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HANDBOOK OF PHYSIOLOGY. 



Tests. There are two chief tests for uric acid besides the microscopic 

 evidence of its crystalline structure: (1) The Murexide test, which con- 

 sists of evaporating to dry ness 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 on addition of ammonium hydrate, a beautiful 

 purple color (ammonium. purpurate, C 8 H 4 (NH 4 )N 5 6 ), deepened on 

 addition of caustic potash, takes place. (3) Schiff's test consists of dis- 

 solving the uric acid in sodium carbonate solution, and of dropping 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. 



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

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

 naturally in the urine of man, in a quantity equal or rather exceeding 

 that of the uric acid. 



The quantity of hippuric acid excreted is increased by a vegetable 



FIG. 254. 



FIG. 254. Various forms of uric acid crystals. 

 FIG. 255. -Crystals of hippuric acid. 



diet. It appears to be formed in the body from benzoic acid or from 

 some allied substance. The benzoic acid unites with glycin, probably in 

 the kidneys, and hippuric acid and water are formed thus, C 7 H 6 2 (Ben- 

 zoic acid) + C 2 H 6 NO a (Glycin) = C 9 H 8 N0 3 (Hippuric acid) + H 2 

 (water). It may be decomposed by acids into benzoic acid and glycin. 



Properties. It is a colorless and odorless substance of bitter taste, 

 crystallizes in semi-transparent rhombic prisms (Fig. 255). It is more 

 soluble in cold water than uric acid, and much more soluble in hot 

 water. It is soluble in alcohol. 



(4) Pigments. The pigments of the urine are the following: 1. 

 Urochrome, a yellow coloring matter, giving no absorption band; of 

 which but little is known. Urine owes its yellow color mainly to the 

 presence of this body. 2. Urobilin, an orange pigment, of which traces 

 may be found in nearly all urines, and which is especially abundant in 

 the urines passed by febrile patients. It is characterized by a well- 



