URIC ACID. 591 



words, the chief factor which determines the precipitation of uric acid 

 is the degree of acidity of the urine. Eoberts has found that two other 

 agencies exert an influence over this precipitation the pigmentation 

 of the urine, and its comparative richness or poverty in salines. Other 

 things being equal, a specimen which is poor in pigments on the one 

 hand, or in neutral salts on the other, will exhibit a special tendency 

 to deposit its uric acid in crystals. But while the question of acidity 

 affects that stage of the process which consists in the change from 

 bin rates to quadriurates, the pigmentation and percentage of salts 

 affect rather the change from quadriurate to free acid. The urinary 

 pigments and the neutral salts inhibit the decomposition of quadri- 

 urates by water. 



* f , 



FIG. 52. Upper half, ammonium urate. Lower half, sodium urate. 



Upon standing, some specimens of urine deposit urates, not as amorphous 

 quadriurates, but as crystalline biurates. Ammonium urate is frequently to be 

 seen in the deposit from alkaline urine in the form of roughly dumb-bell-shaped 

 masses; and in concentrated specimens sodium urate forms the so-called thorn- 

 apple crystals (Fig. 52). 



Isolation of uric acid from the urine. If the urine be acidified with 

 hydrochloric acid, much of its uric acid separates in pigmented crystals, 

 which tend to adhere to the sides of the vessel. These can be easily 

 identified by the microscope. But for the purpose of applying the 

 characteristic tests, a supply of uric acid may be more conveniently and 

 quickly obtained by adding crystals of ammonium chloride to the 

 urine till near saturation, and then a few drops of strong ammonia. 

 The precipitate which falls is at once filtered off, washed from the filter 

 with a little hot water, and warmed with a few drops of hydrochloric 



