CHEMICAL BASIS OF THE ANIMAL BODY. 167 



(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 

 potassium 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, NH 4 MgP0 4 + 6 H 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 3 . 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 containing 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 

 nitrate 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. 



By similar treatment uric acid is readily obtained from fowl's 

 excrement, a convenient source of the acid. 



Identification of uric acid. The crystalline forms afford some clue, 

 but are so numerous that some forms which may at any time present 

 themselves are scarcely characteristic. The rhombic tables, 'dumb-bell' 

 and ' whetstone ' crystals are on the whole most characteristic. 



i. Murexid test. The suspected substance is treated in a porcelain 



