THE URINE 



45i) 



classes of salts the normal urates (Na 2 U) and the acid urates (NaHU). 

 The chief urates present in the urine are acid sodium urate (C 5 H 3 NaN 4 0) 

 and normal sodium urate (C.HUNa^O.,). Sometimes ammonium urate 

 also occurs. Urates are frequently deposited from concentrated urine 

 as a pinkish deposit coloured by uroerythrin. Such a deposit is 

 soluble on heating or addition of alkali. Urates are sometimes amor- 

 phous, sometimes crystalline, as '' thorn apple-,'' fan-shaped clusters of 

 prismatic needles (Fig. 222). To obtain uric acid quickly, a consider- 

 able quantity of urine (about 100 c.c.) is taken, ammonia added till 

 the reaction is alkaline, and then the urine saturated with ammonium 

 chloride. Ammonium urate is precipitated, and from this precipitate 

 uric acid may be obtained by the addition of acid. If hydrochloric 



FIG. 222. SJDICM URATE. x 350. 



acid be added to urine, and the urine be left to stand for twenty-four 

 hours, crystals of uric acid fall out, usually highly pigmented with 

 urorosein, and known as the brick-dust or cayenne-pepper deposit. 

 Under the microscope, the crystals appear shaped like whetstones, 

 barrels, wedges, rosettes, and coloured reddish-yellow (Fig. 223). 

 Uric acid is sometimes passed in acid urines, and known as gravel. 

 Stones forming in the bladder and kidney are often found to be com- 

 pose:! of uric acid or urates. 



Uric acid is almost insoluble in water. It i* held in solution in the 

 urine partly by the alkaline phosphates, and partly by the urea 

 present. It is readily soluble in alkalies. When evaporated with 

 nitric acid, it leaves a yellowish residue, which turns purple on adding 

 ammonia, and blue with caustic potash (the murexide test). Urates 

 also give this murexide test, which depends upon the fact that a sub- 

 stance alloxantin is formed from uric acid, and combines with 

 ammonia to form ammonium purpurate. 



