166 URIC ACID. 



Salts of Uric acid. Of these the most important are the acid 

 urates of sodium, potassium, and ammonium ; these salts are fre- 

 quently still called ' lithates,' the term ' lithic ' acid being used 

 for uric acid. The sodium salt which is the most common con- 

 stituent of many urinary sediments crystallises in many different 

 forms, these not being characteristic, since they are almost the 

 same for the corresponding compounds of the other two bases. It 

 is very sparingly soluble in cold water (1 in 1100 or 1200), more 

 soluble in hot (1 in 125). It is the principal constituent of several 

 forms of urinary sediment, and composes a large part of many 

 calculi ; the excrement of snakes contains it largely. The potas- 

 sium resembles the sodium salt very closely, as also does the 

 compound with ammonium; the latter occurs generally in the 

 sediment from alkaline urine. 



FIG. 20. (Krukenberg after Kiihne.) 



Urinary sediment from alkaline urine. The large crystals consist 

 of ammonio-magnesium phosphate (triple phosphate, XH 4 MgP0 4 -f- 

 6H 2 0). A few crystals (octahedra) of calcium oxalate are also shown. 

 The remaining crystals represent the form of acid ammonium urate, 

 C 5 H 3 (NH 4 )N 4 O3. The rounded objects are urinary fungi. 



Preparation. The amount of uric acid in mammalian urine is 

 too small to make it a source of the acid. Crystals may however 

 be readily obtained from human urine by adding to it 2 3 p. c. 

 of strong hydrochloric acid and letting it stand for one or two 

 days in a cool place. The crystals form on the sides, of the con- 

 taining vessel. 



On the large scale it is usually prepared from guano, or from 

 snake's excrement. From the latter it is obtained by boiling with 

 caustic potash (1 part alkali to 20 of water) as long as ammonia 

 is evolved ; in the filtrate a precipitate of acid urate of potassium 

 is formed by passing a current of carbonic acid ; this salt is then 

 washed, dissolved in caustic potash, and decomposed by carefully 

 filtering its solution into an excess of dilute hydrochloric acid. 



