186 HIPPURIC ACID. 



The acid is usually prepared by the above decomposition of 

 hippuric acid, which is readily effected by a short boiling with 

 mineral acids or, less readily, with caustic alkalis. It is also 

 obtained by the dry distillation of gum-benzoin from which the 

 acid separates by sublimation. The sublimed acid generally 

 crystallises in fine needles which are light and glistening. It is 

 soluble in about 200 parts of cold or 25 of boiling water and very 

 soluble in alcohol, ether, and petroleum-ether, 1 in which latter 

 hippuric acid is insoluble. When precipitated from solutions, either 

 by cooling or the addition of acids to its salts in the cold, the 

 crystalline form is usually much less distinct. 



Apart from the crystalline form benzoic acid is characterised by 

 its property of readily subliming, even at 100, thus resembling 

 leucin and differing markedly from hippuric acid. As a result of 

 this it passes off freely in the vapours arising from its boiling 

 aqueous solutions, so that in concentrating fluids, such as urine, in 

 which its presence is conjectured, they should be first rendered 

 alkaline with sodium carbonate, thus forming a non-volatile salt. 

 Benzoic acid may be additionally recognised by the following test : 

 when treated with a little boiling nitric acid and evaporated to 

 dryness, the residue thus obtained yields, on further heating, an 

 unmistakable odour of nitrobenzol. 



When introduced into the body benzoic acid is readily and 

 largely converted into hippuric acid, while at the same time small 

 quantities of succinic acid may at the same time make their 

 appearance. The chief interest in the acid centres in the above 

 relationship to hippuric acid, a fact discovered by Wbhler in 1824 

 and specially interesting as being the first known instance of a 

 well defined synthesis effected by the animal body, and the start- 

 ing-point for the disproval of Liebig's views as to the funda- 

 mental difference in the metabolic processes of animal and plant 

 tissues. 



2. Hippuric acid. C 7 H 6 2 . [C 6 H 5 . CO . NH . CH 2 . COOH.] 

 (Benzoyl-gly cin. ) 



This acid is found in considerable quantities (1'5 2*5 p.c.) in 

 the urine of herbivora, and also, though to a much smaller amount 

 (0*1 1*0 grm. per diem) in the urine of man. It is undoubtedly 

 formed in the body by the union, with dehydration, of benzoic 

 acid and glycin (see 419.) This mode of its formation may be 

 readily observed out of the body by heating together dry benzoic 

 acid and glycin in sealed tubes to 160. 



C 6 H 5 .COOHH-CH 2 (NH 2 ).COOH=C 6 H S .CO . NH.CH 2 .COOH4-H 2 0. 



1 Petroleum-ether consists ordinarily of a mixture of the more volatile hydro- 

 carbons obtained by distillation during the fractionating of crude petroleum, and 

 boils up to about 120. The most volatile petroleum-ether boils up to about 80. 



