CHEMICAL BASIS OF THE ANIMAL BODY. 1261 



Identification of uric acid. The crystalline forms afford 

 some clue, but are so numerous that some forms which may 

 at any time present themselves are scarcely characteristic. 

 The rhombic tables, 'dumb-bell' and 'whetstone' crystals, are 

 on the whole most characteristic. 



i. Murexide test. The suspected substance is treated in a 

 porcelain dish with a few drops of strong nitric acid and evapo- 

 rated carefully to dryness, by preference on a water-bath. The 

 residue thus obtained will, if uric acid is present, be of a 

 yellow or more frequently red colour, which turns to a brilliant 

 reddish purple on exposure to the vapours of ammonia. On 

 the subsequent addition of a drop of caustic soda the colour 

 is changed to a reddish blue. This disappears on warming, 

 whereas the similar colour obtained by the above process from 

 guanine does not. This is an important means of distinguish- 

 ing betwen the two substances. 



The test depends on the formation of murexide, which is the acid 

 ammonium salt of purpuric acid, the acid itself being unknown in 

 the free state. Uric acid is decomposed when heated with nitric 

 acid, yielding alloxan and then alloxantin; by the action of ammonia 

 the latter is converted into murexide, (NH 4 )C 8 H 4 iSr 5 6 -)- H 2 0. 



The murexide test is so striking and characteristic that it 

 suffices completely for the identification of uric acid. The fol- 

 lowing tests may be applied in confirmation if required, but not 

 for the purposes of initial detection. 



ii. Schiff's reaction. The substance is dissolved in sodium 

 carbonate, and a drop is then placed on filter paper previously 

 moistened with nitrate of silver. A yellow or almost black 

 coloration, due to the formation of metallic silver by reduction 

 of its nitrate, is at once obtained. 



iii. When a solution of uric acid in caustic soda is boiled 

 with a small amount of Fehling's fluid, reduction occurs with 

 production of a greyish precipitate of urate of cuprous oxide. 

 If the copper salt is in excess red cuprous oxide is obtained. 



Estimation of uric acid in solutions (urine'). The accurate 

 quantitative determination of uric acid is a matter of some 

 difficulty; for details some standard work should be consulted. 



An inspection of the constitutional formula of uric acid 

 suggests at once that it contains the residues of two molecules 

 of urea. This corresponds to the fact that nearly all the possible 

 decompositions of uric acid yield either a molecule of urea along 

 with the more specific product of the decompositions, frequently 

 itself derivative of urea, or else some substance which can by 

 further change be decomposed into urea and some other product 

 which is as before frequently a derivative of urea. The close 

 chemical relationship of urea and uric acid is thus clearly 

 shewn. 



