of Binary Liquid Mixtures. 301 



mixture is equal to the theoretical value (dL = 0), the 

 vapour-pressure curve is a straight line (Diag. I.). 



Diagram 'I, 



Vap 



press 



SL 



= 



,^-'^' 



^f^ 





poi 



X 



1 — > 



;«.- c «) 



Diagram II. 



SL negative 





^tD- 



Molar fractions of two components. 



(ii.) I£ the molecular latent heat of vaporization is less 

 than the theoretical (i.e. dL is negative) the vapour pressure 

 is greater than the theoretical (Diag. II.). 



(iii.) If the molecular heat of vaporization is greater than 

 the theoretical (i. e. ^L is positive) the vapour pressure is less 

 than the theoretical (Diag. III.). 



Diagram III. 

 9L positive 



Diagram IV, 



If, as the relative molecular concentration of the two 

 components is varied, the value of BL changes from 

 positive to negative or vice versa, the curve will cross over 

 the diagonal (as in Diag. IV.). In this case ~dh is at first 

 positive, and then changes over to negative. 



As yet there have been no measurements made by which 

 the above equation [17] can be subjected to a quantitative 

 test. But its general soundness can be shown indirectly 

 from surface-tension data. It is well known that there is a 



