Part 5
Our tumbler of water, if put out of doors on a cold winter’s night, would gradually cool until it assumed a temperature of 39° throughout. Cooling below this temperature, the water so cooled would gradually accumulate at the surface by reason of its less density, and its temperature would fall till the thermometer placed in it marked 32°. As soon as this upper water cooled ever so little below 32°, a film like glass would form on its surface by the conversion of the coldest fluid water into solid water or =ice=. And if all the water cooled down to the same degree it would all gradually change into the same kind of substance.
In this condition water is solid. It occupies space, offers resistance, has weight and transmits motion as the water did, but if you shake it out of the tumbler in a cold place it retains its form without the least change. If you press it, it proves to be exceedingly hard and unyielding; and, if the pressure is increased, it becomes crushed and breaks like glass. It may thus be crushed to powder, and the ice powder can be formed into heaps as if it were sand.
Just as any quantity of steam has exactly the same weight as the water which was converted into it by heat; so the ice has exactly the same weight as the water which has been converted into it by taking away heat.
41. =Ice has less Specific Gravity than the Water from which it was formed.=
But though the ice in the tumbler has the same weight as the water had, it has not the same volume. The expansion which began at 39° goes on, and when water passes into the solid state its volume is about 1/11th greater than it was at 39°. Taking water at this temperature as 1·0, ice has a specific gravity of 0·916.
But although water in freezing expands only to this small amount, it resembles steam in the tremendous force with which it expands. If you fill a hollow iron shell quite full of water, screw down the opening tight, and then put it in a cold place where the water may freeze, the water as it freezes will burst the iron walls of the shell. You know that when the winter is severe, the pipes by which water is brought to a house often burst. This is because the water in them freezes, and, being unable to get out of the pipe, bursts it, just as you may burst a jacket that is too tight for you by stretching yourself. Among the bare hill-tops, or on the face of cliffs exposed to the weather, the strongest and hardest rocks are every winter split and broken, just as if quarrymen had been at work at them. In the summer the rain-water gets into the little cracks and rifts in the stone and lodges there. Then the winter comes with its cold and freezes the water. And the water bursts the rocks asunder just as it bursts our waterpipes.
42. =Hoar Frost is the Gaseous Water which exists in the Atmosphere, condensed and converted into Ice Crystals.=
In the winter-time you often notice, on a clear sharp night, that the tops of the houses and the trees are covered with a white powder called =hoar frost=; and, on the windows of the room when you wake up, you see most beautiful figures, like delicate plants. Take a little of the hoar-frost, or scrape off some of the stuff that makes the window look like ground glass, and you find that it melts in your hand and turns to water. It is in fact ice. And if you look at the figures on the window pane with a magnifying glass you will see that they are made up bits of ice which have a definite shape, and are arranged in a regular pattern. Each of these definitely shaped bits of ice has been formed in the following way. The air in the room is much warmer than that outside, and there is mixed with it nearly as much water, derived from the breath and the evaporation of moist surfaces, as can maintain itself in the gaseous state at the temperature. The windowpanes, being thin, are cooled by the outside air, and of course the gaseous water inside the room, when it comes in contact with the cold windowpanes, becomes condensed on them into fine drops of cold water. The panes becoming colder and colder, these minute drops at last freeze, and the water not only becomes solid, but it =crystallises=; that is to say, the little solid masses take on more or less regular geometrical forms with flat faces, inclined to one another at constant angles, so that they resemble bits of glass cut according to particular fixed patterns. All ice is in fact crystalline, but in ice which has been formed from thick sheets of water, the crystals are so packed together that they cannot be distinguished separately.
43. =When Ice is warmed it begins to change back into Water as soon as the Temperature reaches 32°.=
A lump of ice brought out of the open air in very cold weather may have a temperature of 30°, or 20°, or lower. If such a lump is brought into a warm room it gradually becomes warmer, but remains unchanged otherwise, until it has risen to 32°. Then it begins to melt, and remains at 32° as long as it is melting; and the water which proceeds from it is at first also at 32°.
If you were to throw a lump of ice into the middle of a hot fire, so long as a particle of ice remained as such, it would have a temperature of 32° and no more. This is a fact exactly parallel to that which is observed when water is raised to the boiling-point. So long as any of the water remains unconverted into steam it becomes no hotter. Moreover the steam itself is at first at 212°.
