HIPPURIC ACID. 29 



cupric oxide and other metallic oxides and salts. When uric acid is moistened 

 with nitric acid, the excess of acid gently evaporated, and the residue after 

 cooling breathed on, and then held over strong ammonia, a bright red colour 

 is produced, which is due to the formation of murexide ; if potash or soda be 

 added instead of ammonia, the colour produced is violet. 



Allantoin exists, along with uric acid and urea, in the 

 urine discharged during the first few days of life. 



Hippuric acid, which in the urine of many herbivorous 

 mammalia replaces uric acid, also occurs in human urine 

 in very small proportion. It is of importance as affording 

 a channel for the discharge of glycin from the organism. 

 Taurin, in like manner, appears in the urine as Tauro- 

 carbamate. The body cystic oxide or cystin, which also 

 contains sulphur, occurs occasionally either as a crystalline 

 deposit or as a concretion. Its physiological relations are 

 unknown. 



Hippuric acid (C 9 H 9 NO 8 ) occurs in very small proportions (less than o f i 

 per cent. ) in human urine or in that of the carnivora, but so abundantly as 

 alkaline hippurates in that of herbivora, that on the addition of hydrochloric 

 acid it crystallizes out. It is obtained by boiling the urine of the horse or the 

 cow with milk of lime, filtering, concentrating the filtrate, and adding hydro- 

 chloric acid. It crystallizes in four-sided prisms, which have their edges bevelled 

 off at the ends. Hippuric acid is scarcely soluble in cold water, more readily 

 in hot, but its salts are very soluble. It appears in the urine of man and 

 other non-herbivorous animals, whenever benzoic acid (C 7 H 6 O 2 ) enters the 

 organism, glycin being taken up and water given off. C 7 H 6 O 2 -|-C 2 H 3 (NH 2 ) 

 O 2 =C 9 H 9 NO 3 +H 2 O. On the other hand, it very readily undergoes decom- 

 position, yielding benzoic acid and glycin whenever urine containing it becomes 

 putrid. In the formation of hippuric acid from benzoic acid in the living 

 organism the glycin produced in the liver takes part, but it has not yet been 

 proved that the process by which it is normally produced in such large 

 quantity in herbivora is of the same kind ; it has, however, been shown 

 that sufficient sources of benzoyl exist in the food of such animals. As 

 regards the origin of hippuric acid in the carnivora and in man nothing is 

 known. In all animals of which the urine contains much hippuric acid (e.g. t 

 in the horse), " indigo-producing substance " is also present in relatively large 

 quantities. 



The urine also contains an organic base, creatinin, the 

 percentage of which depends upon the quantity of creatin 

 taken as food. 



