THE KIDNEYS AND URINE. 



361 



the latter is the more likely opinion; but the quantity of either uric acid 

 or urates in the blood is probably too small to allow of this question being 

 solved. 



Owing to its existence in combination in healthy urine, uric acid for 

 examination must generally be precipitated from its bases by a stronger 

 acid. Frequently, however, when excreted in excess, it is deposited in a 

 crystalline form (Fig. 248), mixed with large quantities of ammonium or 

 sodium urate. In such cases it may be procured for microscopic exami- 

 nation by gently warming the portion of urine containing the sediment; 

 this dissolves urate of ammonium and sodium, while the comparatively 

 insoluble crystals of uric acid subside to the bottom. 



The most common form in which uric acid is deposited in urine, is that 

 of a brownish or yellowish powdery substance, consisting of granules of 



FIG. 248. Various forms of uric acid crystals. 



FIG. 349. Crystals of hippuric acid. 



ammonium or sodium urate. "When deposited in crystals, it is most 

 frequently in rhombic or diamond-shaped laminae, but other forms are not 

 uncommon (Fig. 248). When deposited from the urine, the crystals are 

 generally more or less deeply colored, from being combined with the 

 coloring principles of the urine. 



There are two chief tests for uric acid besides the microscopic evidence 

 of its crystalline structure: (1) The Murexide test, which consists of 

 evaporating to dryness a mixture of strong nitric acid and uric acid in a 

 water bath. This leaves a yellowish-red residue of Alloxan (C 4 H 2 N 2 4 ) 

 and urea, and -this, on addition of ammonium hydrate, gives a beautiful 

 purple (ammonium purpurate, C 8 H 4 (NH 4 ) N fi 6 ), deepened on addition 

 of caustic potash. (2) Schiff's test. Dissolve the uric acid in sodium 

 carbonate solution, and drop some of it on a filter paper moistened with 

 silver nitrate, a black spot appears, which corresponds to the reduction 

 of silver by the uric acid. 



Hippuric Acid (C 9 H 9 N0 3 ) has long been known to exist in the urine 

 of herbivorous animals in combination with soda. It also exists naturally 



