of the Surface Layers of Liquids. 275 



the Theory of the Ideal Dilute Solution makes no state- 

 ment; and 



IT - * (26) 



where P is the vapour-pressure of the pure solvent. 

 Substituting the values of p , p u and dp given by equations 

 (25) and (26) in equation (23) we obtain the result 



-W*,). • • • • («> 



R0 \d Pl 



which differs from the well-known result deduced by 

 Milner * only in respect to the more precise definition of the 

 " surface excess." We thus see that the equation applies to 

 the case of a dilute solution of a volatile substance in contact 

 with the vapour phase. 



We will consider the question of liquid mixtures in 

 Part II., when we deal with the case of a solution in contact 

 with a third substance. Practically all the experimental 

 data available for illustrating the theory relate to this latter 

 case, which is a special case of a ternary system. 



Case IV. Liquid Phase in contact with Vapour Phase (only 

 one component volatile) . 



If Ci is involatile equation (22) becomes 

 •n _ PiPo f dr 



J- un\ — ' I -^— 



Po 



-(a < 28) 



If the solvent vapour behaves as an ideal gas of molecular 

 weight m , we have 



_m oPl p/dT\ 



Ll(Q) -~nr\d P )a 



(29) 



In the case of an ideal dilute solution equation (27) is 

 obviously valid. 



The University of Leeds. 

 Feb. 3rd, 1916. 



* Loc, cit. 



U2 



