But the simplest drop of water, in itself, and apart from its mechanical relations to other matter, is really a very complex and a very wonderful thing; not at all likely to be "self-caused." Water is made up, we know, of oxygen and hydrogen—two elementary colourless, formless gases. Now we can easily divide the one drop into two, and, without any great difficulty, the two into four, and (perhaps with the aid of a magnifying glass) the four into eight, and so on, as long as the minute particle still retains the nature of water. In short, we speak of the smallest subdivision of which matter is capable without losing its own nature, as the molecule. All matter may be regarded as consisting of a vast mass of these small molecules.
Now, we know that all known matter is capable of existing either in a solid, liquid, or gaseous form, its nature not being changed. Water is very easily so dealt with. Some substances, it is true, require very great pressure or very great cold, or both, to alter their form; but even carbonic acid, oxygen, and hydrogen, which under ordinary conditions are gases, can with proper appliances be made both liquid and solid. Pure alcohol, has, I believe, never been made solid, but that is only because it is so difficult to get a sufficient degree of cold: there is no doubt that it could be done.
It might be supposed that the molecules of which dead matter (whether solid, liquid, or vapourous) is composed, were equally motionless and structureless. But it is not so: every molecule in its own kind is endowed with marvellous properties. In the first place, every molecule has a double capability of motion. In the solid form the molecules are so packed together that, of course, the motion is excessively restricted; in the liquid it is a little easier; in the gaseous state the molecules are in a comparatively "open order." In most substances that are solid under ordinary conditions, by applying heat continuously we first liquefy and ultimately vapourize them. In those substances which under ordinary conditions are gas (like carbonic acid, for instance), it is by applying cold, with perhaps great pressure as well, that we induce them to become liquid and solid; in fact, the process is just reversed. As we can most easily follow the process of heating, I will describe that. First, the solid (in most cases) gets larger and larger as it progresses to liquefaction, and when it gets to vapour, it suddenly expands enormously. Take a rod of soft iron, and reduce it to freezing temperature: let us suppose that in that condition it measures just a thousand inches long. Then raise the temperature to 212 degrees (boiling point), and it will be found to measure 1,012 inches. Why is that? Obviously, because the molecules have got a little further apart. If you heat it till the iron gets liquid, the liquid would also occupy still more space than the original solid rod; and if we had temperature high enough to make the melted iron go off into vapour, it would occupy an enormously increased space. I cannot say what it would be for iron vapour; but if a given volume of water is converted into vapour, it will occupy about 1,700 times the space it did when liquid, though the weight would not be altered.
It may here be worth while to mention that it is not invariably true that a substance gets contracted, and the molecules more and more pressed together, as it assumes a solid form. There is at least one exception. If we take 1,700 pints of steam, the water, as I said, on becoming cool enough to lose the vapourous form, will shrink into a measure holding a single pint; if we cooled lower still, it will get smaller and smaller in bulk (though of course not at all at the same rate) till it arrives at a point when it is just going to freeze; then suddenly (7 degrees above the freezing point) it again begins to expand. Ice occupies more space than cold water; its molecules get arranged in a particular manner by their crystallization.
On the admission of an intelligent Creator providing, by beneficent design, the laws of matter, it is easy to give a reason for this useful property. It prevents the inhabitants of northern climates being deprived of a supply of water. As it is, the solid water or ice expands, and, becoming lighter, forms at the top of the water, and the heavier warmer water remains below. But if ice always got denser and sank, the warmer liquid would be perpetually displaced and so come up to the surface, where it would freeze and sink in its turn. In a short time, then, all our water supplies would (whenever the temperature went down to freezing, which it constantly does in winter) be turned into solid ice. This would be a source of the gravest inconvenience to the population of a cold climate. If we deny a designing mind, the alternative is that this property of water is a mere chance.
But to return to molecules. Molecules are endowed with an inherent faculty of motion; only under the conditions of what we call the solid, they are so compressed, that there is no room for any motion appreciable to the senses. Even if the solid is converted into vapour, the molecules are still much restrained in their movements by the pressure of the air. But of late years, great improvements (partly chemical, partly mechanical) have been made in producing perfect vacua; that is to say, in getting glass or other vessels to be so far empty of air, that the almost inconceivably small residue in the receptacle has no perceptible effect on the action of a small quantity of any substance already reduced to the form of gas or vapour introduced into it. Dr. W. Crookes has made many beautiful experiments on the behaviour of the molecules of attenuated matter in vacua. The small quantity of vapour introduced contains only a relatively small number of molecules, which thus freed from all sensible restraint within the limits of the glass vessel used, are free to move as they will; they are observed to rush about, to strike against the sides of the vessel, and under proper conditions to shine and become radiant, and to exhibit extraordinary phenomena when subjected to currents of electricity. So peculiar is the molecular action thus set up, that scientific men have been tempted to speak of a fourth condition of matter (besides the three ordinary ones, solid, liquid, and gaseous), which they call the ultra-gaseous or radiant state of matter.
This marvel of molecular structure seems already to have removed us sufficiently far from the idea of a simple inert mass, which might be primordial and self-caused. But we have not yet done. Even imagining the extreme subdivision[[10]] of the particles in one of Dr. Crookes' vacuum globes, the particles are still water. But we know that water is a compound substance. The molecule has nine parts, of which eight are hydrogen and one oxygen—because that is the experimentally known proportion in which oxygen and hydrogen combine to form water. As we can (in the present state of our knowledge) divide no farther, we call these ultimate fragments of simple or elementary substance atoms.
Every substance, however finely divided into molecules, if it is not a simple substance, must therefore have, inside the molecular structure, a further atomic structure. And in the case of unresolvable or "elementary" substance, the molecule and the atom are not necessarily the same. For though there is reason to believe that, the molecule of these does consist, in some cases, of only one atom—in which case the atom and the molecule are identical; in other cases, the molecule is known to consist of more than one atom of the same element; and the atoms are capable of being differently arranged, and when so arranged have different properties or behaviour, though their nature is not changed. This property is spoken of by chemists as allotropism. No chemist on earth can detect the slightest difference in constitution between a molecule of ozone and one oxygen; but the two have widely different properties, or behave very differently. There is thus a great mystery about atoms and their possible differences under different arrangement, which is as yet unsolved. Those who wish to get an insight into the matter (which cannot be pursued farther here) will do well to read Josiah Cooke's "The New Chemistry," in the International Scientific Series. The mind is really lost in trying to realize the idea of a fragment of matter too small for the most powerful microscope, but existing in fact (because of faultless reasoning from absolutely conclusive experiments), and yet so constituted that it is practically a different thing when placed in one position or order, from what it is when placed in another.
Turning from this mystery, as yet so obscure, to what is more easily grasped, we shall hardly be surprised to learn, further, that every kind of, atom obeys its own laws, and that while atoms of one kind always have a tendency to combine with atoms of other kinds, it is absolutely impossible to get them to combine together except on certain conditions.
The difference between combination and mixture is well known. Shake sand and sugar in a bag for ever so long, but they will only mix, not combine or form any new substance even with the aid of electric currents; but place oxygen and hydrogen gas under proper conditions, and the gases will disappear, and water (in weight exactly equal to the weight of the volume of gases) will appear in their place.