HIPPURIC ACID. 855 



hydrocyanic and formic acids. It exists in large quantity in the 

 urine of herbivorous mammifers, and was first derived from 

 that of the horse ; hence its name. 



To prepare it the fresh urine of the cow or horse is evaporated 

 by a gentle heat, care being taken not to allow it to enter into 

 ebullition ; it is then made sharply acid by means of hydro- 

 chloric acid, and left to itself. The hippuric acid which crys- 

 tallizes from this liquid is coloured and impure, but may be 

 purified by adding a little hydrochloric acid to it with portions 

 of chloride of lime till the odour and colour disappear. 



Hippuric acid crystallizes in pretty large semi-transparent 

 prisms of four sides and dihedral summits, which have a slightly 

 bitter taste, and redden litmus strongly. They fuse by heat 

 into an oily liquid without loss of weight ; at a higher tempe- 

 rature the acid undergoes decomposition and is converted into 

 benzoic acid and benzoate of ammonia, which distil over in red 

 drops, diffusing an agreeable odour like that of the Tonquin 

 bean, which is occasioned by an oily product of the reaction ; 

 while towards the end of the distillation hydrocyanic acid 

 appears and a porous residue of charcoal. Cold water dissolves 

 TOO- of hippuric acid, hot water dissolves it very abundantly; it 

 is more soluble in alcohol, and very slightly soluble in 

 ether. 



Hydrochloric acid dissolves hippuric acid without decompos- 

 ing it; nitric acid converts it almost immediately into benzoic 

 acid. Peroxide of manganese and sulphuric acid convert it, 

 with the aid of heat, into carbonic acid, ammonia and benzoic 

 acid. Boiled with puce coloured oxide of lead, hippuric acid 

 is converted into benzamide and carbonic acid. 



After the urine of the horse is left to itself for a long time, or 

 evaporated rapidly, it contains benzoic acid and no hippuric 

 acid. M. Liebig expresses an opinion that hippuric acid is % not 

 a product of the animal economy, but comes from the food of 

 the animals, in which it may exist ready formed. For it is 

 observed that the urine of horses living on green food, always 

 furnishes hippuric acid, while the urine of animals fed on dry 

 herbs or hay, which has undergone a kind of fermentation, 

 contains only benzoic acid. Mr. Alexander Ure has since made 

 the curious observation that benzoic acid taken internally by 

 man is discharged in the urine as hippuric acid, while the pro- 



