EXAMINATION OF URINARY CALCULI. 129 



1, Residue : Uric acid. Confirm by making the murexide 

 test (page 101). Uric acid calculi vary in size, are quite hard, 

 and are usually colored reddish yellow or brown. 



2. Filtrate: may contain ammonium chloride. To test 

 for ammonia warm some of the filtrate with sodium carbonate 

 solution: ammonia is evolved and may be detected by its 

 odor, alkaline reaction, etc. 



II. THE POWDER TURNS BLACK BUT DOES NOT BURN. 



A slight blackening always results when the calculi are 

 heated, due to the presence of organic matter. A small por- 

 tion of the finely powdered calculus is digested by warming 

 with dilute hydrochloric acid (1 : 2) : effervescence indicates 

 the presence of carbonates. 



(a) Complete Solution. Uric acid is not present. 



(6) Incomplete Solution. The residue may contain uric 

 acid, proteids, epithelium, etc. The general appearance or 

 a microscopical examination usually enables us to decide 

 what is present. The presence of the uric acid may be 

 readily confirmed by the murexide test. 



In any case the solution is to be further investigated. 

 Warm a small portion of the filtered solution with an excess 

 of sodium carbonate solution and test for ammonia (see 

 above). Dilute the main quantity of the liquid with water, 

 filter, make faintly alkaline with ammonia, cool the fluid in 

 case it has become hot from the addition of the ammonia, and 

 acidify with acetic acid. Either an approximately clear solu- 

 tion results or a turbid one, which gradually deposits a white 

 pulverulent precipitate. 



The yellowish-white flakes which are seen in the approxi- 

 mately clear solution consist of ferric phosphate. Prove this 

 by filtering and dissolving the washed precipitate in hydro- 

 chloric acid: the solution is colored blue on the addition of 

 potassium ferrocyanide. 



