1028 MODIFICATIONS OF PROTEIN. 



tein-compound is completely decomposed, and ammonia, 

 carbonic acid, formic acid, and three azotised bodies, are formed : 

 leucin, protid and erythroprotid. The alkaline solution is neu- 

 tralised with sulphuric acid, poured off from the sulphate of 

 potash which precipitates, evaporated to dryness, and the mass 

 boiled with alcohol. Erythroprotid, is first deposited, as a 

 reddish brown extractiform mass. This substance, as it exists 

 in combination with oxide of lead, is expressed by C 13 H 8 NO 5 , 

 Later, the leucin separates in a crystalline state. Protid, 

 C 13 H 9 NO 4 , which is a yellowish uncrystallizable brittle sub- 

 stance, remains in solution, with formiate of potash. The 

 leucin crystallizes in brilliant plates, like cholesterin, is inodor- 

 ous and tasteless, and sublimes unchanged at 338. It is but 

 slightly soluble in water, and still less soluble in alcohol. It is 

 not decomposed by alkalies. It combines with 1 atom of the 

 protohydrate of nitric acid, and becomes nitroleucic acid, which 

 forms crystalline salts containing 1 atom of base, without losing 

 its leucin. The same substance is also formed by the digestion 

 of a protein-compound in sulphuric acid. 



CASEIN. 



The curd or coagulable portion of milk has been named 

 casein and also caseum; it is a principle having considerable 

 analogy to albumen, coagulable by rennet but not by a boiling 

 temperature. Sweet milk contains its whole casein in solution, 

 with globules of fat in a state of suspension, which last rise to 

 the surface in the form of cream, or are separated, by agitation 

 of the milk, in the form of butter. The milk contains also in 

 solution a considerable quantity of lactine or sugar of milk 

 (page 755), to which the casein stands in the relation of a fer- 

 ment. The latter soon begins, probably after being affected 

 like other ferments by the air, to convert the lactine into lactic 

 acid (page 809). Milk thus spontaneously becomes sour in 

 open vessels, and its casein is at the same time coagulated by 

 combining with lactic acid (Fremy). 



Casein combines with other acids, besides the lactic, and is 

 best prepared according to Braconnot, by adding dilute sulphuric 

 acid to skimmed milk ; a white coagulum is formed, the sulphate 

 of casein insoluble in water, which may be collected and washed 

 upon a filter. It is afterwards digested with carbonate of lead, 