44. =Ice the solid, Water the liquid, and Steam the gas, are three states of one natural object; the Condition of each State being a certain Amount of Heat.=
Ice, liquid water, and steam, are three things as unlike as any three things can well be. What do we mean then by saying that they are states of one substance, water?
What we really mean is that if we take a given quantity of water, say a cubic inch, and change it first into ice and then into steam, there is something which remains identically the same through all these changes. This something is, in the first place, the weight of the material substance. The water weighs 252½ grains, the ice into which it is converted weighs 252½ grains, and the steam produced from it weighs 252½ grains. In the second place, the same force would cause the ice, the water, and the steam, to move with the same rapidity; and, when set in motion, they would produce the same effect upon anything movable against which they struck.
In the third place, when you study chemistry, you will learn that the ice, the steam, and the liquid water, would yield the same weight of the same two gases, =oxygen= and =hydrogen=, and nothing else. Every one cubic inch of water, 1,700 cubic inches of steam, and 1/111 cubic inch of ice, yield 281/18 grains of hydrogen, with 2248/18 grains of oxygen, and nothing else. (See § 50.)
As there is not the slightest difference in weight between a given quantity of water and the ice, or the steam, into which it may be converted, it is clear that the heat which is added to or taken from the water to give rise to these several states, can possess no weight. If then heat is a material body, it must be devoid of weight—and hence, in former times, heat was called an =imponderable= substance. It was thought to be a kind of fluid, called =caloric=, which had no weight, and which drove the particles of bodies asunder, when it entered them as they were heated, and let them come together as it left and they grew cool.
45. =The Phenomena of Heat are the Effects of a rapid Motion of the Particles of Matter.=
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=.[4]
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.
A hundred and fifty years ago it was universally believed that water was as much an element as mercury. But water is now well known to be a compound. In fact, as has already been said, the particles of water may be very readily broken up or =decomposed= (in what way, you will learn when you study chemistry) into two totally distinct substances, =oxygen= and =hydrogen=, which are gaseous at all known temperatures, though by combining vast pressure with extreme cold they have recently been liquefied. Each of these gases, according to our hypothesis, consists of particles, and since these can by no known means be further broken up, they are considered to be =atoms= like those of mercury.
Nine parts by weight of pure water always yield eight of oxygen and one of hydrogen. The hypothetical particle, or molecule of water, therefore, must be composed of atoms of oxygen and hydrogen having this relative weight; and chemists have grounds for believing that one atom of oxygen and two atoms of hydrogen exist in each molecule of water. If this be so, the structure of water must be more complicated than we thought at first; and each particle of water (the molecule) must be a system composed of three separate atoms.
50. =Elementary Bodies are neither destroyed nor is their Quantity increased in Nature.=
It has been seen that when a cubic inch of water is dissipated by heat, it is not destroyed, but that it merely changes its form from the fluid to the gaseous state, while its weight remains unaltered. If the same cubic inch of water is decomposed into oxygen and hydrogen gases, the water is indeed destroyed, but the matter of which it consisted remains unchanged in weight. If the water weighed 252·5 grains, the oxygen gas will weigh 224·45 grains and the hydrogen gas will weigh 28·05 grains. And nothing that man has been able to do has affected the weight of a given quantity of either of these gases. So far as we know, elementary bodies retain their weight under all circumstances, and can be traced by it whatever shape they may take. If this is true it follows that, in the order of nature, matter is =indestructible=: the quantity of it neither increases nor diminishes.
Hence it follows that natural things and artificial things resemble one another in one respect. It is true of both that the matter of which they are composed is never destroyed and never increased; and therefore the order of events in nature as much consists in the joining together and putting apart of natural bodies by natural agencies, as the order of events in the artificial world consists in the joining together and the putting apart of natural bodies by human agencies.
51. =Simple Mixture.=
In order to learn the manner in which water may be broken up into its elements or decomposed, you must turn to the Primer on Chemistry. But as a preliminary to the study of that science, it may be useful to consider some simple cases of composition and decomposition which are exemplified by water.