MOLECULAR ASSOCIATION OF LIQUIDS 77 

 According to p. 28 the expression for Mathias' rule is 



where D L and D v are the densities of the liquid and vapour 

 respectively. If the latter is negligible by comparison with 

 the former, as is the case at some distance from the critical 

 temperatures, at which also the Eotvos-Ramsay rule first 

 becomes applicable, then 



D L = a-bT. 



From this it appears, according to p. 65, in conjunction 

 with the rule of Deprez-Trouton for corresponding states, 

 that we get the simple result 



Ai 



= const., 



i. e. the diminution of the latent heat with temperature, 

 when not too near the critical point, has a constant value 

 for those substances in which a formation and decomposi- 

 tion of double molecules do not occur. 



But the latent heat of evaporation is associated with the 

 surface energy 1 , since if we neglect the influence of the 

 vapour, a molecule, in passing from the interior of the 

 liquid to the surface escapes from one-half of the internal 

 attraction, while on evaporation it escapes altogether. If 

 L is the internal latent heat of evaporation, for which the 

 law of Deprez and Trouton holds, 



yM* = \L, 

 so that 



Ay if/ 

 A y = const. 



in accordance with the law of Eotvos and Ramsay. 



6. OPTICAL RELATIONS. 



We have restricted ourselves, in dealing with the rela- 

 tions between physical properties and constitution, to what 

 can be approximately brought under a single point of view 



1 Stefan, Wied. Ann. 29. 655. 



