436 THE POPULAR SCIENCE MONTHLY 



minute particles suspended in liquids, and liquids are very much lesa 

 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 ex- 

 hausted if desired. This apparatus is shown in the photograph on p. 426. 

 When the air was at atmospheric pressure, the smallest particles pro- 

 duced 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 sur- 

 rounded by a dense swarm of bombarding molecules, it is like a foot- 

 ball 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 parti- 

 cles 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 pre- 

 cisely 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 dimen- 

 sions. 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, 



