486 ALBUMEN. 



they are heated for a long time with dilute sulphuric 

 acid, and when they are carefully fused with potassium 

 hydroxide. In the first case aspartic acid (p. 160) and 

 glutamic acid (p. 163) are formed at the same time 

 from many protein compounds. When oxidized with 

 dilute sulphuric acid and manganese peroxide, or 

 potassium bichromate, they yield numerous products : 

 formic, acetic, and other acids of the same series, ben- 

 zoic acid, oil of bitter almonds, and aldehydes of the 

 fatty acids, prussic acid, acetonitrile, and homologous 

 nitriles, etc. 



The most important varieties of protein compounds 

 are : 



1. Albumen. Three modifications of albumen, 

 differing slightly from each other in their properties, 

 have been distinguished: Vegetable albumen, albumen 

 of serum, and albumen of eggs. Vegetable albumen is 

 contained in nearly all vegetable juices ; albumen of 

 serum in blood-serum of vertebrate animals, in lymph, 

 chyle, in the transudates and pathological cystic fluids, 

 in urine in diseases of the kidneys, abundantly in 

 colostrum, and in small quantity in milk. Albumen 

 of eggs is contained only in birds' eggs. Vegetable 

 albumen cannot be obtained in a pure, uncoagulated 

 condition from vegetable juices. Serum-albumen is 

 obtained most readily from blood-serum or hydrocelic 

 fluid by diluting with twenty volumes of water, and 

 precipitating the protein compounds, which accompany 

 the albumen, by the careful addition of acetic acid or 

 continued passage of carbonic anhydride into the solu- 

 tion. The liquid, filtered off after twenty-four hours, 

 is evaporated at 40, and separated by dialysis from 

 the salts ; or precipitated with lead acetate, and the 

 precipitate decomposed with carbonic anhydride. 

 Pure serum-albumen forms a clear, not very tenacious 

 liquid, from which it can be precipitated with alcohol. 

 Directly after being thrown down this precipitate is 

 soluble in water, but, in a few minutes, it is converted 

 into the coagulated condition. It is not precipitated 

 by carbonic anhydride, dilute mineral acids, and tar- 



