﻿Attraction of Unlike Molecules. 13 



diffusion have hitherto found fair agreement between the 

 results of experiment at ordinary temperatures and the kinetic 

 theory of forceless perfectly restitutional spherical molecules. 

 The immediate effect of the result on our present inquiry is 

 to render illusory the hope of obtaining values of ^2 for the 

 various pairs of gases for which Loschmidt and v. Obermayer 

 have found values of the diffusion-coefficient at only one 

 temperature near 284, for as regards these values we have 

 just seen that the molecules behave almost as if forceless. 

 It may be suggested that the failure of B to prove constant 

 is due to inadequacy of Stefan's theory of diffusion, but 

 the expressions for the diffusion-coefficient given by Meyer 

 and Tait gave on trial about the same results as Stefan's ; so 

 that the failure of B to prove constant is not due to any 

 peculiarity of Stefan's theory. We have to go deeper for the 

 reason, and in doing so have to open up a very important 

 department of molecular dynamics of which at present we know 

 but little, namely, the nature of collisions between molecules. 

 Hitherto in the kinetic theory it has been assumed that the 

 forces called into play during the collision of two molecules 

 are such as they w r ould be if the molecules were perfectly 

 restitutional spheres, and the assumption seems to have 

 worked well as regards the general phenomena of gases ; but 

 in reality it was not required there, and could be replaced by 

 the assumption that the translatory kinetic energy of a number 

 of molecules is a constant fraction of their total kinetic energy 

 The usual assumption of perfect restitutionality causes no 

 difficulty in connexion with the theory of the viscosity of a 

 single gas, because the nature of the collisional forces be- 

 tween molecules is not directly involved in that theory ; but 

 in the theory of diffusion, as well as in that of the character- 

 istic equation of the element gases, the forces involved in 

 collision enter as an essential element of the calculation. 

 Now in the paper on the Viscosity of Gases and Molecular 

 Force, in connexion with the theory of the characteristic 

 equation of the element gases, just such a discrepancy as we 

 have encountered in diffusion cropped up between the 

 behaviour of actual gases and the theory of a medium com- 

 posed of attracting perfectly restitutional spheres ; and it was 

 pointed out that in some way, which at present must be called 

 accidental, the departure from perfect restitutionality in the 

 collisions compensated for a certain effect of molecular attrac- 

 tion in such a way as to make the molecules behave in one 

 respect as if they were forceless. It seems desirable, there- 

 fore, to bring out clearly the parallelism of the two cases. 

 In the theoretical characteristic equation of a medium 



