SURFACES OF DISCONTINUITY 559 



solution in water was greater than in the interior. Zawidski 

 (Zeit. physik. Chem., 35, 77, (1900) and 42, 612, (1903)) pre- 

 pared saponin foams and showed by means of measurements of 

 the refractive index that the saponin content in the foam was 

 higher than in the original solution. Analogous qualitative 

 information was obtained by Ramsden (Zeit. physik. Chem., 

 47, 336, (1904)) on the accumulation and consequent precipita- 

 tion of protein at surfaces. C. Benson (J. Phys. Chem., 7, 532, 

 (1903)) examined foams from aqueous solutions of amyl alcohol 

 and also observed excess concentration of the alcohol in the 

 foam. An important investigation was made by S. R. Milner 

 (Phil. Mag., 13, 96, (1907)) on solutions of acetic acid and 

 sodium oleate. He used the Gibbs equation in its simple form 

 to calculate the surface excess in the first case and brought out 

 the important fact that the surface excess for a normal solution 

 of acetic acid is only about 15 per cent less than what it is for 

 a solution eight times as concentrated. In the case of sodium 

 oleate, its high capillary activity causes the surface tension to 

 fall so rapidly that the (<r, c) curve quickly becomes nearly 

 parallel to the c-axis, and only very doubtful values of r could 

 be obtained. A rough experimental method gave as the surface 

 excess 0.4 mgm. per square meter, which Milner regarded as a 

 "moderately good estimate" for it at the moment of formation 

 of the bubbles of air which were passed through the oleate solu- 

 tion; but he was of the opinion that this was "very much less 

 than the ultimate value of the excess." He concluded that 

 there was an irreversible process here which actually caused the 

 solute to come out of solution in the surface in consequence 

 of excessive adsorption. As we have pointed out above, if 

 such is the case the theoretical conditions for an application of 

 Gibbs' equation do not hold under these circumstances. 



Actually the first attempts at a quantitative verification of the 

 equation were made by W. C. M. Lewis at the suggestion of 

 Donnan (Phil. Mag., 15, 499, (1908) and 17, 466, (1909)). In 

 one set of experiments an oil-water interface was used and 

 solutes were chosen so as to be insoluble in the oil phase and 

 very capillary-active in the aqueous phase. Sodium glyco- 

 cholate, however, yielded results for the direct measurement of 



