CHEMICAL BASIS OF THE ANIMAL BODY. 1271 



given by xanthine, but does not respond to Weidel's test. (See 

 above, p. 1265.) 



Capranica'' s reactions, (i) A yellow crystalline precipitate 

 on the addition of a saturated aqueous solution of picric acid 

 to a solution of guanine-hydrochloride ; insoluble in cold water, 

 (ii) An orange-coloured crystalline precipitate, very insoluble 

 in water, on the addition of a concentrated solution of potas- 

 sium chromate. (iii) Prismatic yellowish-brown crystals on 

 the addition of a concentrated solution of ferricyanide of potas- 

 sium. Xanthine and hypoxanthine when similarly treated do 

 not yield the last two precipitates. 



Several new crystalline substances of the xanthine series have 

 recently been isolated from urine, in which, however, they occur in 

 most minute quantities. Of these episarkine, C 4 H 6 N 3 (?), is very 

 slightly soluble in water. It forms a double salt with nitrate of 

 silver, which is very insoluble in nitric acid. It does not give the 

 ordinary reactions of xanthine, but yields a colour as of murexide 

 (see p. 1201) with hydrochloric acid and potassium chlorate and sub- 

 sequent exposure to ammonia vapours. Epiguanine, to which the 

 formula C 10 H 13 N 9 O 2 is assigned, is also but slightly soluble in water. 

 It yields crystalline compounds with acids and salts, of which the 

 double salt with platinum is peculiarly characteristic. It also gives 

 the ordinary reaction for xanthine with nitric acid and caustic soda. 



8. Guanidine. CN 3 H 5 . NH 2 



NH = C 



I 

 NH 2 . 



Although this substance does not occur in the free state in 

 any tissue or fluid of the animal body, it is of considerable 

 interest, for it has been obtained by the direct oxidation of 

 proteids and may be made to yield urea by treatment with boil- 

 ing dilute sulphuric acid or baryta-water. NH:C(NH 2 ) 2 + 

 H 2 0=(NH 2 ) 2 CO + NH 3 . Further, it affords a connecting 

 link between the xanthine series and creatine (p. 1247), the 

 latter substance being, as already stated, methylguanidinacetic 

 acid, while guanidine is itself the chief product of the oxida- 

 tion of guanine. 



Xanthine derivatives. 



There can be but little doubt that the xanthine bodies (and uric 

 acid) are typically products of the downward, excretionary nitro- 

 genous metabolism of animals. The alkaloidal principles of plants, 

 in this case theobromine and caffeine, may be similarly regarded as 

 excretionary products, and are hence found collected in those parts 

 of the plant which are more immediately or ultimately cast off, viz. 

 the leaves, seeds and bark. The facts already stated render the 



