428 



THE CHEMICAL COMPOSITION OF THE BODY [CH. XXVIII. 



They enter into colloidal solution in water, in dilute saline solu- 

 tions, and in saturated solutions of sodium chloride and magnesium 

 sulphate. They are, however, precipitated by saturating 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. 



4. The Globulins. 



The globulins give the same general tests as the albumins ; they 

 are coagulated by heat, but differ from the albumins mainly in their 

 solubilities. This difference in solubility may be stated in tabular 

 form as follows : 



Globulins are more readily salted out than albumins; they 

 may therefore be precipitated, and thus separated from the albumins 

 by saturation with such salts as sodium chloride, or better magnesium 

 sulphate, or by half saturation with ammonium sulphate. 



The typical globulins are also insoluble in water, and so may be 

 precipitated by removing the salt which keeps them in solution. 

 This may be accomplished by dialysis (see p. 424). Their temperature 

 of heat-coagulation varies considerably. The following are the 

 commoner globulins : fibrinogen and serum globulin in blood, egg 

 globulin in white of egg, paramyosinogen in muscle, and crystallin in 

 the crystalline lens. We must also include under the same heading 

 certain proteins which are the result of enzyme coagulation on 

 globulins, such as fibrin (see blood) and myosin (see muscle). 



The most striking and real distinction between globulins and 

 albumins is that the former on hydrolysis yield glycine, whereas the 

 albumins do not. 



5. The Sclero-proteins. 



These substances form a heterogeneous group of substances, 

 which are frequently termed albuminoids. The prefix sclero indicates 



