14 HIPPUEIC ACID. 



Uric acid is soluble in caustic potash; the alkaline solution 

 is precipitated by acids; the precipitate is formed of micro- 

 scopic crystals. 



When uric acid is dissolved in nitric acid, and the acid 

 solution is evaporated to dryness, and the residue is exposed 

 to the vapours of ammonia, a beautiful purple colour is 

 developed; this is known as the Murexide test for uric acid, 

 and the re-action is characteristic of its presence. 



Uric acid, when boiled with peroxide of lead, yields urea 

 amongst other products. 



Uric acid is a bibasic acid, and forms acid and neutral 

 salts with bases, only the former of which occur in the urine. 

 Urine, when allowed to cool, very often copiously deposits a 

 sediment of a yellowish or reddish colour, which by chemical 

 examination is known to be composed of a mixture of 

 urates of ammonia, magnesia, and lime. The deposit occurs 

 in consequence of the solubility of the urates not being as 

 great in cold as in hot water, and there having just been a 

 sufficient amount of water to hold them in solution at the 

 temperature of the body; when the temperature falls, they are 

 precipitated. Generally this depends upon the quantity of 

 water which is taken into the system being deficient. It is 

 often noticed after active exercise, when, a great propor- 

 tion of the superfluous water of the body having been got 

 rid of by sweating, the water of the urine is scanty. The 

 means of detecting these deposits will be afterwards described, 

 when speaking of the urine in disease.* 



Hippuric acid, C 18 H 8 N0 5 HO, is constantly found in 

 the urine of the horse, ox, and all other herbivorous animals, 

 as well as in that of man, but is not found in that of the car- 

 nivora. 



* From what has preceded it will be inferred that these deposits 

 never occur in the urine of the herbivora. 



