URINE. 117 



stand, covered, for twenty-four to forty-eight hours. A red or 

 reddish-brown sediment of uric acid then separates, consisting 

 of crystals of the forms represented in fig. 23a, and 23b. 



2*. [Hippuric acid is regarded by Liebig 1 as an invariable 

 constituent of ordinary human urine. "All the urine taken 

 in this country from individuals living upon a mixed animal 

 and vegetable diet, contains hippuric as well as uric acid, and 

 about the same proportion of both acids. Hippuric acid may 

 be obtained in the following manner, even from proportionally 

 small amounts of fresh urine : Fresh urine is evaporated in a 

 water-bath to the consistence of a syrup ; it is then mixed with 

 some hydrochloric acid, and agitated with its own volume of 

 ether, which latter substance dissolves the hippuric acid. It 

 usually happens that the mixture does not separate spontane- 

 ously, but that the ether remains inclosed by the fluid, like 

 froth the separation of the ether takes place immediately upon 

 adding to the mixture, after having allowed it to stand at rest 

 for an hour, one twentieth part of its volume of alcohol. In 

 this case the froth disappears, and the fluid separates into two 

 layers ; the upper layer contains the hippuric acid in solution; 

 but besides it also contains urea, owing to the addition of the 

 alcohol. This upper layer is carefully removed by means of a 

 pipette or syphon, and agitated with small portions of water; 

 the water removes the alcohol and the urea, whilst the hippuric 

 acid remains in solution in the ether. By evaporating the 

 ethereal solution the hippuric acid is obtained in crystals. The 

 crystals produced are usually of a yellowish or brown colour, 

 arising from the presence of a resinous substance, which may be 

 easily and completely removed by means of charred blood. 2 



1 Lancet, June 1844. 



2 [The following is a simple method of obtaining pure crystals of hippuric acid 

 from human urine. Evaporate the urine till there is a copious deposition of salts. 

 Add strong alcohol, and place the mixture in a stoppered bottle. With the aid of a 

 gentle heat, (for instance, by placing the bottle in warm water), we ensure the solu- 

 tion of the urea, the lactates (if any are present) and the hippurates in the alcohol, 

 whilst the urates remain with the insoluble constituents. When the supernatant 

 fluid is perfectly clear, it must be decanted, evaporated veiy nearly to dryness, and 

 redissolved in hot water. If a stream of chlorine be passed through the aqueous 

 solution, the urea is destroyed ; and by gradual concentration, and the addition of a 

 little free mineral acid, we obtain crystals of hippuric acid.] 



