340 PHYSIOLOGICAL CHEMISTRY. 



which dissolves in water, giving a beautiful reddish-violet liquid 

 with copper sulphate and alkali (biuret reaction). On heating with 

 baryta-water or caustic alkali, urea splits into carbon dioxide and 

 ammonia with the addition of water ; this also takes place by the 

 action of micro-organisms, which produces the so-called alkaline 

 fermentation of urine. The same decomposition is produced when 

 urea is heated with concentrated sulphuric acid. An alkaline solu- 

 tion of sodium hypobromite decomposes urea into nitrogen, carbon 

 dioxide, and water according to the equation 



COIST 2 H 4 + SNaOBr = 3NaBr + C0 2 +2H 2 + N 2 . 



Urea forms with many acids crystalline combinations. Among 

 these the one with nitric acid and the one with oxalic acid are the 

 most important. 



UEEA NITRATE, CO(NH 2 ) 2 .HN0 3 . This combination on crys- 

 tallizing quickly forms thin rhombic or six-sided, overlapping 

 tiles, colorless plates, whose point has an angle of 82. On slowly 

 crystallizing, larger and thicker rhombic pillars or plates are ob- 

 tained. This combination is rather easily soluble in pure water,, 

 but is considerably less soluble in water containing nitric acid; it 

 may be obtained by treating a concentrated solution of urea with 

 an excess of strong nitric acid free from nitrous acid. On heating 

 this combination it volatilizes without leaving a residue. 



This compound may be employed with advantage in detecting small 

 amounts of urea. A drop of the concentrated solution is placed on a micro- 

 scope-slide and the cover-glass placed upon it ; a drop of nitric acid is then 

 placed on the side of the cover-glass and allowed to flow under. The forma- 

 tion of crystals begins where the solution and the nitric acid meet. Alkali 

 nitrates may crystallize very similarly to urea nitrate when they are con- 

 taminated with other bodies ; therefore, in testing for urea, the crystals must 

 be identified as urea nitrate by heating and by other means. 



UREA OXALATE, 2.CO(NH 2 ) 2 .H 2 C 2 4 . This compound is more 

 sparingly soluble in water than the nitric-acid compound. It is- 

 obtained in rhombic or six-sided prisms or plates on adding a 

 saturated oxalic-acid solution to a concentrated solution of urea. 



Urea also forms combinations with mercuric nitrate in variable 

 proportions. If a very faintly-acid mercuric-nitrate solution is 

 added to a two-per-cent solution of urea, and the mixture carefully 

 neutralized, a combination is obtained of a constant composition 



