HIPPURIC ACID. 195 



now be carefully evaporated, and the residue which, besides free 

 acids, also contains fatty matters, must be treated with water in 

 in order to remove the latter. It sometimes happens that on the 

 addition of the water, crystals of hippuric acid at once separate 

 from the above extract-like mass; but whether this be the 

 case or not, this ethereal extract must be warmed with water, and 

 allowed to percolate through a previously well moistened filter ; 

 the filtered acid fluid may then either be gently concentrated by 

 warmth, or, if its quantity be very small, it may be left to sponta- 

 neous evaporation in a watch-glass ; crystals of hippuric acid very 

 soon separate, whose form must be determined by the microscope. 

 If much hippuric acid be present, it will sometimes separate from 

 the syrupy residue by the mere addition of hydrochloric acid, and 

 can be distinguished from uric acid and other crystalline substances 

 by the microscope. 



Physiological Relations. 



Occurrence. Hippuric acid was first recognised by Liebig as an 

 independent acid in horses 3 urine where it had previously been mis- 

 taken for benzoic acid; it has been subsequently found in the urine of 

 many graminivorous animals, as, for instance, oxen, elephants, goats, 

 hares, sheep, &c. It is, however, singular that, according to Wohler, 

 it is entirely absent in the urine of calves while suckling, although the 

 fluid contains allantoine, uric acid, and urea, (see p. 176.) In the 

 urine of the pig neither Boussingault,* nor von Bibra,f could 

 discover any hippuric acid. Liebigf was the first who recognised 

 its presence in healthy human urine, in which it principally occurs 

 after the use of vegetable food : according to him it exists in human 

 urine, in about the same quantity as uric acid, while according to 

 Bird, the hippuric acid most commonly stands to the uric acid 

 in the ratio of 1 : 3. 



I have already remarked in p. 83. that benzoic acid never occurs 

 in fresh horses 5 urine, and that it is merely a product of the 

 decomposition of that fluid ; I can, however, perfectly confirm the 

 observation of Schmidt, || that hippuric acid is occasionally, although 



* Ann. de Chim. et de Phys. 3 Ser. T. 15, pp. 97-104. 



f Ann. de Ch. u. Pharm. Bd. 53, S. 98-112. 



$ Ibid. Bd. 37, S. 257. 



London Medical Gazette, vol. 34, p. 685: [In his Urinary Deposits, &c. 

 3rd edit., p. 96, this opinion is considerably modified. We there find that " its 

 quantity in health is not constant, and always, unless after the ingestion of benzoic 

 or cinnamic acid, very much less than has been stated." G. E. D.] 



U Entwurf u. s. w. S. 39. 



o 2 



