URIC ACID. 
59i 
words, the chief factor which determines the precipitation of uric acid 
is the degree of acidity of the urine. Roberts has found that two other 
agencies exerl an influence over this precipitation — fche 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 
biurates 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. 
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 
