750 UREA AND ITS ALLIES. [APP. 



wards that of rhombic tables 1 . When impure it crystallises readily, 

 but then possesses a yellowish or brownish colour. In water it is very 

 insoluble (1 in 14,000 or 15,000 of cold water); aether and alcohol 

 do not dissolve it appreciably. On the other hand, sulphuric acid takes 

 it up without decomposition, and it is also readily soluble in many salts 

 of the alkalis, as in the alkalis themselves. Ammonia however scarcely 

 dissolves it. 



Salts of Uric acid. Of these the most important are the acid 

 urates of sodium, potassium, and ammonium. The sodium salt crystallises 

 in many different forms, these not being characteristic, since they are 

 almost the same for the corresponding compounds of the other two 

 bases. It is very insoluble in cold water (1 in 1100 or 1200), more 

 soluble in hot (1 in 125). It is the principal constituent of several 

 forms of urinary sediment, and composes a large part of many calculi ; 

 the excrement of snakes contains it largely. The potassium resembles 

 the sodium salt very closely, as also does the compound with 

 ammonium ; the latter occurs generally in the sediment from alkaline 

 urine. 



Preparation. Usually from guano, or snake's excrement. From 

 guano by boiling with caustic potash (1 part alkali to 20 of water) as 

 long as ammonia is evolved. In the nitrate a precipitate of acid urate 

 of potassium is formed by passing a current of carbonic anhydride ; 

 this salt is then washed, dissolved in a caustic potash and decomposed 

 by carefully pouring its solution into an excess of hydrochloric acid. 



The presence of uric acid is recognized by the following tests. The 

 substance having been examined microscopically, a portion is evaporated 

 carefully to dry ness with one or two drops of nitric acid. The residue 

 will, if uric acid is present, be of a red colour, which on the addition of 

 ammonia turns to purple. This is the murexide test, and depends 

 on the presence of alloxan and alloxantin in the residue. Schiff 2 has 

 given a delicate reaction for uric acid. The substance is dissolved 

 in sodic carbonate, and dropped on paper moistened with a silver 

 salt. If uric acid be present a brown stain is formed, due to the 

 reduction of the carbonate of silver. An alkaline solution of uric acid 

 can, like dextrose, reduce cupric sulphate, with precipitation of the 

 cuprous oxide. 



Uric acid resists very largely the action of even strong acids and 

 alkalis, exhibiting in this respect a marked difference from urea. It 

 might therefore perhaps be supposed that urea residues do not pre-exist 

 in uric acid ; nevertheless by oxidation uric acid does give rise not only 



1 See Ultzmann and K. B. Hoffman, Atlas der Harnsedimente, Wien, 1872. 



2 Ann. d. Chem. u. Pharm. Bd. 109, S. 65. 



