460 Prof. Liebig on the Constitution of 



These proportions correspond exactly with the formula of 

 benzoic acid. 



With respect to the question of the presence of acetic acid 

 and benzoic acid in fresh urine, no doubt can be entertained 

 that the latter is not contained, as benzoic acid, in fresh urine, 

 since it has been proved by the experiments of Ure and Keller, 

 that crystallized benzoic acid becomes in the organism con- 

 verted into hippuric acid, and appears in the urine as hippu- 

 rate of soda. Now, as we also know that the hippuric acid 

 contained in the urine of herbivorous animals passes into de- 

 composition upon the putrefaction of their urine, and that 

 benzoic acid is one of the products of this decomposition, we 

 may safely and positively assume that the benzoic acid in pu- 

 trid human urine has the same origin, and that consequently 

 it must exist in fresh urine in the form of hippuric acid. This 

 has been fully confirmed by the results of minute examina- 

 tions. All the urine taken in this country from individuals 

 living upon a mixed animal and vegetable diet, contains hip- 

 puric acid, besides uric acid, and about the same proportion 

 of both acids. Hippuric acid may be obtained in the follow- 

 ing manner, even from proportionally small amounts of fresh 

 urine : — Fresh urine is evaporated in a water-bath to the con- 

 sistence of a syrup ; it is then mixed with some hydrochloric 

 acid, and agitated with its own volume of aether, which latter 

 substance dissolves the hippuric acid. It usually happens 

 that the mixture does not separate spontaneously, but that the 

 a&ther remains inclosed by the fluid, like froth ; the separation 

 of the aether takes place immediately upon adding to the mix- 

 ture, after having allowed it to stand for an hour, one-twen- 

 tieth part of its volume of alcohol. In this case the froth dis- 

 appears, and the fluid separates into two layers ; the upper 

 layer contains the hippuric acid in solution, but it also con- 

 tains urea, owing to the addition of the alcohol. This upper 

 layer is carefully removed by means of a pipette or siphon, 

 and agitated with small portions of water; the water removes 

 the alcohol and the urea, whilst the hippuric acid remains in 

 solution in the aether. By evaporating the asthereal solution 

 the hippuric acid is obtained in crystals, which are usually of 

 a yellowish or brown colour, imparted to them by the pre- 

 sence of a resinous substance, which may be easily and com- 

 pletely removed by means of charred blood. 



In its pure state the hippuric acid obtained from human 



