CH. xxv.] THE PROTBIDS. 397 



Classification of Proteids. 



Both animal and vegetable proteids can be divided into the 

 following 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 PROTEIDS. 



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 satu- 

 rating their solutions 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 therefore be precipitated 



1. By removing the salt by dialysis (see p. 394). 



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

 are ammonium sulphate (half-saturation) or magnesium sulphate 

 (complete saturation). This method is often called "salting out." 



The globulins are coagulated by heat ; the temperature of heat 

 coagulation varies considerably. The following are instances : 



(a) Fibrinogen I . 



;.( ,. iii-/ i u i- \ m blood-plasma. 



(/>) Serum globulin (paraglobulin) j 



(c) Myosinogcn in muscle. 



('/) Crystallin in the crystalline K-ns. 



If we compare together these two classes of proteids, the most 

 important of the native proteids, we find that they all give the 

 same general tests, that all are coagulated by heat, but that they 



