PROTEINS 143 



silver, etc., contained in such precipitates depend upon the concen- 

 trations of the solutions of albumin and of electrolyte, and that 

 precipitates of constant constitution are always obtained under the 

 same conditions. FR. N. SCHULZ and R. ZSIGMONDY showed that 

 crystallized egg albumin which had adsorbed colloidal metallic gold, 

 recrystallized with it. 



As the result of such observations we become very sceptical con- 

 cerning the "purity" of proteins. However, it is just such explana- 

 tion of earlier errors which shows us upon what facts we may really 

 depend, and gives to science a new method and, in part, a new course. 



Before we describe the few proteins which have been studied colloid- 

 chemically, we shall consider briefly some of their general properties. 



One of the most characteristic properties of many proteins is 

 coagulation. It may be brought about, either by a rise of temper- 

 ature (heat coagulation) or by chemical means. 



Most of the coagulations due to the salts of the light metals and 

 some of those due to the alkaline earths are reversible, i.e., the 

 coagulations reverse themselves by the addition of more water. Heat 

 coagulation and coagulation due to many of the salts of the heavy metals 

 are irreversible. The coagulations due to alcohol, acetone and ether 

 are intermediate, that is, the coagulation produced is at first soluble 

 in water but becomes insoluble after a while. Globulin which has 

 been preserved for a time in pure water behaves in a similar way, 

 for it then becomes less soluble in salt solutions. 



Though reversible coagulation may be viewed as a purely physical 

 salting out (see under this heading) a chemical change must be assumed 

 in the cases of irreversible coagulation. Many heavy metals form 

 insoluble complexes with albumin (see p. 157) - 1 Irreversible coagu- 

 lation by heat, alcohol, etc., may be explained, possibly, by a chemical 

 transformation. The fact that the H ion concentration diminishes 

 after heat coagulation is in favor of this view (SORENSEN and JURG- 

 SEN,* H. CHICK and C. J. MARTIN,* GUAGLIERIELLO*) . In the case 

 of heat coagulation, water appears to enter the albumin molecule, 

 because absolutely dry hemoglobin and egg albumin may be heated 

 to 120 C. without losing their solubility in water (H. CHICK and 

 C. J. MARTIN*). Possibly this is the initial stage of hydrolysis, since 

 according to BERCZELLER* the surface tension of salt-poor albumin 

 solutions is temporarily depressed upon boiling, just as occurs upon 

 hydrolysis by pepsin, trypsin, etc. Irreversibly coagulated albumi- 

 nous pellicles may be formed merely by shaking with air (see p. 34). 



1 [SANSUM has shown that after the absorption of a lethal dose of 4 mg. per kilo 

 of mercuric chlorid, no treatment avails. Jour. Am. Med. Association, vol. 70, 

 p. 824. Tr.] 



