CHEMICAL BASIS OF THE ANIMAL BODY. 179 



Paraxanthin gives Weidel's reaction but not the ordinary xanthin 

 test with nitric acid and caustic soda. 



An inspection of Fischer's formula for xanthin shows the possibility 

 of the existence of at least two isomeric di-methyl derivatives of this 

 base according to the replacement by methyl CH 3 of the hydrogen 

 atoms in the three NH groups which it contains. Of these one has for 

 some time been known as theobromin ; paraxanthin is probably another 

 isomer, and more recently Kossel has described a third, theophyllin. 

 By substitution of (CH 3 ) for hydrogen in the third (NH) group 

 trimethyl-xanthin or catfei'n is obtained. In connection with the 

 isomeric relationship of paraxanthin and theobromin it is of great 

 interest to observe that the physiological action of the two bases is the 

 same \ 



4. Carnin. C 7 H 8 N 4 O 3 . 



Closely allied in composition to the preceding base, but as yet of 

 unknown constitution, carnin occurs only as a constituent of ' extract 

 of meat ' of which it forms about one per cent. 2 , although it has been 

 stated to occur also in urine (?) 3 . 



It is prepared by precipitating extract of meat with baryta-water, 

 avoiding all excess of the precipitant. The nitrate from this is now 

 precipitated with basic acetate of lead, which carries down all the 

 carnin. This precipitate is repeatedly boiled with water which 

 dissolves out the lead salt of carnin, which is then decomposed by 

 sulphuretted hydrogen, and the carnin obtained by concentration of 

 the aqueous filtrate from the sulphide of lead 4 . 



It crystallises in white masses composed of very small irregular 

 crystals ; it is soluble with great difficulty in cold, readily soluble in hot 

 water, insoluble in alcohol and in ether. It unites with acids and salts 

 to form crystallisable compounds. Of these the more important are the 

 salts with basic lead acetate, soluble in boiling water, and with nitrate 

 of silver, insoluble in strong nitric acid and ammonia. Carnin gives 

 Weidel's reaction when only a small amount of chlorine water is 

 employed, but the test fails if any excess is used. 



Carnin bears an interesting relationship to hypoxanthin into- which 

 it may be converted by treatment with chlorine or nitric acid, or still 

 more readily by bromine. 



C 7 H 8 N 4 O 3 + Br 2 = C 5 H 4 N 4 O . HBr + CH 3 Br + C0 2 . 



The latter may be isolated from its hydrobromic acid salt by means of 

 caustic soda. 



1 Salomon, Verh. d. physiol. Gesell Berlin. Arch. f. Physiol. 1887, S. 582. 



2 Weidel, Ann. d. Chem. u. Pharm. Bd. CLVIII. (1871), S. 353. 



3 Pouchet, Journ. de Therap. T. vn. (1880), p. 503. 



4 Krukenberg u. Wagner, Sitzb. d. phys-med. Gesell. Wurzburg, 1883, No. 4. 



