108 CHEMICAL STATICS 



ing to Hardy, non-ionic and it can be coagulated by still further 

 addition of the salt.* 



The first kind of precipitation appears to be undoubtedly 

 chemical in character and in mechanism. The mechanism of 

 coagulation is, however, far from clear, and for the attainment 

 of an adequate understanding of this phenomenon we shall 

 doubtless have to wait until the physico-chemical theory of solu- 

 tions in general has reached a more mature stage of development 

 than it has at present. At least three possibilities exist. 



(i) The coagulation of proteins by salts is a purely physical 

 phenomenon due either to an alteration in the electrical condition 

 of the protein (Bredig, Billitzer, Freundlich) or to a physical 

 alteration in the nature of the solvent. 



(ii) The coagulation of proteins by salts is partly a physical 

 and partly a chemical phenomenon depending upon the forma- 

 tion of various compounds between the protein and the salt, 

 and upon their varying solubilities in salt solutions (Spiro, 

 Galeotti). 



(iii) The coagulation of proteins by salts is indirectly a chemi- 

 cal phenomenon, attributable to a disturbance in the chemical 

 equilibrium between the protein and its solvent (Hofmeister, 

 Pauli). 



In reviewing the various factors which have been ascertained 

 to be of importance in determining the precipitation and coagu- 

 lation of proteins by electrolytes we shall incidentally discuss the 

 applicability of these several hypotheses. 



2. Earlier Investigations on the Significance of the State of 

 Hydration of the Proteins in Relation to their Coagulation by 

 Salts. The fact that proteins can be thrown out of solution by 

 the addition thereto of inorganic salts appears to have first been 

 pointed out by Claude Bernard (2), who employed, among others, 

 magnesium sulphate, sodium sulphate and ammonium carbonate. 

 As early as 1854, Virchow (76) suggested that the inorganic 



* An interesting example of the dependence of the ionization of a protein 

 upon concentration (i.e., available mass of water) of the medium in which it 

 is dissolved is afforded by serum-globulin dissolved in solutions of sodium 

 citrate. In solutions containing low concentrations of sodium citrate the 

 protein is ionized and drifts in an electric field, while the same protein when 

 dissolved in more concentrated sodium citrate solution is found to be no longer 

 ionized (-9). 



