194 CONJUGATED ACIDS. 



separates in a colourless state. Moreover, by mere, but often 

 repeated, boiling of horses 5 urine, and of the hippuric acid separated 

 from it with milk of lime, we may obtain it free from colour. 



Perfectly fresh urine must be used, since horses' urine, even at 

 an ordinary temperature, very soon begins to decompose ; and it 

 then no longer yields hippuric but benzoic acid. 



Tests. Hippuric acid presents such characteristic properties, 

 that if it be once pretty well freed from other substances, it can 

 scarcely be confounded with any other acid, except, perhaps, ben- 

 zoic acid, if the latter be contaminated with organic colouring, and 

 nitrogenous matters ; since in the pure state, the two acids act so 

 differently when exposed to heat that it is impossible to confound 

 one with the other. 



When they occur in an impure state, they may be distinguished 

 from one another by attention to the following points. 



Hippuric acid, which is far less soluble in ether than benzoic 

 acid, crystallises from hot saturated solutions in needles or prisms, 

 while benzoic acid crystallises in scales. The latter often causes 

 such a solidification of the whole fluid, that the vessel after cooling 

 may be inverted without the escape of a single drop. Further, on 

 the addition of acids to solutions of their salts, hippuric acid is at 

 once precipitated in needles or spangles, while benzoic acid gives 

 rise to a milky turbidity before it crystallises. On rapidly evapo- 

 rating an acid fluid in a basin covered over with paper, delicate 

 glistening scales may be observed on its lower surface if benzoic 

 acid be present, but not if hippuric acid alone be present in the 

 fluid. The microscope, however, affords the best means of dis- 

 tinguishing these acids from one another, by comparing their crys- 

 talline forms in accordance with the directions given in pp. 83, and 

 188. With such an examination, it is impossible that these acids 

 can be confounded. 



In order to detect small quantities of hippuric acid in animal fluids, 

 we must be especially careful that such fluids are fresh, since if this 

 be not the case, the hippuric acid will have become changed into ben- 

 zoic acid, which on evaporation for the most part escapes with the 

 aqueous vapour; if, however, the animal fluid be still perfectly un- 

 decom posed, it must be evaporated to almost the consistence of a 

 syrup and then extracted with alcohol of specific gravity, 0.83 ; a 

 little oxalic acid must be added to the alcoholic extract during its 

 evaporation, which must be continued till it assumes a syrupy con- 

 sistence ; the residue must then be extracted with ether to which 

 |th of its volume of alcohol has been added. This extract must 



