THE URINE 



457 



When solid urea is heated in a dry test-tube, it first melts and then 

 goes solid again. During the process ammonia gas is given off, and 

 the body known as biuret is formed, which, with a little copper sulphate 

 and some caustic potash, gives a pink colour. 



< NH2 



Biuret 



If still further heated, a body known as cyanuric acid (C 3 N 3 H 3 O 3 ) 

 is formed, which does not give the biuret test. 



-Urea is precipitated from solution as a dense white precipitate by 

 the addition of mercuric chloride. 



The presence of urea in urine may be shown by concentrating the 

 urine by evaporation over a water-bath, and adding concentrated 

 nitric or oxalic acid, when crystals of urea nitrate or oxalate will be 



Fi3. 219. UREA NITRATE. 



FIG. 220. UREA OXALATE. 



deposited. Crystals of urea may be obtained by rubbing the urea 

 nitrate into a paste with barium carbonate as a result, barium 

 nitrate and urea are formed, with the evolution of carbon dioxide, 

 and then extracting with alcohol, and allowing the alcohol to 

 evaporate. Crystals of urea may also be obtained directly from 

 concentrated urine by extracting with acetone, and allowing the 

 acetone to evaporate. On heating with acids under pressure, urea is 

 decomposed into ammonia and carbon dioxide. It is also split thus 

 by the ferment urease present in certain bacteria and in the soya bean. 

 When fuming nitric acid is added to urine, an evolution of carbon 

 dioxide and nitrogen takes place, owing to a decomposition of the urea, 

 as expressed by the equation 



CO< NH! 



