UEINE 119 



a few drops of acetic acid distinguishes it from albmnin, for which it is 

 liable to be mistaken. 



Some of the facts described in the foregoing exercises have been ab-eady 

 dwelt upon in the preceding lesson. They are, however, conveniently grouped 

 together here, as all involve the use of the microscope. 



We have now studied urea, the principal nitrogenous constituent 

 of urine, at some length. There are stiU left for our consideration a 

 number of other niorogenous constituents, the most important of which 

 are uric acid, hippuric acid, and creatinine. 



UBIC ACID 



Tlric acid (C5N4H4O3) is in mammals the mediiim by which only a 

 small quantity of nitrogen is excreted from the body. It is, however, 

 in birds and reptiles the principal nitrogenous constituent 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 ciystals which form are deeply 

 tinged with urinary pigment, and though by repeated solution in 

 caustic soda or potash, and reprecipitation 

 by hydrochloric acid, they may be obtained 

 fairly free from pigment, pure uric acid is 

 more readily obtained from the soUd urine of 

 a serpent or bird, which consists principally 

 of the acid ammonium urate. This is dis- 

 solved in soda, and then the addition of 

 hydrochloric acid produces as before the 

 crystalMsation of m-ic 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 wric acid assumes when precipitated from hmnan urine, 

 either by the addition of hydrochloric 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. 43). 



The murexide test which has just been described among the 



