﻿Attraction of Unlike Molecules. 15 



and the difference between the diffusion case and that of the 

 collisional virial may perhaps lie in the fact that the mechanism 

 does not operate in the same manner as regards the mass 

 motion of diffusion and the molecular motion of heat. But 

 the whole question of molecular collision is so large a one 

 that it will require considerable research to itself; from the 

 glimpse we have got into it, it appears that the momentum 

 imparted to a molecule during a collision, instead of being u> 

 as calculated on the assumption that the molecules are smooth 

 perfectly restitutional spheres, is /*//,, where h is a parameter 

 characteristic of the pair of molecules colliding, and which 

 we have found empirically in the case of diffusion to be 

 approximately proportional to 1 4- iC 2 /T. It may be worth 

 while noting a certain regularity in the departure from strict 

 proportionality, or in the departure of 10 3 B/(1 + 1 C 2 /T) from 

 constancy : for the two triatomic molecules C0 2 and N 2 its 

 value is least, namely, *94 ; for the triatomic C0 2 with di- 

 atomic N 2 and H 2 it is 1*11 and 1*12 ; while for the three 

 diatomic pairs 2 with H 2 , N 2 , and CO, it is 1*17, 1*17, and 

 1-16. 



With Loschmidt's and v. Obermayer's diffusion-coefficients 

 for a number of pairs of gases at about 15° C, we can test 

 more extensively our empirical relation that at about that 

 temperature the diffusion-coefficient is proportional to 



(1/M 1 + 1/M 2 )V{(M 1 /3 1 )72 + (M 2 /3 2 )*/2} 2 . 



The data are available for sixteen pairs including the six 

 already considered, and as the experimenters have reduced 

 their results to values at 0° C. by the approximate formula 



D 2 z 3 /D T =(273/T) 2 , 



which is near enough to the truth for small differences 

 between T and 273, we will take the values D 273 as suitable for 

 our present purpose. The additional values required for 

 (M/^)/2are:— 



CH,. C 2 H 4 . C 2 H 6 . SO„. 



P 1'59 1-53 1-4 (near) '55 



M 16 28 30 64 



(M/3)V2... 1-47 1-75 1-74 1-63 



With these and the values already given, and the values of D 

 reproduced from v. Obermayer and Loschmidt, the values of 



(1/Mj + l/M 2 )V{(M 1 ft)72 4- (M 2 /3 2 )'/2} 2 D 



