The Romance of Modern Chemistry

INVISIBLE SUBSTANCES pin head are thirty million times as numerous as the human beings on the face of the globe, it will be seen that a gas molecule is quite the smallest thing we can think of. Not less surprising than the size of the mole- cules is the rate at which they move. If it came to a race between an express train and a molecule of oxygen, the train would be hopelessly out of it; for the oxygen molecule slips along at the rate of about twenty miles a minute. Now why should liquids be so different from gases, so much more easily visible, so much more tangible, so much less changeable in their volume ? The key to the difficulty lies in the recognition that in a liquid the molecules are much closer together than in a gas. Just as one heavenly body attracts another, so a inole- ctile is subject to the attractive force of the surround- ing molecules, and it is only because the molecules of a gas are relatively so far away from each other that the attraction may be neglected in this case. In a liquid, however, where the molecules, although still endowed with the power of rapid motion, exert a power- ful attraction on each other, it becomes very difficult for an individual molecule to escape through the surface of the liquid. There is, as it were, a social force exerted which seeks to prevent the individual molecule desert- ing the community. Those molecules which attempt to escape through the surface with a rush and so evapo- rate have to run the gauntlet of the crowded molecules around, and most of them are prevented. Thus it comes that a liquid is not at liberty to expand to any extent like a gas; a liquid has a definite volume, whereas a gas, like the Vicar of Bray, adapts itself to suit the circumstances in quite a remarkable manner. 49 D