Chemical Attraction between Atoms from Physical Data. 101 



Table V. 





\ 



(2 s/m^f 32-96 



Cpi-Pa) 4 ' 



Methyl formate 



2753 



26-93 



333 



41-76 



68-77 



Carbon tetrachloride 



399 

 465 

 62-92 



Ether 





expression ap 2 for the intrinsic pressure of a liquid, where 

 a is a constant which depends only on the nature of the 

 liquid. From the above equation we see that van der Waals' 

 constant a — which he determines by means of his equation 



1/3 



of state— is equal to F-£^ (2^/^)2. This is roughly the 



case. But the above law of attraction cannot be exactly 

 true since the attraction must be a function of the tem- 

 perature ; moreover it gives a formula for the latent heat 

 which does not agree with the facts *. However, it is 

 probably a nearer approximation to the actual facts than 

 most of the other definite laws that have been proposed. 



At the critical state of a liquid the average kinetic energy 

 of a molecule must be approximately equal to its latent heat 

 of evaporation into a vacuum. If this were not so, then only 

 the molecules having a velocity above a certain limit could 

 escape from the liquid, and it could be in equilibrium with 

 vapour of a different density than the liquid. The internal 

 heat of evaporation of a molecule from a liquid in the critical 

 state into a vacuum is, according to the general law of 

 molecular attraction, equal to 



A >tef(v^ 



where A } is a constant. This is proportional to the critical 

 temperature, and we therefore have 



Tc-H'gJaVm,) 2 , 



where H 2 is a constant. This equation the writer has already 

 * Ibid. p. 677. 



