CHEMICAL BASIS OF THE ANIMAL BODY. 187 



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 recog- 

 nised 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 unmistakeable 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 Wohler in 1824 and specially interesting as being the 

 first known instance of a well defined synthesis effected by the animal 

 body and the starting-point for the disproval of Liebig's views as to the 

 fundamental 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-glycin. ) 



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 (O'l 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 . COOH + CH 2 (NH,). COOH - C 6 H 5 . CO. NH . CH 2 . COOH+H 2 O. 



Its constitution is further characteristically shown by its production 

 by the action of benzamide on monochlor-acetic acid : 



C 6 H 5 . CO. NH 2 + CH 2 C1. COOH = C 6 H 5 . CO . NH . CH 2 . COOH. + HC1. 



and also by that of benzoyl-chloride on glycin l : 



C 6 H 5 . CO . Cl + CH 2 (NH 2 ). COOH- C 6 H 5 . CO . NH. CH 2 . COOH +HC1. 



It may be readily obtained from the urine of horses or cows, more 

 particularly when they are out to grass. The perfectly fresh 1 * urine 



1 Baum, Zt. /. physiol. Chem. Bd. ix. (1885), S. 465. 



2 To avoid fermentative decomposition into benzoic acid and glycin. 



