THE ETIIER AXD PONDERABLE MATTER 



due to Daniel Bournelli, who advanced it early in the 

 eighteenth century. The idea, then little noticed, had 

 been revived about a century later by William Hera- 

 path, and again with some success by J. J. "Waterston, 

 of Bombay, about 18i6; but it gained no distinct foot- 

 ing until taken in hand by Clausius in 1857 and by 

 Maxwell in 1859. 



The investigations of these great physicists not only 

 served fully to substantiate the doctrine, but threw a 

 flood of light upon the entire subject of molecular dy- 

 namics. Soon the physicists came to feel as certain of 

 the existence of these showers of flying molecules mak- 

 ing up a gas as if they could actually see and watch 

 their individual actions. Through study of the viscosity 

 of gases that is to say, of the degree of frictional oppo- 

 sition they show to an object moving through them or 

 to another current of gas an idea was gained, with the 

 aid of mathematics, of the rate of speed at which the 

 particles of the gas are moving, and the number of col- 

 lisions which each particle must experience in a given 

 time, and of the length of the average free path trav- 

 ersed by the molecule between collisions. These meas- 

 urements were confirmed by study of the rate of diffusion 

 at which different gases mix together, and also by the 

 rate of diffusion of heat through a gas, both these phe- 

 nomena being chiefly due to the helter-skelter flight of 

 the molecules. 



It is sufficiently astonishing to be told that such 

 measurements as these have been made at all, but the 

 astonishment grows when one hears the results. It ap- 

 pears from Maxwell's calculations that the mean free 

 path, or distance traversed by the molecules between 

 collisions in ordinar}^ air, is about one half-millionth of 



243 



