556 



THE URINE. 



[ CH. xxxvii. 



between 'ammonium carbonate, ammonium carbamate and urea 

 The loss of one molecule of water from ammonium carbonate 

 produces ammonium carbamate ; the loss of a second molecule of 

 water produces urea 



Q = C< O.NH 4 



[Ammonium carbonate] 



[Ammonium carbamate] 



[Urea or carbamide] . 



Uric Acid. 



Uric Acid (C 5 N 4 H 4 3 ) is in mammals, the medium by which 

 a very small quantity of nitrogen is excreted from the body. It 

 is, however, in birds and reptiles the principal nitrogenous con- 

 stituent of their urine. It is not 

 present in the free state, but is 

 combined with bases to form 

 urates. 



It may be obtained from human 

 urine by adding 5 c.c. of hydro- 

 chloric acid to 100 c.c. of the 

 urine, and allowing the mixture to 

 stand for twelve to twenty-four 

 hours. The crystals which form 

 are deeply tinged with urinary 

 pigment, and though by repeated 

 solution in causti? soda or potash, 

 and precipitation by hydrochloric 

 acid, they may be obtained fairly 

 free from pigment, pure uric acid is more readily obtained from 

 the solid urine of a serpent or bird, which consists principally 

 of the acid ammonium urate. This is dissolved in soda, and 

 then the addition of hydrochloric acid produces as before the 

 crystallisation of uric acid from the solution. 



The pure acid crystallises in colourless rectangular plates or 

 prisms. In striking contrast to urea it is a most insoluble sub- 

 stance, requiring for its solution 1,900 parts of hot and 15,000 

 parts of cold water. The forms which uric acid assumes when 

 precipitated from human urine, either by the addition of hydro- 

 chloric acid or in certain pathological processes, are very various, 

 the most frequent being the whetstone shape ; there are also 

 bundles of crystals resembling sheaves, barrels, and dumb-bells 

 (see fig. 433). 



The murexide test is the principal test for uric acid. The test 

 has received the name on account of the resemblance of the colour 



Kg. 433. Various forms of uric acid 

 crystals. 



