EXCRETION. 



477 



&copic evidence of its crystalline structure: (1) The Murexide test, 

 \v hich consists of evaporating to dryness a mixture of strong nitric acid 

 ADd uric acid in a water bath. This leaves a yellowish-red residue of 

 4-Hoxan (4112^204) and urea, and on addition of ammonium hydrate, a 

 beautiful purple color (ammonium purpurate, CsH^NH^NsOe), deep- 

 ened on addition of caustic potash, takes place. (2) Schiff's test con- 

 sists of dissolving 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. 



Hippuric Acid (CgHgNOs) has long been known to exist in the 

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



Fig. 801. Various forms of uric acid crystals. 



Fig. 302. Crystals of hippuric acid. 



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 

 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, CtHeC^ 

 (Benzoic acid) + C 2 H 5 N0 2 (Glycin) = C 9 H 9 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. 302). It is more 

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

 water. It is soluble in alcohol. 



Pigments. The pigments of the urine are the following: 1. Uro- 

 chrome, a yellow coloring matter, giving no absorption band; of which 

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

 ence 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-marked 

 spectroscopic absorption band at the junction of green and blue, best 



