UREA. 157 



March and, taking up 2 atoms of water of crystallisation) is also 

 obtained by the direct union of the constituent parts, and forms, 

 as far as the unaided eye can perceive, long thin plates or prisms ; 

 under the microscope it is usually seen in hexagonal plates, similar 

 to those of nitrate of urea, interspersed occasionally with four-sided 

 prisms with planes of truncation proceeding from the broader sides 

 of the rectangular section. The form of this oxalate, like that of 

 the nitrate of urea, belongs to the monoclinometric system. This 

 salt has an acid taste, dissolves at 16 in 22*9 parts of water and in 

 62*5 of alcohol ; it is precipitated from its aqueous solution by 

 an excess of oxalic acid. On exposure to heat it is decomposed 

 into carbonate of ammonia and cyanuric acid. 



Like glycine, urea also unites with salts, which hold it in such 

 firm combination, that not only does no decomposition ensue when 

 their solutions are boiled, but even oxalic and nitric acids fail to 

 separate the urea from some of their compounds (Werther*). 



On mixing concentrated solutions of urea and nitrate of silver, 

 there are formed thick prisms with a rhombic base which are readily 

 soluble in water and alcohol =C 2 H 4 N 2 O. 2 .AgO.NO 5 . On the 

 addition of a solution of soda to the solution of these crystals, a 

 yellow precipitate is obtained = 5 AgO + 2C 2 H 4 N 2 O 2 . Besides 

 these, Werther has also obtained the following combinations: 

 C 2 H 4 N 2 O 2 + 2AgO.NO 5 ; CaO.NO 5 + 3C 2 H 4 N 2 O 2 ; MgO.NO 5 + 

 2C 2 H 4 N 2 O 2 ; NaO.NO 5 + C 2 H 4 N 2 O 2 + 2HO; NaCl + C 2 H 4 N 2 O 2 + 

 3HO, crystallising in deliquescent rhombic prisms ; 2HgCl + 

 C 2 H 4 N 2 O 2 , flat prisms glistening like mother-of-pearl. Urea can- 

 not be separated from the solutions of these compounds either by 

 nitric or oxalic acid. 



Products of its metamorphosis. Biuret, C 4 H 5 N 3 O 4 , is, as we 

 have already mentioned, the chief product (together with cyanuric 

 acid) which is obtained on heating pure urea or its nitrate to a tem- 

 perature of 152 170 ; the cyanuric acid is precipitated by basic 

 acetate of lead from the aqueous solution of the fused product, and 

 the excess of lead removed by sulphuretted hydrogen ; the biuret 

 is then obtained by the evaporation of the solution. It forms small 

 crystal which dissolves readily in water, and still more readily in 

 alcohol ; it exerts no action on vegetable colours, does not combine 

 with bases, and dissolves unchanged in concentrated sulphuric and 

 nitric acids ; with sulphate of copper and potash it yields a red 

 solution. Its rational formula = H 3 N + 2C 2 HNO 2 . 



Preparation. Urea not only occurs preformed in the animal 



* Journ. f. pr .Ch. Bd. 35, S. 51-66. 



