184 THE PROTEIN SUBSTANCES. 



ably the nucleic-acid residue is combined with the imide hydrogen at 

 position 7. 1 



HANS FISCHER 2 has found that the combinations of the purine bases 

 with diazobodies are azo pigments, and that the diazo group enters at the 

 8th position. The purine bases substituted at the 8th position do not 

 combine with diazo bodies any more than those substituted at the 7th 

 position, and FISCHER concludes from this, contrary to BURIAN, that the 

 nucleic acid molecule, which also does not react with diazo bodies, with 

 the formation of pigments, can have the purine bases combined in the 

 8th as well as the 7th position. 



The nuclein bases form crystalline salts with mineral acids, which, 

 with the exception of the adenine salts, are decomposed by water. They 

 are easily dissolved by alkalies, while with ammonia their action is some- 

 what different. They are all precipitated from acid solution by phos- 

 photungstic acid; they also separate as silver compounds on addition 

 of ammonia and ammoniacal silver-nitrate solution. These precipitates 

 are soluble in boiling nitric acid of 1.1 specific gravity. All purine bodies 

 are also precipitated by FEHLING'S solution (see Chapter XV) in the pres- 

 ence of a reducing substance such as hydroxylamine (DRECHSEL and 

 BALKE). Copper sulphate and sodium bisulphite may also be used to 

 advantage in their precipitation (KRUGER). 3 This behavior of the 

 purine bases serves just as well as the behavior with the silver solution 

 for their precipitation and preparation. 



HN CO 



Xanthine, Cs^N^, = OC C NIL (2, 6-dioxypurine) , 4 is found 



I II >CH 



HN C W 



in several cellular organs. It occurs in small quantities as a physio- 

 logical constituent of urine, and it occasionally has been found as a urinary 

 sediment, or calculus. It was first observed in such a stone by MARCET. 

 Xanthine is found in larger amounts in a few varieties of guano (Jarvis 

 guano) . 



Xanthine is amorphous, or forms granular masses of crystals, or may 

 also, according to HoRBACZEWSKi, 5 separate as masses of shining, thin, 

 large rhombic plates with 1 mol. water of crystallization. It is very 



1 In regard to the disputed views see Steudel, Zeitschr. f. physiol. Chem., 48; 

 Burian, ibid., 42 and 51. 



2 Zeitschr. f . physiol. Chem., 60. 



3 Balke, Zur Kenntnis der Xanthinkorper, Inaug.-Diss. Leipzig, 1893; Kriiger, 

 Zeitschr. f. physiol. Chem., 18. 



4 In regard to the synthesis of xanthine and other purines see E. Fischer, foot-note 

 5, page 181. 



5 Zeitschr. f. physiol. Chem., 23. 



