340 CONSIDERATION OF CARBON COMPOUNDS. 



49. ALBUMINOUS SUBSTANCES OR PROTEIDS. 



Occurrence in nature. Albuminous substances form the chief 

 part of the solid and liquid constituents of the animal body ; 

 they occur in blood, tissues, muscles, nerves, glands, and all 

 other organs; they are also found in small quantities in nearly 

 every part of plants, and in larger quantities in many seeds. 

 They have never yet been formed by artificial means, but are 

 almost exclusively products of vegetable life, and undergo but 

 little change when consumed as food and assimilated by animals. 



General properties. The various proteids resemble one another 

 closely in their properties. Their composition is so complex 

 that, as yet, no chemical formula has been assigned to them 

 with any certainty. The percentage composition and other 

 reasons have led to a formula represented by C 144 H 224 N 36 44 S 2 , 

 which represents about the average composition of the proteids. 

 The percentage composition is shown in the following figures: 



Carbon, 51.5 per cent, to 53.6 per cent. 

 Hydrogen, 6.7 " 7.1 " 



Nitrogen, 15.6 " 17.4 " 



Oxygen, 21.8 " 24.0 " 



Sulphur, 0.8 " 1.5 " 



Of other properties may be mentioned : 



1. They are amorphous, colorless, odorless, and nearly taste- 

 less substances, incapable of dialysis. 



2. They easily undergo that decomposition known as putre- 

 faction. 



8. Some are soluble in water, others only in water containing 

 alkalies, acids, or other substances, whilst some are insoluble. 



4. They are not volatile without decomposition. 



5. The soluble proteids are converted into insoluble modifica- 

 tions either by heating to 60 or 70, or by the addition of 

 mineral acids, alcohol, or certain metallic salts. This process 

 of converting soluble into insoluble proteids is called coagulation; 

 and proteids when once coagulated will not return to their 

 original soluble form without suffering some alteration. 



6. They are converted into peptones by the action of gastric 

 juice. (See later.) 



