ALBUMINOUS SUBSTANCES OR PROTEIDS. 341 







Analytical reactions. 



1. Proteids are colored yellow by warm nitric acid, the color 

 changing to orange with ammonia. 



2. Milton's reagent colors them purple-red on heating. This 

 reagent is a solution of mercuric nitrate, containing some excess 

 of nitric acid; it is best made by dissolving 1 part of mercury in 

 2 parts of nitric acid of a specific gravity of 1.42, and diluted 

 with 2 volumes of water. 



3. A few drops of cupric sulphate solution and then potassium 

 hydrate added, give a violet color. 



4. A few drops of solution of 1 part of cane-sugar in 4 parts 

 of water and then strong sulphuric acid added, produce a purple 

 color. 



5. They are often precipitated by highly diluted acids, but 

 redissolved by boiling with strong hydrochloric acid, forming a 

 violet-red solution. The precipitated proteids are also generally 

 dissolved by caustic alkalies. 



Classification. Our present unsatisfactory state of knowledge 

 regarding proteids, the close resemblance which they show in 

 properties, and the difficulties which are met with in separating 

 them in a pure state, make it difficult to arrange these bodies 

 properly. Three principal groups may, however, be distin- 

 guished; they are: 



/. True albumins. If. Fibrins. 



Serum-albumin. Blood-fibrin. 



Egg-albumin. 

 Plant-albumin. 



Muscle fibrin. 

 Plant-fibrin or gluten. 



Soluble in water and j Insoluble in water; solu- 

 precipitated (coagulated) ble in water containing i-er- 



///. Caseins. 



Milk-casein. 

 Serum-casein. 

 Plant-casein or legumin. 

 Not coagulated by heat, 

 but by diluted acids, and 



by warming their solu- tain neutral salts, alkalies, al?o by the mucous mem- 



tion to 60 or 70. 



or acids. They become gen- [ brane of the stomach of 

 efally insoluble as soon as > the calf (rennet), 

 they leave the organism. 



True albumins occur in the whites of birds' eggs, in milk, in 

 the plasma of the blood, chyle, lymph, in the juices of muscles, 

 in serous and nutritive liquids ; they are also found in the juices 

 of plants. Solutions of albumin become turbid at 60, and are 

 coagulated at or below 75; the presence of a little free acid 



