CONCRETIONS. 431 



becomes whitened by incineration, or, finally, it may become 

 blackened without, or with very slight diminution in size. 



1. If the tested portion disappear entirely, it may consist of 

 uric acid, urate of ammonia, or both, hippurate or benzoate of 

 ammonia, uric oxide, cystin, cholesterin, biliphsein, fibrin, albu- 

 men, or hair. 



a. It is uric acid if, when carbonized by exposure to heat 

 on platinum foil, it gives off a peculiar animal odour resembling 

 hydrocyanic acid, and diminishes to a scarcely visible residue ; 

 when a portion of the concretion heated with nitric acid dis- 

 solves therein with effervescence, and, after evaporation nearly 

 to dryness, assumes, on the addition of ammonia, a beautiful 

 purple tint; and when it dissolves thoroughly in a weak solution 

 of caustic potash or its carbonate, and at the same time is 

 insoluble in water, alcohol, and dilute hydrochloric acid. 



b. It is urate of ammonia (which seldom occurs alone) if it 

 behaves before the blowpipe, and with nitric acid, in just the 

 same manner as uric acid, but at the same time, evolves an 

 ammoniacal odour when heated on platinum foil, and develops 

 a considerable amount of free ammonia on being triturated with 

 caustic potash ; and if it dissolves in boiling water. 



c. It is uric or xanthic oxide if it burns without the pecu- 

 liar odour of uric acid, if it dissolves without effervescence in hot 

 nitric acid, and the evaporated solution when treated with ammonia 

 assumes, not a rich purple, but a dark yellow colour and if, 

 finally, it is insoluble in a dilute solution of carbonate of potash. 



d. It is cystin if it burns before the blowpipe with a blue 

 flame, emitting at the same time, a pungent acid odour; if when 

 treated with nitric acid it assumes (instead of a purple or yellow) 

 a brown tint; if it dissolves in a dilute solution of carbonate of 

 potash, and in caustic ammonia, and if it crystallizes from 

 the latter in six-sided plates, easily recognized under the micro- 

 scope. 1 



e. It contains benzoate of ammonia if alcohol extracts a 

 substance which, after evaporation is soluble in water, and if, 



1 [In order to detect the presence of cystin a portion of the suspected calculus 

 should be dissolved in a strong solution of caustic potash, and a solution of acetate 

 of lead added in excess ; the liquid must then be heated to the boiling point. If cystin be 

 present insoluble sulphuret of lead is formed, which at first gives the liquid the aspect 

 of ink, but is shortly precipitated, while oxalate of ammonia remains in solution.] 



