114 HANDBOOK OF PHYSIOLOGY. 



vii. Boiled with strong hydrochloric acid, they give a violet red 



coloration, 

 viii. With cane sugar and strong sulphuric acid, on heating, they 



give a, purplish coloration, 

 ix. They are precipitated on addition of citric or acetic acid, 



and picric acid ; or citric or acetic acid, and sodium tungstate ; 



or citric or acetic acid, and potassio-mercuric iodide ; and with 



many other metallic salts in solution and by alcohol. 



Varieties. Proteids are divided into classes, chiefly on the basis of 

 their solubilities in various reagents. Each class, however, if it contains 

 more than one substance, may often be distinguished by other properties 

 common to its members. Not every one of the proteids enumerated is 

 contained in the animal tissues, some are used as food. 



(1.) Native- Albumins. These substances are soluble in water and in 

 saline solutions, and are coagulated, i.e., turned into coagulated proteid, 

 on heating. 



(2.) Albuminates. These are soluble in acids or alkalies, insoluble in 

 saline solutions and in water, and not coagulated on heating. 



(3.) Globulins. These are soluble in weak saline solutions, in dilute 

 acids and alkalies, and insoluble in water and in strong solutions of neu- 

 tral salts. They are coagulated on heating. 



(4.) Proteases. These are soluble in water and dilute saline solutions, 

 precipitated by saturation with ammonium sulphate ; precipitated but not 

 coagulated by alcohol ; precipitated by picric acid : cannot be coagulated 

 by heat. 



(5.) Peptones. These are soluble in water, saline solutions, acids, or 

 alkalies; not precipitated on saturation with any neutral salt; they are 

 not coagulated on heating. 



(6.) Coagulated Proteids. These are of two classes, either coagulated 

 by (a) action of ferments, or (b) heat. These are soluble only in gastric 

 or pancreatic fluids, forming peptones, or (with difficulty) in strong acids 

 and alkalies. 



Native- Albumins. Of native-albumins there are several varieties: 

 (a) egg-albumin ; (b) serum-albumin ; (c) lact-albumin, etc. 



Egg Albumin is contained in the white of the egg. 



When in solution in water it is a transparent, frothy, yellowish fluid, 

 neutral or slightly alkaline in reaction. It gives all of the general pro- 

 teid reactions. It yields 8 per cent of argenin, 22.6 per cent of leucin, 

 and 2 per cent of tyrosin. 



At a temperature not exceeding 40 C. it is dried up into a yellowish, 

 transparent, glassy mass, soluble in water. At a temperature of 70 C. 

 it is coagulated, i.e., changed into a new substance, coagulated proteid, 



