16 EXTRACTIVE AND PIGMENTARY MATTERS. 



its way into the urine ; and it was therefore assumed that the 

 food upon which the herbivora are fed must contain benzoic 

 acid; the incorrectness of this hypothesis has been proved 

 by the fact, that the urine of cows contains as much hippuric 

 acid when they are fed on hay as on mangel wurzel ; now the 

 latter substance has been carefully analyzed, and found to 

 contain no benzoic acid. It is probable that benzoic acid is 

 a peculiar product of the oxydation of the tissues in the herbi- 

 vora, and that it is afterwards converted into hippuric acid. 



It has been stated by some chemists, that the urine of 

 horses which are subjected to hard work contains no hip- 

 puric acid, but much benzoic acid instead; the last part of 

 the statement seems doubtful, though it would appear that 

 hard work checks the excretion of hippuric acid, and in- 

 creases the quantity of urea excreted. This is a question 

 which is entirely unsettled, as different chemists have made 

 the most contradictory statements. 



The quantity of hippuric acid present in the urine varies 

 greatly; from the analyses which have been published by 

 Von Bibra, Boussingault, and other chemists, it would appear 

 that urea and hippuric acid bear a certain relation to one 

 another, for in those analyses where the quantity of urea 

 is small, the quantity of hippuric acid is almost always large, 

 and vice versa. The hippuric acid varies from 5 to 15 parts 

 in 1000 of urine of healthy horses. 



The term extractive matters is applied by chemists to cer- 

 tain of the constituents of urine, which are soluble in alcohol 

 and water, and the nature of which has not been thoroughly 

 ascertained. Such substances as creatine, creatinine, lactic 

 acid, benzoic acid, colouring matters, &c., are comprised 

 under this term. 



The pigmentary, or colouring matters of the urine, have 

 such extremely complex chemical relations, and their ascer- 



