196 CONJUGATED ACIDS. 



very rarely, entirely absent, and that in its place there is found an 

 oily matter which when heated with caustic alkalies yields ben- 

 zine. 



Attempts have been made to refute Liebig^s assertion that hip- 

 puric acid always exists in human urine, at least after the use of 

 vegetable food ; but although I formerly did not succeed in detect- 

 ing this acid in my own urine during a purely vegetable diet, I 

 have since very frequently convinced myself, both from experi- 

 ments both on large and on small quantities of urine, that this 

 acid is constantly present during the use of a mixed diet. The 

 presence of hippuric acid may, however, readily escape our notice 

 if we evaporate the acid urine too rapidly, after the acid has been 

 converted into benzoic acid ; on the other hand, we need be under 

 no apprehension that the hydrochloric acid which is added will 

 decompose the hippuric acid, as in order to effect any change on it, 

 a very concentrated acid and prolonged boiling are required. 



Hippuric acid is not found in the urine of carnivorous animals, 

 but it has probably not been sought for with sufficient care and 

 attention. In the urine of tortoises neither J. Miiller and Magnus,* 

 nor Marchandf could detect hippuric acid ; I have, however, con- 

 vinced myself with the greatest certainty, and on many occasions, 

 that hippuric acid is present in addition to uric acid in the urine 

 of Testudo grceca. 



Magnus was unable even to find uric acid in the urine of Tes- 

 tudo nigra s. elephantopus, while Marchand found uric, but no hip- 

 puric acid in the urine of T. tabulata ; I probably worked with 

 much larger quantities, and certainly always used fresh urine. My 

 specimens of Testudo graca were fed with lettuce and other vegeta- 

 bles. The urine may be easily collected by placing the animal on 

 its back in a dish; when the bladder is moderately filled, the 

 animal very soon spontaneously passes its urine, which, besides 

 alkaline urates and hippurates, contains free hippuric acid. Without 

 the preliminary addition of a stronger acid, we may obtain the 

 hippuric acid in a crystalline state by the addition of water to the 

 ethereal extract, and sufficiently pure to admit of our accurately 

 studying its behaviour when exposed to heat, its solubility, &c. ; 

 if, however, oxalic or hydrochloric acid were used in the process, in 

 the manner which has been already explained, we should obtain 

 much larger quantities of hippuric acid. 



In morbid human urine I have almost always been able to detect 



* Mailer's Archiv. 1835. S. 214. 



t Journ. f. pr. Ch. Bd. 34. S. 244-247. 



