302 Vapour Pressures of Binary Liquid Mixtures. 



very close connexion between the latent heat of vaporization 

 o£ a liquid and its surface-tension, liquids having a high 

 latent heat, for instance, also having a high surface-tension *. 

 Abnormally high vapour pressures are therefore to be asso- 

 ciated also with abnormally low surface-tensions, and vice 

 versa. F. P. Worley has already advanced this view on 

 experimental grounds f. Corresponding to the three cases 

 just given for the differences in latent heats, this investigator 

 has given the three following rules for the connexion between 

 the vapour pressures of binary mixtures and their surface- 

 tensions : — 



" (i.) If at any given temperature the vapour pressures of 

 mixtures of two liquids agree with the values calculated by 

 the rule of admixture in molecular proportions, the surf ace- 

 tensions of the mixtures agree with those calculated by the 

 formula 



s^ViSi+vA*. 



" (ii.) If the vapour pressures are greater than those 

 calculated, then the surface-tensions are less than those 

 calculated. 



*' (iii.) If the vapour pressures are less than those calcu- 

 lated, the surface-tensions are greater than those calculated/' 



By comparing his own and other surface-tension curves 

 with the vapour-pressure curves of Zawidski and others, 

 Worley has shown that every pair of liquids so far investi- 

 gated comes under the rules laid down. Mixtures of benzene 

 and' ethylene dichloride, for instance, form perfect mixtures 

 and obey the first rule; carbon bisulphide and acetone, 

 benzene and carbon tetrachloride, ether and carbon bisul- 

 phide obey the second rule ; whilst in the same way, water 

 and the alcohols, and pyridine and acetic acid obey the 

 third. 



* So much can be deduced from the Laplace Theory of Capillarity. 

 Also Walden has given the following empirical formula for this 

 relationship, based on a large number of experimental data : 



L = 3'64?;y, 



where L= latent heat per gm. in calories ; v = sp. vol. in c.c. ; y= surface- 

 tension in dynes/cm. 



f Journ. Chem. Soc. Trans, cv. [1] p. 273(1914). 



X Vi and V 2 are the volumes of the two liquids expressed fractionally. 



