400 



THE CHEMICAL COMPOSITION OF THE BODY [CII. XXV. 



ammonium sulphate (half-saturation) or magnesium sulphate (com- 

 plete 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 : 



(6) SSglobulin (paraglobulin) } in blood-plasma. 



(c) Paramyosinogen in muscle. 



(d) Crystallin in the crystalline lens. 



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 differ in 

 their solubilities. This difference in solubility may be stated in 

 tabular form as follows : 



nl TTT ( These products of digestion will be 



C ass III Proteoses I ^ ^ connect ! on with thafc 



Class IV. Peptones j ^.^ 



Class V. Coagulated Proteids. There are two main subdivisions 

 of these : 



(a) Proteids in which coagulation has been produced by heat; 

 they are insoluble in water, saline solutions, weak acids, and weak 

 alkalis; they are soluble after prolonged boiling in concentrated 

 mineral acids ; dissolved by gastric and pancreatic juices, they give 

 rise to peptones. 



(b) Proteids in which coagulation has been produced by fer- 

 ments : i. Fibrin (see BLOOD), ii. Myosin (see MUSCLE), iii. Casein 

 (see MILK). 



Appendix to the class of simple proteids. Albuminates are 

 compounds of proteid with mineral substances. Thus, if a solution 

 of copper sulphate is added to a solution of albumin a precipitate of 

 copper albuminate is obtained. Similarly, by the addition of other 

 salts of the heavy metals other metallic albuminates are obtainable. 



The albuminates which are obtained by the action of dilute acida 

 and alkalis on either albumins or globulins are, however, of greater 

 physiological interest, and it is to these we shall confine our attention, 



