MODIFICATIONS OF PROTEIN. 1023 



SECTION II. 



MODIFICATIONS OF PROTEIN: ALBUMEN, FIBRIN, CASEIN. 

 ALBUMEN. 



This substance forms the white of eggs, whence its name, and 

 is the principal constituent of blood ; it is also found in many 

 fluid secretions, and in nearly all the solids of the animal body. 

 It exists in two conditions, soluble, as it is in the animal fluids, 

 and insoluble or coagulated, when heated to 158. 



Uncoagulated albumen may be prepared by evaporating the 

 clear serum of blood, or white of egg, by a heat of 120, till it 

 dries up and forms a yellowish transparent brittle mass, like gum. 

 This is reduced to powder, and washed successively with ether 

 and alcohol, which dissolve out the fat, salts and other foreign 

 matters in the serum or white of egg. 



Dry albumen first swells up in water, then forms with it a 

 glairy colourless fluid, which is nearly tasteless. At 140 the 

 dry albumen begins to lose its transparency, and at 142 it 

 changes into a white coherent mass, in which the albumen has 

 passed into the insoluble condition. When dissolved in water 

 it coagulates at 158 ; a very dilute solution, however does not 

 become turbid till it is boiled. Albumen is thrown down from, 

 solution, in a coagulated state, by alcohol, creosote, by acids 

 particularly nitric acid ; by metaphosphoric acid, but not by 

 the other hydrates of phosphoric acid, nor by acetic acid. The 

 precipitates with acids are definite compounds of albumen with 

 the latter. Coagulated albumen also forms compounds with 

 acids, which are insoluble in an excess of the acid, but are 

 soluble in water. Dilute hydrochloric acid precipitates albumen, 

 the concentrated acid when heated dissolves the coagulum, of 

 a lilac and then of a deep blue colour, as it also dissolves fibrin 

 and casein. Albumen is precipitated from its soluble com- 

 pounds with acids, by the ferrocyanide of potassium. Coagu- 

 lated albumen dissolves in caustic alkalies and neutralises 

 them ; the solutions are precipitated by soluble metallic salts, 

 and insoluble albuminates of the metals formed. 



A solution of albumen in water is precipitated by acetate of 



