16 THEORY OF COLLOIDAL BEHAVIOR 



trically with other compounds, since aggregates were assumed 

 to react only at their surface an assumption which, as already 

 stated, is not warranted in the case of proteins, since protein gels 

 are freely permeable to crystalloids. It led, however, to another 

 equally fatal idea, that this aggregate formation would explain 

 all the colloidal phenomena. Thus when R. S. Lillie 1 made the 

 important observation that neutral salts depress the osmotic 

 pressure of gelatin solutions, it seemed natural to explain this 

 fact from the precipitating action of salts, by assuming that the 

 addition of salt caused an aggregation of gelatin molecules or ions 

 into larger aggregates. This would lead to a diminution of the 

 number of particles in solution. But it was also found that the 

 addition of salts depresses the viscosity of protein solutions and 

 the swelling of solid proteins. We shall see later that the 

 formation of aggregates out of isolated protein molecules or ions 

 increases the viscosity of a gelatin solution. 2 Hence, if the addi- 

 tion of a salt to a protein solution diminishes its osmotic pressure 

 by causing an increased formation of aggregates the same addi- 

 tion of salt should increase the viscosity of such a solution. The 

 reverse, however, happens, the viscosity of the solution being 

 decreased by the addition of salt. 



There is nevertheless a connection between the phenomena of 

 precipitation and the depressing effect of salts on viscosity, 

 osmotic pressure, and swelling of proteins. Schulze, Linder and 

 Picton, and Hardy had observed that in the precipitation of 

 colloids that ion is active which has the opposite sign of charge 

 from the protein particle, and that the efficiency of the active 

 ion increased with its valency. The same rule applies to the 

 depressing action of salts on the osmotic pressure, viscosity, and 

 swelling of proteins. By trying to explain these latter effects 

 from the precipitating action of salts the colloid chemists put the 

 cart before the horse, and were led into a hopeless contradiction 

 wfth the facts. We shall see that by taking the reverse step, 

 namely, of explaining the precipitating action of salts from their 

 depressing action on osmotic pressure and P.D. of protein solu- 

 tions, everything becomes clear and consistent. But this step 

 could not be taken as long as the belief in the adsorption theory 



MILLIE, R. S., Am. J. Physiol., vol. 20, p. 127, 1907-08. 

 2 LoEB, J., J. Gen. Physiol., vol. 4, p. 97, 1921-22. 



