in daily life, that a hypothesis must be regarded as a means and not as an end; that we may cherish it so long as it helps us to explain the order of nature; but that we are bound to throw it away without hesitation as soon as it is shown to be inconsistent with any part of that order.

48. =The Hypothesis that Water is composed of Separate Particles (Molecules).=

It has been pointed out that we cannot see, and indeed that there is not much hope of our ever being able to see, the separate particles of water, even if water is composed of such particles. But it is perfectly legitimate to suppose that water is made up of such particles, if that hypothesis will enable us to explain the properties of water.

Let us suppose then that any portion of fluid water is really composed of a prodigious number of particles less (and probably much less) than a millionth of an inch in diameter. We may call these particles =molecules=.

Footnote 4:

Diminutive of moles, a mass.

We are justified, in accordance with the general properties of matter (§ 18), in supposing that these molecules tend to approach one another. But the fact that water is slightly compressible justifies the supposition that its molecules are not in actual contact, but that they are separated by interspaces, just as the motes in the air of a dusty room are so separated.

What is it that keeps the molecules apart? We have seen that great mechanical pressure brings them but slightly nearer to one another; hence there is an equivalent resistance of some kind which keeps them apart. This resistance must have the same origin as the sensation which we know as heat, for it has been seen that diminution of heat diminishes the bulk of water; that is, allows the molecules to come closer together; that is, diminishes their tendency to keep asunder. Increase of heat, on the other hand, increases the volume of water; that is to say, drives the molecules further apart, or increases their tendency to keep asunder.

Suppose we call the cause of the tendency of the molecules of water to come together an =attractive force=; and the cause of their keeping apart, which manifests itself to us as the sensation of heat and is, as we have seen, in all probability, a rapid vibratory or whirling motion of the molecules, a =repulsive force=; then, in the liquid state, these forces are so adjusted that the molecules are quite free to move, and yet hold together.

By adding heat the repulsive force is increased, until the molecules are about twelve times as far apart as they were in each direction; while the attractive force is overcome, and the molecules fly off in all directions as soon as they are unconfined. On the other hand, by taking heat away, the repulsive force is diminished, until the molecules become inseparable and the water assumes the solid form.

It is probable that the expansion of fluid water, at a temperature below 39°, depends upon the molecules taking up a peculiar arrangement as they approach one another. If sixteen men are formed into a column, four deep, and each man a foot from the other, the same men may stand closer together and yet form a hollow square, which occupies a larger space. That the molecules of water do take up a particular order in assuming the solid condition, is shown by the crystalline form of ice. Each crystal of hoar-frost owes its shape to the arrangement of its molecules, according to a definite geometrical pattern.

Thus the hypothesis that water is composed of separate molecules, is useful, for it helps us to some extent to explain the properties of water. And, when you study physics and learn the laws of motion, you will find that there is no end to the number of the truths established by observation and experiment, which can be explained by this hypothesis. Hence it may fairly be adopted and employed as a means of picturing to ourselves the order of nature, so long as no facts are discovered which are inconsistent with it.

49. =All Matter is probably made up either of Molecules or of Atoms.=

The same reasons which lead to the adoption of the hypothesis that water is composed of separate particles justify its extension to all forms of matter whatever.

The metal =mercury= or =quicksilver=, for instance, may be supposed to be made up of distinct particles of mercury of extreme minuteness, and according to the temperature, these associate themselves in the solid (frozen mercury), liquid (ordinary quicksilver), or gaseous form (vapour of mercury). To whatever treatment pure mercury may be subjected, we cannot get anything but mercury out of it. The particles of mercury have never been broken up. Hence they are generally termed =atoms=, or particles that cannot be divided; and mercury is said to be an =element=, or a substance which is not compounded of any other substances.

Here is a case in which it is very useful to distinguish between fact and hypothesis. The matter of fact is that, up to the present time, no one has been able to get out of pure mercury anything but pure mercury. The statement that mercury is a simple substance, and therefore never can be broken up into any other substances, is a hypothesis which future observation and experiment may or may not confirm.