CII. XXV.] CLASSIFICATION OF PROTEIDS 399 



Such precipitation is not coagulation. Such a precipitate is produced 

 by saturation with ammonium sulphate. Certain proteids, called 

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

 Thus, serum globulin is precipitated by half-saturation with ammonium 

 sulphate. Full saturation with ammonium sulphate precipitates all 

 proteids but peptone. The globulins are precipitated by certain salts, 

 like sodium chloride and magnesium sulphate, which do not precipitate 

 the albumins. 



The precipitation produced by alcohol is peculiar in that after a 

 time it becomes a coagulation. Proteid freshly precipitated by 

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

 been allowed to stand some weeks 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 proteids from others. 



Classification of Proteids. 



Both animal and vegetable proteids can be divided into the follow- 

 ing classes. We shall, however, be chiefly concerned with the animal 

 proteids : 



If we use the term proteid in the widest sense, the first main 

 subdivision of these substances is into 



A. The Simple Proteids. 



B. The Conjugated or Compound Proteids. 



C. The Albuminoids. 



D. The Protamines. 



We will take these classes one by one. 



A. THE SIMPLE PKOTEIDS. 



Class I. Albumins. These are soluble in water, in dilute saline 

 solutions, and in saturated solutions of sodium chloride and magnesium 

 sulphate. They are, however, precipitated by saturating their solu- 

 tions with ammonium sulphate. Their solutions are coagulated by 

 heat, usually at 70-73 C. Serum albumin, egg albumin, and lact- 

 albumin are instances. 



Class II. Globulins. These are insoluble in water, soluble in 

 dilute saline solutions, and insoluble in concentrated solutions of 

 neutral salts like sodium chloride, magnesium sulphate, and ammonium 

 sulphate. A globulin dissolved in a dilute saline solution may there- 

 fore be precipitated 



1. By removing the salt by dialysis (see p. 396). 



2. By increasing the amount of salt. The best salts to employ are 



