Transition Layer of a Liquid on its Surface Tension. 885 



denotes the density of the liquid and p 2 that of the sur- 

 rounding vapour. The surface-tension is thus approximately 

 given * by 



M-v^-rtgg)'- ■ ■ ■ ^ 



At low temperatures 



2073pi 8 

 m m n 2/3' 



and therefore more generally 



L= 2073^_^ 

 mJISpW v 



Equation (14) may therefore be written 



X,=203-2( Pi -p^^ff . . . . (16) 



'"1 Pc 



Equation (14) may also be applied to mixtures. If two 

 liquids 1 and 2 are mixed in the proportion of n x and n 2 , and 

 no new molecules are formed in the mixture, the value of U 

 is given by 



XJ = Li??i + L 2 ?i 2 ^ 

 % + w 2 



where H denotes the heat of solution per gram of solution, 

 and Lj the internal heat of evaporation of a gram of liquid 1, 

 and L 2 that of a gram of liquid 2. The values of L l9 L 2 

 and H x can usually be obtained and hence the value of X 2 

 calculated. 



The relative concentration of the ingredients in the transi- 

 tion layer of a mixture of substances is not the same as in 

 the interior of the mixture. This property of a mixture is 

 known as adsorption. Its thermodynamical treatment was 

 first given by Willard Gibbs. It follows therefore that 

 (\ 2 — Xj) represents the external work done during the for- 

 mation of the transition layer, and in altering the relative 

 proportion of its ingredients. Equation (14) is therefore 

 very useful, since it furnishes information on the nature of 

 the transition layer in addition to that given by Gibbs's 

 investigation. It may therefore help to clear up the im- 

 portant part played by the surfaces of contact in chemical 

 changes. 



London, July 10, 1912. 



* Phil. Mag-. Jan. 1911, pp. 99-101. 

 t Phil. Mag. Oct. 1910, pp. 66(3-067. 



