THEORIES OF COLLOIDAL BEHAVIOR 113 



of fact the explanations offered in the colloidal literature do not 

 even suffice as qualitative explanations since they are in contra- 

 diction with the facts. 



We have seen in the introduction how the original definition 

 of colloids by Graham, based on the non-diffusion of colloids 

 (through membranes), has of late been abandoned by colloid 

 chemists in favor of the micella or aggregation theory of colloids, 

 according to which the ultimate unit of colloidal matter in 

 solution or suspension is not the isolated molecule or ion, but 

 an aggregate of the latter the micella of Naegeli. Such aggre- 

 gations occur and they play a role in gel formation, precipitation, 

 and to some extent in the viscosity of protein solutions, but they 

 cannot explain the influence of electrolytes on the properties of 

 proteins mentioned, since they have only an indirect connection 

 with colloidal behavior. It will be shown that the aggregates 

 act like membranes blocking the diffusion of the ions constituting 

 the aggregate and this prevention of diffusion is a source of 

 colloidal behavior. 



The depressing effect of the addition of salts to protein solu- 

 tions cannot be harmonized with the aggregation theory. Zsig- 

 mondy suggests that the depressing effect of a neutral salt on the 

 osmotic pressure of a solution of a gelatin salt might find its 

 explanation in the assumption that the addition of salt increases 

 the degree of aggregation and hence diminishes the number 

 of the particles in solution, the diminution in the number of 

 particles leading to the lowering of osmotic pressure. 1 It is 

 undoubtedly true that salts precipitate proteins and that pre- 

 cipitation is due to an increase in aggregation, but the salting 

 out of gelatin from its watery solution is not determined by the 

 ion with the opposite sign of charge to that of the protein ion, 

 while we have seen that the depressing effect of a salt on the 

 osmotic pressure of gelatin solutions is determined by the ion 

 with the opposite sign of charge to that of the protein ion. In 

 other words, the salting out of gelatin from its watery solution 

 is a process of an entirely different character from the lowering 

 of the osmotic pressure of a protein solution by a neutral salt. 

 It is, therefore, impossible to explain the latter process by the 

 former. 



1 ZSIQMONDY, R., "Kolloidchemie," 2nd ed,, p. 342, Leipsic, 1918, 

 8 



