178 CAKNIK 



substances, a crystalline salt with nitrate of silver ; this like the 

 corresponding compound of xanthin is soluble in strong nitric acid 

 (sp. gr. I'l) at 100, and may thus be separated from hypoxanthin. 

 It may be separated from xanthin by means of its greater solubility 

 in cold water, and from heteroxanthin by the difference in the solu- 

 bility of its salts with sodium and hydrochloric acid. 



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 pos- 

 sibility of the existence of at least two isomeric di-methyl deriva- 

 tives 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 ; para- 

 xanthin is probably another isoiner, and more recently Kossel has 

 described a third, theophyllin. By substitution of (CH 8 ) for 

 hydrogen in the third (NH) group trimethyl-xanthin or caffein 

 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. 1 



4. Carnin. C 7 H 8 N 4 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 filtrate 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 nitrate from the sulphide of 

 lead* 



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 



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



2 Weidel, Ann. d. Che'm. u. Pharm. Bd. CLVIII. (1871), S. 353. 

 8 Pouchet, Journ. de Thtrap. T. vn. (1880), p 503. 



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



