158 COMPOUNDS OF THE PROTEINS 



THE PRECIPITATION AND COAGULATION OF PROTEINS 

 BY INORGANIC SALTS. 



The precipitation of proteins, and, indeed, of Colloids in general, 

 may be of two kinds : The first is clearly accompanied by decomposi- 

 tion of the precipitating agent, it will not occur unless the protein is 

 ionized, i. e., migrates under the influence of an electric current; and 

 only 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-ionized, and requires relatively 

 large amounts of the precipitating-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 neverthe- 

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

 alcohol. For this latter type of precipitation we shall henceforth 

 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 addi- 

 tion of salt to the originally salt-free solution which contains ionic 

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

 electric field, first brings about precipitation and then resolution of the 

 protein. In this new solution the protein appears to be invariably 

 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 under- 

 standing of this phenomenon we shall doubtless have to wait until 

 the physicochemical theory of phenomena of solution in general has 

 reached a more mature stage of development than it has at present. 

 There would appear to be no room for doubt, however, that processes 

 of Dehydration play an important and perhaps a decisive part in bringing 

 about coagulation. 



The concentrations of the various inorganic salts which are required 

 to bring about the precipitation of a colloid in solution very greatly 

 depend upon the Electrical Sign of the inorganic ions with which it may 

 chance to be combined, and upon the Valencies of the ions of the salts 

 used for precipitating the colloid. Thus, defining the " Precipitating- 

 power" of a salt as the reciprocal of the concentration, in gram-molecules 

 per liter, necessary to precipitate a given solution of colloidal Sulphide 

 of Arsenic, Schultz found that the relative precipitating-powers of the 

 univalent, divalent and trivalent metals are in the ratios 1 : 30 : 1650. 

 Similar figures were obtained for colloidal Cadmium Sulphide, while 

 Linder and Picton, using colloidal Antimony Sulphide, found that the 

 precipitating-powers of different salts of a given metal are proportional 

 to their equivalent conductivities, i. e., to the concentration of metal 



