URIC ACID. 209 



in 20 parts of water till ammoniacal fumes cease to be evolved. A 

 current of carbonic acid is now passed through the solution till the 

 fluid almost ceases to have any alkaline reaction ; the precipitated 

 urate of potash is washed with cold water till it begins to dissolve ; 

 on now dissolving this potash-salt in a solution of potash, warming 

 it, arid pouring it into an excess of warmed hydrochloric acid, we 

 obtain a precipitate of pure uric acid. 



Tests. Uric acid possesses such characteristic properties, and 

 differs in so many respects from all other substances occurring in 

 the animal body, that it can hardly be confounded with any other 

 substance, unless possibly with xan thine and guanine (see p. 169 and 

 p. 171) ; and from these it may be distinguished with extreme 

 readiness and certainty, by the relation of its alkaline salts towards 

 carbonic acid and the alkaline bicarbonates. Uric acid is, how- 

 ever, principally distinguished from all other organic substances 

 (except perhaps from caffeine) by the murexide test, that is to say. 

 by the purplish red residue which its solution in nitric acid leaves 

 on evaporation ; the further addition of caustic potash should, 

 however, never be omitted, by which a yet more distinct reaction 

 the development of a splendid violet tint is induced. 



All chemical means would, however, frequently fail, and the 

 presence of uric acid would remain undetected, where the quantity 

 of matter to be examined is so small as to afford very slight traces 

 of uric acid, if we were not in possession of the microscope, whose 

 use in physiological chemistry is inestimable. No substance 

 presents such characteristic and so easily determinable crystalline 

 forms under the microscope as uric acid, or crystallises so readily. 

 Hence it may be detected with ease and certainty by all who are 

 moderately familiar with the use of the microscope, and with the 

 various forms which the crystals of uric acid present. Although, 

 to beginners, the form of the crystals of uric acid appears truly 

 protean, yet with some knowledge of crystallography one form 

 may very readily be deduced from another. We must, however, 

 here refer to the admirable analysis of the crystallogenesis and 

 crystallography of uric acid, as given by Schmidt.* For those who 

 are acquainted with crystallography, it is sufficient to give the 

 symbols for the perfect combination of the crystal of uric acid : 



oo P2. ooP. oo P2. ooPoo. OP. 



For the benefit of those who are unlearned in crystallography, 

 we will remark that uric acid when it gradually and spontaneously 



* Entwurf, u. s. w. S. 28-34. 



