Science Primers, Introductory

by Thomas Henry Huxley

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.