A KINETIC THEORY OF MATTER. 225 



experiments 1 on the thermal effect of gases ex- 

 panding from a high pressure vessel through a 

 porous plug, proves the less dense gas to have 

 greater intrinsic potential energy than the denser 

 gas, if we assume the ordinary hypothesis re- 

 garding the temperature of a gas, according to 

 which two gases are of equal temperatures 2 when 

 the kinetic energies of their constituent molecules 

 are of equal average amounts per molecule. 



Think of the thing thus. Imagine a great 

 multitude of particles enclosed by a boundary 

 which may be pushed inwards in any part all 

 round at pleasure. Now station an engineer corps 



1 Republished in Sir W. Thomson's Mathematical and Physical 

 Papers, Vol. I. Article XLIX. p. 381 ; also, see Joule's Collected 

 Papers, Vol. II. p. 216. 



2 That this is a mere hypothesis has been scarcely remarked by 

 the founders themselves, nor by almost any writer on the kinetic 

 theory of gases. No one has yet examined the question : what is 

 the condition as regards average distribution of kinetic energy, 

 which is ultimately fulfilled by two portions of gaseous matter, 

 separated by a thin elastic septum which absolutely prevents 

 interdiffusion of matter, while it allows interchange of kinetic energy 

 by collisions against itself? Indeed I do not know but that the 

 present is the very first statement which has ever been published 

 of this condition of the problem of equal temperatures between two 

 gaseous masses. 



