CHAPTER VI 

 THE COMPOUNDS OF THE PROTEINS (Continued) 



1. General Remarks on the Precipitation of Proteins by 

 Inorganic Salts. It was pointed out by Hardy (24) in his 

 exhaustive communication on globulin that the precipitation 

 of proteins and, indeed, of colloids in general may be of two 

 kinds. The first is clearly accompanied by decomposition of 

 the precipitating agent; it will not occur, as Pauli has demon- 

 strated (56) unless the protein is in some proportion ionized, 

 and relatively small quantities of the precipitating agent are 

 required to bring about the precipitation. The second kind of 

 precipitation, however, whether accompanied by decomposition 

 of the precipitating agent or not, occurs even when the protein 

 is non-ionic and requires relatively large amounts of the pre- 

 cipitating agent. Precipitation of the first kind is, generally 

 speaking, only brought about by electrolytes, while precipitation 

 of the second kind, although as a rule, more readily brought 

 about by electrolytes than by non-electrolytes, may nevertheless 

 be brought about by certain non-electrolytes, for example, by 

 alcohol. 



For this latter type of precipitation we shall henceforth, when- 

 ever possible, reserve the term coagulation.* Both precipitation 

 and coagulation of a protein may be brought about by one and 

 the same inorganic salt. In such a case the gradual addition of 

 salt to the originally salt-free solution which contains ionic pro- 

 tein, i.e., protein which drifts to one electrode or to the other in 

 an electric field, first brings about precipitation and then reso- 

 lution of the protein. In this new solution the protein is, accord- 



* Much confusion exists in the literature on this subject on account of the 

 fact that the distinction between the precipitation of a protein through chemical 

 interaction with the added salt, and its coagulation through the change in the 

 nature of the solvent resulting from the further addition of salt has not invari- 

 ably been recognized. 



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