URFNK K>7 



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

 to be mistaken. 



Some of the facts described in the foregoing exercises have been already 

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

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



URIC ACID 



Uric Acid (C 5 N 4 H 4 O 3 ) is in mammals the medium 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 crystals 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 solid 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 

 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 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. 46). 



The murexide test which has just been described among the 

 practical exercises is the principal test for uric acid. The test has 

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

 purple of the ancients, which was obtained from certain snails of the 

 genus Murex. 



Another reaction that uric acid undergoes (though it is not appli- 

 cable as a test) is that on treatment with certain oxidising reagents 



