188 SOLUTION AND PRECIPITATION OF PROTEINS 



It was, secondly, found that proteins combine with acids and 

 bases according to the stoichiometrical laws of classical chemistry, 

 and that there is neither any need nor, in fact, any room for the 

 assumption that the ions of electrolytes are adsorbed by the protein 

 particles in solution.^ Thirdly, the electrical charges of protein parti- 

 cles which occur in such solutions are due chiefly, if not exclusively, 

 to either the electrolytic dissociation of protein salts or to membrane 

 potentials caused by the Donnan equilibrium.'* Fourthly, the in- 

 fluence of electrolytes on the p.d., the osmotic pressure, and the vis- 

 cosity of protein solutions^ and the swelling of gels^ can be explained" 

 quantitatively on the basis of the Donnan equilibrium. Under 

 these circumstances it became necessary to find out whether or not 

 the influence of electrolytes on the precipitation of proteins can be 

 harmonized with these recent results. 



All the workers who have studied the influence of electrolytes on 

 the precipitation of colloids have come to the conclusion that there 

 are two distinct groups of phenomena. In the one group the precipi- 

 tation requires high concentrations of an electrolyte, while in the 

 second, a low concentration of electrolyte suffices for precipitation. 

 The difference between these two cases is so striking that it has been 

 used for a classification of colloids. Thus, according to Zsigmondy, 

 Noyes^ discriminates between "colloidal solutions" and "colloidal 

 suspensions;" the "colloidal solutions" being characterized by a high 

 degree of viscosity, by a tendency to gelatinize, and by the fact that 

 they are not easily precipi table by electrolytes; while the "colloidal 

 suspensions" are, according to Noyes, characterized by a low order 

 of viscosity, by a lack of tendency to gelatinize, and by the fact that 

 they are easily precipitated by electrolytes.^ 



^Loeb, J., Science, 1920, Hi, 449. 



" Loeb, J., J. Gen. Physiol., 1920-21, iii, 667. 



5 Loeb, J., J. Gen. Physiol., 1920-21, iii, 557, 667, 691, 827; 1921-22, iv, 73, 97. 



« Procter, H. R., /. Chem. Soc, 1914, cv, 313. Procter, H. R., and Wilson, 

 J. A., J. Chem. Soc, 1916, cix, 307. 



^ Noyes, A. A., quoted in Zsigmondy, R., KoUoidchemie, Leipsic, 2nd ed., 

 1918, 29. 



^ DifTerent authors have introduced different terms for these two groups of 

 colloids, Perrin speaks of hydrophilic and hydrophobic colloids; Freundlich of 

 lyophilic and lyophobic, and Wo. Ostwald of emulsoids and suspensoids. 



