376 



Mr. A. Frumkin on the 



Lewis's * investigations have shown that lowering of 

 surface-tension at a liquid/liquid interface is really accom- 

 panied by adsorption, but the observed value oil T was 

 nearly always much greater than the calculated one. Only 

 non-electrohytes of a small molecular weight like caffeine 

 gave satisfactory results. Lewis supposed that the observed 

 discrepancy is due to gelatinization and to electrical effects. 

 Unfortunately, the accuracy of Lewis's method was very 

 limited ; further experimental investigations would be of 

 great interest. 



The reasoning of Gibbs neglects the electric charge of the 

 dividing surface. Goiiy f showed that an analogous equation 

 may be deduced for charged surfaces, if the potential 

 difference between the two phases remains constant when 

 the concentration of the active substance is varied. Gouy 

 deduced therefrom that the electrocapillary curve of a 

 solution becomes larger with increasing dilution by a constant 

 quantity, independent of the value of 7. We will show 

 that, using Gibbs's equation, it is possible to calculate the 

 horizontal distance between the ascending and descending 

 branches of electrocapillary curves at different concentrations, 

 and that the calculated values are in fair agreement with the 

 observed ones. We must first consider w hat form Gibbs's 



Fig. 1. 



\J 



equation will have if we take separate account of the absorp- 

 tion of the anion and the cation. For this purpose let us 

 use an arrangement already employed by Chapman % (fig. 1). 

 A mercury drop and two electrodes from Na and 01 are 



* Phil. Mag. (6) xv. p. 499 (1908) : xvii. p. 466 (1909) ; Zeit. phys. 

 Chem. lxxiii. p. 129 (1910). 

 t Journ. de Phys. (3) x. p. 245 (1901). 

 % Phil. Mag. (6) xxv. p. 475 (1913). 



