Chemical Attraction hetween Atoms from Physical Data. 97 



from the density p l to p 2 , the energy L expended in over- 

 coming the molecular forces is given * by 



m**-m)-®"*-Q : T ?)}- 



where x and x h are the distances of separation of the 

 molecules before and after expansion. The inversion of 

 the Joule-Thomson effect occurs when 



(£)"*<: \)<i l> 



pj 



pf~ 



Pl Pc 

 ' Pl' n Pc" ' 



T' _ T" 



m , — m n 



t 



?■>■ 



L = or (^ 



Therefore if T' and T" are the temperatures of inversion of 

 two different gases, when the experiments are carried out so 

 that the conditions 



are satisfied, then 



from the above equation, or the temperatures of inversion are 

 proportional to the critical temperatures. These conditions 

 have probably been approximately satisfied in the experi- 

 ments on the subject, since gases with a low critical 

 temperature were found to have a low temperature of in- 

 version. A systematic investigation of the Joule-Thomson 

 effect of a gas at different initial densities would furnish some 

 useful information by means of which the exact form of the 

 arbitrary function in the law of attraction might possibly be 

 determined. 



Particular Cases of the General Law of Attraction 



^ 2 fe' f) 



We have seen that it is possible to find an infinite number 

 of formulae for the latent heat and surface tension, each of 

 which corresponds to a definite law of molecular attraction, 

 but none of these laws can be taken to represent the law 

 actually existing without further evidence. All these laws 

 must be particular cases of the general law given above, 

 obtained by giving the arbitrary function in the law definite 



* Phil. Mag. May 1910, p. 794. 

 Phil Mag. S. G. Vol. 21. No. 121. Jan. 19.11. H 



