478 PHYSIOLOGICAL CHEMISTRY. 



Uric acid is not dissolved by heat, nor by acetic or hydrochloric 

 acid, but dissolves on the addition of caustic potash and burns on 

 platinum foil without leaving a residue ; it is recognized by the 

 murexid test. 



Oystin is insoluble in water and alcohol, but soluble in mineral 

 acids and in caustic alkalies; from either solution it is reprecipitated 

 by neutralizing. Cystin contains 26 per cent, of sulphur, which 

 causes the formation of black sulphide of lead when cystin is boiled 

 with caustic potash to which a few drops of solution of lead acetate 

 have been added. 



Urinary calculi are solid deposits of larger or smaller size formed 

 from the urine within the tracts (kidneys, ureter, bladder, and urethra). 



The chemical composition of the calculi is generally that of either 

 of the above-named unorganized sediments, and their nature can 

 easily be determined by using the following method : 



Make a section through the centre of the calculus, scrape some of 

 the substance off, powder it finely, and heat some of it on platinum 

 foil. It may either burn away completely (uric acid, urate of ammo- 

 nium, cystin, xanthin) or may be partially combustible (urates or 

 oxalates), or may be incombustible (chiefly phosphates). A slight 

 blackening occurs generally, even in heating a calculus consisting of 

 incombustible matter, and is due to the presence of traces of organic 

 urinary constituents. 



If completely combustible, digest a little of the powder with dilute 

 hydrochloric acid ; cystin and xanthin are dissolved, uric acid remains 

 undissolved. Apply murexid test for uric acid, the above-mentioned 

 lead test for cystin, and for xanthin test by dissolving a little of the 

 calculus in nitric acid and evaporating to dryness, when in the 

 presence of xanthin a bright-yellow residue will be left, which becomes 

 violet-red when treated with caustic potash. In case uric acid has 

 been found, it may be in combination with ammonia, which may be 

 verified by heating the powder with a little caustic potash, when 

 ammonia gas is liberated, which may be recognized by its action on 

 red litmus-paper, odor, etc. 



If partially combustible or incombustible, digest some of the powder 

 with dilute hydrochloric acid. If it dissolves completely, uric acid is 

 not present. If a residue be left, apply the murexid test. To a 

 portion of the solution add ammonium molybdate and heat; a yellow 

 precipitate indicates phosphoric acid. To another portion add am- 

 monia water and then excess of acetic acid ; a white pulverulent 



