ETHER AND PONDERABLE MATTER 



spherical, and elastic, we may, if we please, say the 

 particles are centres of force, of which the action is 

 insensible except at a certain very small distance, 

 when it suddenly appears as a repulsive force of very 

 great intensity. It is evident that either assumption 

 will lead to the same results. For the sake of avoiding 

 the repetition of a long phrase about these repulsive 

 bodies, I shall proceed upon the assumption of per- 

 fectly elastic spherical bodies. If we suppose those 

 aggregate molecules which move together to have a 

 bounding surface which is not spherical, then the 

 rotatory motion of the system will close up a certain 

 proportion of the whole vis viva, as has been shown by 

 Clausius, and in this way we may account for the value 

 of the specific heat being greater than on the more 

 simple hypothesis." l 



The elaborate investigations of Clerk-Maxwell served 

 not merely 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 

 a that is to say, of the degree of f fictional oppo- 

 sition they show to an object moving through them or 

 : i other current of gas an idea was gained, with the 

 f mathematics, of the rate of speed at which the 

 the gas are moving, and the number of col- 

 08 which e;ich particle must experience in a given 

 time, and of the length of the average free path t 

 ersed by the molecule between collisions, These n 



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