CHEMISTRY OF THE LEUCOM AINES. 315 



urine by Hofmeistee's method. The urine, acidulated 

 with hydrochloric acid, is precipitated with phosphotungstic 

 acid; the precipitate is decomposed by warming with 

 baryta, filtered, and the filtrate is freed from barium by the 

 cautious addition of sulphuric acid. The solution is then 

 made alkaline with ammonium hydrate, any traces of phos- 

 phates that appear are filtered off, and finally it is precipi- 

 tated by addition of ammoniacal silver nitrate. The pre- 

 cipitate which forms consists of the silver compounds of the 

 xanthine bodies, and is purified by dissolving in hot nitric 

 acid, as given on page 285. Xanthine has been shown to 

 be formed at the same time with guanine, adenine, and 

 hypoxanthine, in the decomposition of nuclein by means of 

 dilute acids. It may, therefore, be prepared from cellular 

 organs according to the method given under Adenine. The 

 method of its preparation from tea-leaves is also given 

 elsewhere. 



Xanthine is a white, granular, amorphous body, and is 

 deposited from hot aqueous solution on cooling in colorless 

 floccules, or as a fine powder, which, under the microscope, 

 is seen to consist of rounded granules. When occurring in 

 calculi, it forms compact, moderately hard, yellow or brown 

 fragments, which, on being rubbed with the finger-nail, 

 assume a wax-like appearance. It is difficultly soluble in 

 cold water (about 14,000 parts), alcohol, and ether ; some- 

 what more soluble in boiling water (about 1200 parts). It 

 is soluble in alkalies and alkali carbonates, not bicarbon- 

 ate, and from these solutions it is precipitated on neutral- 

 ization with acids, or by passing carbonic acid. In warm 

 ammonia it dissolves more readily than does uric acid or 

 guanine, and on cooling the ammonium compound recrys- 

 tallizes. It acts as a weak base, and as a weak acid ; with 

 salts of the heavy metals it forms difficultly, soluble or 

 insoluble compounds. Its basic properties, however, are 

 weaker than those of hypoxanthine or guanine. 



"When xanthine is evaporated with nitj-ic acid it leaves a 

 lemon-yellow residue (hence its name), which is not changed 

 by ammonium hydrate — distinction from uric acid — but 

 with potassium hydrate becomes yellowish-red, on heating 

 purple-red. When added to a mixture of bleaching powder 



