CHEMICAL BASIS OF THE ANIMAL BODY. 151 



formation of a sodium salt of benzoyl-cystin when it is shaken up 

 with a few drops of benzoyl chloride 1 . 



FIG. 13. CYSTIN CEYSTALS. (After Funke.) 



Cystin is insoluble in either water, alcohol or ether, readily soluble in 

 ammonia, differing in this respect from uric acid, also in many alkaline 

 carbonates and in mineral acids. Its solutions are strongly Isevorota- 

 tory, (a) D = 205'9 in hydrochloric acid 11 '2 p.c. 2 or if the acid is 

 dilute (a) D = -214. 3 



Apart from the characteristic crystalline form and its solubility in 

 ammonia, the fact that cystin is one of the few crystalline substances, 

 occurring physiologically, which contains sulphur, renders its detection 

 very easy. Thus when boiled with caustic alkalis a sulphide of the 

 alkali is obtained which gives a dark stain on silver foil ; also a brown 

 or black colouration appears when cystin is boiled in a test-tube with 

 a solution of oxide of lead in caustic soda 4 . 



AMIDO-ACIDS OF THE OXALIC SERIES. 



1. Carbamic acid. NH 2 (COOH). 



Carbonic acid is more usually classed at the head of the acids of the 

 glycolic (lactic) series. It exhibits however a remarkable difference from 



1 Goldmann u. Baumann, Zt. f. physiol. Chem. Bd. xn. (1888), S. 254. 

 Udransky u. Baumann, Ibid. Bd. xv. (1891), S. 87. 



2 Mauthner, Zt. f. physiol. Chem. Bd. vn. (1883), S. 225. Of. Drechsel, Arch. 

 f. Physiol. Jahrg. 1891, S. 247. 



3 Baumann, loc. cit. S. 303. 



4 The following literature may be additionally consulted on the occurrence of 

 cystin in urine. Zt. f. physiol. Chem. Bde. ix. 129, 260 ; xn. 254 ; xiv. (1889), 109. 

 Virchow's Arch. Bd. c. (1885), S. 416. Maly's Jahresb. 1886, S. 465. Berl klin. 

 Wochensch. 1889, No. 16. Zt. f. klin. Med. Bd. xvi. (1889), S. 325. 



