426 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXV11I. 



Strong acids such as nitric acid ; picric acid ; acetic acid and 

 potassium ferrocyanide ; acetic acid and excess of a neutral salt such 

 as sodium sulphate, when these are boiled with the protein solution ; 

 salts of the heavy metals such as copper sulphate, mercuric chloride, 

 lead acetate, silver nitrate, etc. ; tannin ; alcohol ; saturation with 

 certain neutral salts such as ammonium sulphate. 



It is necessary that the words coagulation and precipitation should 

 in connection with proteins be carefully distinguished. The term 

 coagulation is used when an insoluble protein (coagulated protein) is 

 formed from a soluble one. This may occur : 



1. When a protein is heated heat coagulation ; 



2. Under the influence of an enzyme; for instance, when a 

 curd is formed in milk by rennet or a clot in shed blood by the fibrin 

 ferment enzyme coagulation ; 



3. When an insoluble precipitate is produced by the addition of 

 certain reagents (nitric acid, picric acid, tannin, etc.). 



There are, however, other precipitants of proteins in which the 

 precipitate formed is readily soluble in suitable reagents such as saline 

 solutions, and the protein continues to show its typical reactions. 

 This is not coagulation. Such a precipitate is produced by satura- 

 tion with ammonium sulphate. Certain proteins, called globulins, 

 are more readily precipitated by such means than others. Thus, 

 globulins are precipitated by half - saturation with ammonium 

 sulphate. Full saturation with ammonium sulphate precipitates all 

 proteins but peptone. The globulins are precipitated by certain 

 salts, such as sodium chloride and magnesium sulphate, which do 

 not precipitate the albumins. This method of precipitation is called 

 "salting out." 



The precipitation produced by alcohol is peculiar in that after a 

 time it becomes a coagulation. Protein freshly precipitated by 

 alcohol is readily soluble in water or saline media ; but after it has 

 been allowed to stand some time under alcohol it becomes more and 

 more insoluble. Albumins and globulins are most readily rendered 

 insoluble by this method ; proteoses and peptones are never rendered 

 insoluble by the action of alcohol. This fact is of value in the 

 separation of these proteins from others. 



CLASSIFICATION OF PKOTEINS. 



The knowledge of the chemistry of the proteins, which is slowly 

 progressing under Emil Fischer's leadership, will, no doubt, in time 

 enable us to give a classification of these substances on a strictly 

 chemical basis. The following classification must therefore be 

 regarded as a provisional one, which, while it retains the old 

 familiar names as far as possible, yet attempts also to incorporate 

 some of the new ideas. 



