PROTEIDS 159 



incineration is regarded as the quantity of cellulose. The solu- 

 tions used should be exactly of the strength specified, and the 

 sodium hydrate pure. 1 



226. Proteids, The various proteids differ much in their ele- 

 mentary composition, but the percentages of the elements average 

 about as follows : 



,; C H N O S P 



52 7 16 22 .5-2 0.3-1.5 



The constitutional formula of proteids is as yet unknown. 

 Proteids have the following properties in general : 



(a) They are all insoluble in ether, and most of them insoluble 

 in alcohol. Most proteids are soluble in water. Other more gen- 

 eral solvents are dilute and concentrated saline solutions, weak 

 acids and weak alkalies. They are all decomposed by the action 

 of concentrated mineral acids or alkalies and by the action of 

 certain enzymes. 



(b) Some proteids when in solution are coagulated by heat. 



(c) Proteids (with the exception of hydrated varieties, such as 

 proteoses and peptones) are indiffusible ; that is, they are incapable 

 of passing through dead animal membranes. 



(d) All proteids in solution are laevo- rotatory. 



(e) With certain mineral reagents they give characteristic 

 color reactions. 



(/) Most proteids are precipitated by salts of the heavy metals, 

 by picric acid, by acetic acid and potassium ferro-cyanide, by satu- 

 ration with certain neutral salts, as ammonium sulphate and by 

 strong acids. 



Proteids are divided with reference to their origin into two 

 classes, animal and vegetable, and as in both cases they are often 

 found combined with other bodies, they are further classified on 

 this basis as simple or compound. It may be said here that the 

 compound proteids offer many exceptions to the general proper- 



1 Wiley, H. W. Principles and Practice of Agricultural Analysis. 3 : 1897. 

 Hammarsten-Mandel. Text-Book of Physiological Chemistry. 1899. 



