THE CHEMICAL COMPOSITION OF THE BODY. 113 



varied in the case of each particular substance. The chief of these are as 

 follows : 



i. Xantho-Proteic Reaction. The addition of strong nitric 

 acid, drop by drop, to a solution of any proteid produces a 

 flocculent precipitate which dissolves in an excess of the acid. 

 The solution becomes canary yellow in color; when heated, 

 this color is more marked ; when cooled, the addition of am- 

 monia in excess changes the color to orange. The nitric acid 

 decomposes the proteid to a certain extent and then unites 

 with the decomposition products, forming, among other 

 things, xanthoproteic acid which gives the yellow color. The 

 ammonia unites with this and forms ammonium xanthoprote- 

 ate which gives the orange color. 



ii. Biuret (Piotrowski's) Reaction. With a trace of cupric 

 sulphate and an excess of potassium or sodium hydrate pep- 

 tones and proteoses give a rose red ; with ammonia instead of 

 the fixed alkalies, a blue coloration. Most proteids, however, 

 give a violet (pinkish purple) color; the color is due to re- 

 duced copper, cuprous hydroxide being formed along with 

 other compounds of red, yellow, and blue colors. 



iii. Millon's Reaction. With Millon's reagent (a solution of 

 mercuric nitrate) proteids give a heavy white precipitate of 

 mercuric albuminate which, with an excess of the reagent, be- 

 comes brick red when heated. This test is said to be due to 

 the presence of ty rosin, an aromatic compound in the proteid 

 molecule : it is generally used for solids though it may be 

 used for liquids also. With all substances containing the 

 C 6 H 5 OH group, e.g., carbolic acid, this reagent gives the same 

 color reaction, though no precipitate is formed, the solution 

 itself becoming red. 



iv. Ammonium Sulphate Reaction. They are, with the ex- 

 ception of peptone, entirely precipitated from their solutions 

 by saturation with ammonium sulphate. 



Many of the proteids give, in addition, the following tests : 



v. With excess of acetic acid, and potassium ferrocyanide, a white 



precipitate, 

 vi. With excess of acetic acid and a saturated solution of sodium 



sulphate, on boiling, a white precipitate. This test is often 



used to get rid of all traces of proteids, except peptones, from 



solutions. 

 8 



