594 RICE 



ART. L 



discovered empirically by McLeod (Trans. Faraday Soc, 19, 38, 

 (1923)) which holds accurately for a great number of organic 

 liquids over a wide range of temperature. In it C is a con- 

 stant, different for each liquid, and the relation is of great value 

 in comparing densities. 



As stated, these relations all have an important bearing on 

 molecular complexity in liquids, a problem which still awaits 

 solution. In applying them it is assumed that M changes with 

 temperature since with increasing temperature polymerized 

 molecules tend to dissociate into the simple molecules which 

 exist in vapor, and the assumed truth of the expressions enables 

 relative values of M to be found at each temperature. Although 

 too great reliance cannot be placed on the conclusions deduced, 

 Bennett and Mitchell (Zeit. phijsik. Chem., 84, 475, (1913) and 

 Bennett {Trans. Chem. Soc, 107, 351, (1915)) have shown that 

 the ''total molecular surface energy" 



(-9 



(Mvy 



is a better quantity to use for this purpose than the "free 

 molecular surface energy" 



of Eotvos, and that this leads to more consistent conclusions 

 concerning molecular association. 



35. Heat of Adsorption 



Returning to Gibbs work, the reader will find on pages 271, 

 272 a reference to the "amount of heat necessary to keep the 

 phases from altering while the surface of discontinuity is ex- 

 tended." If dcr/dt is negative, as appears to be the general 

 rule, this heat is positive and if not supplied the temperature of 

 the surface will fall, causing an increase of tension. Actually, if 

 da/dt were positive, an increase of tension would also occur since 

 in this case the heat would be negative, so that if transfer of 

 heat were prevented the surface would warm up. Now this 

 heat must be carefully distinguished from "heat of adsorption," 



