CHAPTER XIV 

 THE STABILITY OF PROTEIN SOLUTIONS 1 



A. THE STABILITY OF WATERY AND AQUEOUS SOLUTIONS OF 



GELATIN 



1. It is difficult to discuss the problem of the stability of 

 colloidal solutions satisfactorily as long as we do not possess a 

 complete theory of the solution of crystalloids. In a general way 

 we can say that there seem to exist two different kinds of forces 

 by which substances can be kept in solution, first, the general 

 forces active in all solutions and which are supposed to be the 

 forces of attraction between solvent and solute; and second, the 

 special forces such as the mutual repulsion of the particles due to 

 electrical charges. These latter special forces are supposed to 

 become of significance only when the general forces of attraction 

 between solute and solvent are comparatively feeble. 



It was noticed long ago that colloids in general and proteins in 

 particular behave very differently in regard to the concentration 

 of salt required for precipitation, some requiring very high con- 

 centrations of salt for this purpose and others comparatively low 

 concentrations. 2 There is apparently no transition between the 

 two extremes. It was formerly believed that these differences 

 in the concentration of salts required could be used for the classi- 

 fication of colloids, and some authors divide the proteins or 

 colloids in general into two groups, those which exist in the form 

 of suspensions ("suspensoids," "lyophobic" or " hydrophobic " 

 colloids), and those which exist in the form of solutions ("emul- 

 soids," "lyophilic" or " hydrophilic " colloids). The former are 

 precipitated by low concentrations, the latter only by high con- 

 centrations of salt. It is of more interest to know the reason 

 why the precipitation of one type requires high and of the other 



1 LoEB, J., and LOEB, R. F., J. Gen. PhysioL, vol. 4, p. 187, 1921-22. 

 2 HARDY, W. B., J. PhysioL, vol. 33, p. 251, 1905-06. 



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