XVIII.] ORGANIC CONSTITUENTS OF THE URINE. 119 



7. Other Reactions of Urea. Make a strong watery solution 

 of urea, and with it perform the following tests : 



(a.) Allow a drop to evaporate on a slide, and examine the 

 crystals which form (fig. 60, a). 



(/>.) To a strong solution of urea add pure nitric acid = a precipi- 

 tate of urea nitrate (fig. 60, b). 



(/.) To a strong solution of urea add ordinary nitric acid tinged 

 yellow with nitrous acid, or add nitrous 

 acid itself ; bubbles of gas are given off, igj 



consisting of carbon dioxide and nitro- f) fl ~^ 

 gen. 11 S\ O 



\ j /l ^^ 



(d.) Add caustic potash, and heat. The 1*~1 ^ \/ 



urea is decomposed, ammonia is evolved, and ^ Sr$ Q ^^^\ ^ 

 Ammonium carbonate is formed : CONJEL \ \ \ n 



J */> x < 



Mercuric nitrate gives a greyish-white jj^N |7 I"I Q(\ 

 cheesy precipitate. ~\ ^J U LJ -. X^ 



8. With Crystals of Urea perform O* 



the following experiments : ^ 



(a.) Biuret Reaction. Heat a crystal * 



in a hard tube; the crystal melts, Fia 6 ^KmUrin? alate f 

 ammonia is given off, and is recognised 



by its smell and its action on litmus, while a white sublimate of 

 cyanuric acid (C 3 H 3 N 3 3 ) is deposited on the upper cool part of 

 the tube. Heat the tube until there is no longer an odour of 

 ammonia. Allow the tube to cool, add a drop or two of water to 

 dissolve the residue, a few drops of caustic soda or potash, and a 

 little very dilute solution of cupric sulphate = a pink colour (biuret 

 reaction). Two molecules of urea yield one of biuret. 



TO 

 J NH 2 



= 



(b.) Place a large crystal of urea in a watch-glass, cover it with a saturated 

 freshly prepared watery solution offurfiirof, and at once add a drop ol strong 

 hydrochloric acid, when there occurs a rapid play of colours, beginning with 

 yellow and passing through green, purple, to violet or brown. This test 

 requires care in its performance. 



9. Quantity. An adult excretes 30 to 40 grams (450 to 600 

 grs.) daily ; a woman less, and children relatively more. It varies, 

 however, with 



(a.) Nature of the Food. It increases when the nitrogenous matters are 



