Mr. Fownes on the Preparation of Hippuric Acid. 383 



may be again dissolved in hot water, a little chlorine passed, the 

 solution supersaturated with carbonate of soda, digested with animal 

 charcoal, and once more decomposed by an acid. 



It is to be observed, that hippuric acid only crystallizes in a 

 distinct and characteristic manner when pure, or at least when in 

 a condition approaching that state ; under other circumstances it 

 usually separates either as short radiated needles, or as a granular 

 crystalline powder. The latter happens when soluble salt is 

 present. 



If the urine, instead of being quite fresh, is at all ammoniacal, 

 then during the evaporation a very large quantity of ammonia is 

 disengaged, accompanied by slow effervescence, and the liquid 

 affords, as Liebig has already pointed out, benzoic acid only, with- 

 out a trace of hippuric. 



The great density of the urine of the cow is a remarkable circum- 

 stance ; one sample, affording much hippuric acid, gave the sp. gr. 

 of 1*0325, which is considerably higher than that of human urine. 

 This density is chiefly due to a most prodigious quantity of urea, 

 which is easily extracted from the brown liquid remaining after the 

 separation of the hippuric acid, by the aid of a hot strong solution of 

 oxalic acid, which throws down the slightly soluble oxalate. This 

 can be decomposed by chalk, and the urea extracted without ha- 

 ving recourse to alcohol. Besides these two substances, hippuric 

 acid, or rather hippurate of an alkali, and urea, cow-urine contains 

 a little uric acid, phosphates and other salts in tolerable abun- 

 dance. 



The constant occurrence of so much urea in the urine of all ani- 

 mals, both granivorous and flesh- eating, tends greatly to strengthen 

 the opinion, that it is by this channel almost alone that the removal 

 of those portions of the azotized constituents of the body, which 

 have been worn out, as it were, or in the act of undergoing decay, is 

 effected. It is well known that such substances, by ordinary putre- 

 faction, furnish carbonate of ammonia ; but in the body this process 

 seems to have been modified in such a manner, that in place of that 

 substance, urea or carbamide is generated, which is destitute of the 

 irritating power upon the organs which a corresponding quantity of 

 the ammoniacal salt would possess. 



It has been suggested that hippuric acid is not a direct product 

 of the animal system, but is formed by the union of benzoic acid or 

 its elements with those of lactate of urea, the benzoic acid being 

 present in the food, and the recent experiments of Mr. Garrod cer- 

 tainly countenance the opinion. But these attempts to detect ben- 

 zoic acid in the food of these animals were in the hands of Liebig 

 quite unsuccessful, and it seems unlikely that it would be found at 

 any rate in considerable quantity in such substances as grains and 

 mangel-wurzel, which, with the addition of a little hay, consti- 

 tuted the food of the cows from which such an abundant supply of 

 hippuric acid was obtained. 



There is only one other point which requires notice, and that is 

 1 the nature of the change which hippuric acid so readily undergoes 



