CHEMICAL BASIS OF THE ANIMAL BODY. 177 



Reactions. The discrimination of members of the xanthin group is 

 not easy since, from their close relationship, they yield many reactions 

 in common. The following are characteristic of xanthin. 



i. Weidel's reaction 1 . The substance is warmed with freshly 

 prepared chlorine-water and a trace of nitric acid as long as any gas 

 is evolved : it is then carefully evaporated to dryness and, if xanthin 

 is present, the residue turns pink or purplish-red on the access of 

 ammonia fumes. Carnin gives a similar colouration if but little 

 chlorine-water is used, while guanin and adenin do not. 



ii. Hoppe-Seyler* s reaction. When xanthin is introduced into 

 some caustic soda with which some chloride of lime has been mixed, 

 each particle of the substance surrounds itself with a dark green ring 

 which speedily turns brown and then disappears. 



iii. Strecker's test\ When evaporated to dryness on porcelain with 

 nitric acid a yellow residue is obtained which turns reddish-yellow on 

 the addition of caustic soda or potash (not of ammonia) and reddish- 

 violet on subsequent warming. Distinctive from uric acid. 



iv. Xanthin is more readily soluble in ammonia than is uric acid. 

 v. Xanthin yields in solution in dilute nitric acid a characteristic 

 crystalline compound with nitrate of silver, which differs from the 

 similar compound of hypoxanthin both in the forms which it presents 

 and in its greater solubility in nitric acid of sp. gr. I'l at 100. It 

 is therefore used as a means of separating xanthin and hypoxanthin. 



FIG. 25. CRYSTALS OP XANTHIN SILVER-NITRATE, C 5 H 4 N 4 2 . AgNO s . 

 (Krukenberg after Kiihne.) 



vi. The compound of xanthin with hydrochloric acid is far less 

 soluble in water than are the similar compounds of hypoxanthin and 

 guanin, and hence affords a further means of separating these bases. 



By treatment with hydrochloric acid and potassium chlorate xanthin 

 is converted into alloxan and urea (Fischer). 



1 Ann. d. Chem. u. Pharm. Bd. CLVIII. (1871), S. 365. This reaction was given 

 by its author for hypoxanthin, but apparently in error. Cf. Kossel, Zt. f. physiol. 

 Chem., Bd. vi. (1882), S. 426. Salomon, Ber. d. d. chem. Gesell. 1883, S. 198. 



2 Ann. d. Chem. u. Pharm. Bd. cvm. (1858), S. 146. 



F. m 



