128 Dr. W. Gregory on the Preparation of Hippuric Acid. 



There cannot, I think, be any longer a doubt that C4 N H4 O3 

 is the true formula of glycocoll, and Hor^ford has, in esta- 

 bUshing this point, at the same time confirmed and explained 

 in the most satisfactory manner the observation of Dessaignes. 



The researches of Horsford, however, have also demon- 

 strated that glycocoll is in itself one of the most interesting 

 compounds known to chemists, and it is evident that the fur- 

 ther study of this most singular body will lead to very va- 

 luable results. 



I have already stated that glycocoll is best obtained from 

 hippuric acid, but as soon as I began to prepare for this pur- 

 pose a considerable quantity of hippuric acid, I found, as all 

 who have done so must have found, that the operation as pre- 

 scribed in books is not only tedious and troublesome, but un- 

 certain. 



The usual process consists in evaporating the urine of the 

 horse or cow at a moderate temperature to about one-eighth 

 of its bulk, and adding hydrochloric acid, when on standing 

 a few hours, crystals of impure hippuric acid are deposited. 

 But it is well-known that if the temperature should rise too 

 high, although still to a point short of boiling, the hippuric 

 acid will partially or totally disappear, and benzoic acid will 

 be found in its place. Now when we bear in mind that the 

 urine contains but little hippuric acid, it is evident that to 

 obtain this acid in quantity we must operate with a very large 

 bulk of urine, and those who have done so well know how 

 tedious the evaporation is, since if we attempt to hasten it 

 by raising the temperature, we run the risk of losing the 

 whole ; and this indeed frequently happens. 



The impure, highly-coloured acid first obtained has been 

 purified by different chemists in a great variety of ways. 

 Some have used chloride of lime ; but this method is not 

 easily managed, and often converts the whole into benzoic acid. 



The last and by far the best method of purification is that 

 of Schwarz, who boils the impure acid with an excess of milk 

 of lime, and strains the alkaline liquid from the undissolved 

 lime. It passes rapidly and clear through calico, and the lime 

 retains the colouring matter, so that the addition of acid to 

 the filtered liquid causes the deposition of crystals of hippuric 

 acid nearly white. Schwarz recommends the addition of 

 chloride of calcium to the filtered or unfiltered liquid, and the 

 precipitation of the lime as carbonate by carbonate of potash 

 or soda, when the precipitated carbonate of lime carries with 

 it the last traces of colouring matter. I have not found this 

 necessary, as a repetition of the process with the milk of lime 

 never fails to yield colourless crystals. 



