Page:Popular Science Monthly Volume 80.djvu/440

436 minute particles suspended in liquids, and liquids are very much less suited to any convincing and accurate test of the kinetic hypothesis than are gases. Apparently the very great advantages of observing minute suspended particles in a gas at very low pressures, where the motions ought to be enormously increased, had not been appreciated, or, at least, had not been utilized, perhaps because the means had not before been at hand for keeping such particles in suspension. Accordingly, the plates M and N, shown in Fig. 1, with the atomizer attached, were placed inside a large brass cylinder, which could be sealed air tight and exhausted if desired. This apparatus is shown in the photograph on p. 426. When the air was at atmospheric pressure, the smallest particles produced by the aspirator showed clearly the incessant wiggling motions which are called, after their discoverer in liquids, the "Brownian movements." But, when the pressure was reduced to seven or eight millimeters of mercury (about 1/100 of an atmosphere), these motions had increased so enormously in violence that it was difficult to follow the smallest particles as they dashed hither and thither like wrigglers in a water barrel. The reason that reducing the pressure brings out the motion so much more clearly is obviously this; When the oil drop is surrounded by a dense swarm of bombarding molecules, it is like a football in a melee of densely packed players who are kicking it on all sides at once, but are unable to send it any appreciable distances. But when it gets out into the open, where the players are scarce, it begins its spectacular flights. Precisely so with the oil drops, and no football game was ever more spectacular or more fascinating than the behavior of one of these oil drops at low pressures. The fact that the motions increase in violence the rarer the gas becomes and the smaller the particles are taken (size being indicated by the speed with which a given particle settles under gravity) is obviously just what ought to happen. There can not then be the slightest doubt that what these oil drops are doing, namely, dancing about violently in all sorts of directions, is precisely what the molecules themselves are doing in a much more excited way for it would be absurd to suppose that the increased speed and the increased distance of the motions as size and mass diminish do not go on after the particles cease to be visible and shrink to molecular dimensions. From the standpoint of a molecule which is darting hither and thither with the speed of a rifle bullet, our dancing oil drops must look like snails crawling about with languorous slowness. But to us they have served their purpose, for they have enabled our minds to see the invisible molecular world doing in a large way just exactly what the oil drops are doing in their small way. They have proved the kinetic theory of matter even to the man on the streets.

But in order literally to pile Ossa upon Pelion in support of this hypothesis, let us next turn to a rigorously quantitative demonstration,