The Romance of Modern Chemistry

INVISIBLE SUBSTANCES still holds the field. We first assume the atomic nature of matter; that is, we suppose that if we had a micro- scope powerful enough, we should find that an apparently continuous and homogeneous piece of matter is really discontinuous, consisting ultimately of tiny, separate, and distinct specks or molecules, just as what looks like a single homogeneous black mound in the distance may turn out on closer inspection to be a heap of separate cannon balls. Then we suppose further (and this seems very unlikely at first) that in the case of a liquid or a gas the ultimate particles are in a state of continual motion. The particles or molecules of a gas are to be pictured as rushing hither and thither at a very high speed, constantly colliding with one another and with the walls of the containing vessel. The pressure which the gas exerts on the walls of the containing vessel—a pressure which we may easily measure—is due simply to the impacts delivered by the myriads of moving molecules. Each molecule, as it comes up to the wall of the containing vessel, delivers its blow, and rebounds with undiminished speed, to continue its zig-zag course among the other molecules. If part of the wall of the containing vessel is removed, then the molecules immedi- ately rush ahead and occupy whatever space is offered to the gas. Although the picture just outlined of the conditions which prevail in a gas may seem somewhat improbable to the reader, it has been found capable of giving an excel- lent interpretation of the varied behaviour of gases. But that is another story, and would lead us too far. The molecules of a gas are very small compared with the spaces between them. When it is remembered that the molecules in a volume of gas about the size of a 48