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.