108 Microscopic Crystals. 



MICROSCOPIC CRYSTALS (URIC ACID AND ITS 

 DERIVATIVES). 



BY GEORGE S. BRADY, M.R.C.S., 

 Secretary to the Tyneside Naturalists' Field Club. 



{With a Tinted Plate) 



A paper on Crystallization in the November part of the Intel- 

 lectual Observer, noticing the experiments of M. Kuhlmann, 

 reminds me of a remarkable group -of salts, very little known 

 except to purely medical or chemical students, but exceedingly 

 interesting in a microscopic point of view, some short account 

 of which may be acceptable to the general reader. 



The compounds to which I refer are derived from uric (or 

 lithic) acid, a substance met with abundantly in most animal 

 excretions, but most plentifully in those of the boa constrictor, 

 and other serpents, the semi-solid excrement of which consists 

 almost entirely of this acid, in combination with ammonia. 

 From this impure urate of ammonia the acid is obtained by the 

 following process : — The material to be operated on is dissolved 

 in boiling water by the addition of caustic potash, until the 

 liquid becomes alkaline. The hot solution of urate of potash 

 thus produced is then filtered, and treated with an excess of 

 hydrochloric acid, which at once precipitates the pure uric acid 

 in a heavy crystalline form. 



The forms assumed by uric acid crystals vary very remark- 

 ably, according to the character of the solutions from which 

 they are produced, being much influenced by the nature of the 

 precipitating agent, the organic matter held in solution, and 

 various other circumstances. Like other crystals, however, the 

 finest specimens are produced the most slowly. Those occur- 

 ring naturally in human and other secretions are often strongly 

 coloured of a deep reddish brown, and have on this account 

 obtained the name of " Cayenne Pepper ,3 crystals, a designa- 

 tion, however, which may be held to have also a covert refer- 

 ence to the gouty subjects in which they most frequently occur. 

 Some of the forms in which this acid crystallizes are represented 

 at Fig. 2 of the accompanying plate. 



When uric acid is added gradually to strong nitric acid, it 

 is dissolved with effervescence, and with considerable evolution 

 of heat, owing to which, indeed, the operation may become 

 unmanageable, if not cautiously and slowly conducted. In the 

 warm liquid there shortly appear crystals of a compound called 

 alloxan and, on cooling, these increase greatly in numbers, so 

 that from 100 parts of pure uric acid, 90 to 105 parts of hydrated 



