ORGANIC CONSTITUENTS OF THE URINE. 93 



(d.) 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, 

 consisting of carbon dioxide and 

 nitrogen. 



(e.) Put some of the urea nitrate 

 precipitate obtained in 4 (.) into the 

 test-tube A (Fig. 23), and some lime Fig. 23. 



water in B. Add nitrous acid to A. 



Cork the tube. The precipitate dissolves. CO 2 and K" are 

 given off, the C0 2 makes the lime water in B white. 



Urea. Nitrous Acid. Carbon Dioxide. Nitrogen. Water. 



CON 2 H 4 + 2(HNO. 2 ) = CO 2 + N 4 + 3H 2 O 



(f.) Mercuric nitrate gives a greyish-white, cheesy pre- 

 cipitate. 



(g.) Add caustic potash, and heat. The urea is decom- 

 posed, and ammonia is evolved. 



8. With Crystals of Urea perform the following experiments : 



(a.) Biuret Reaction. Heat a crystal in a hard tube ; the 

 crystal melts, ammonia is given off, and is recognised by its 

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

 cyanuric acid is deposited on the upper cool part of the tube. 

 Heat the tube until there is no longer an odour of am- 

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

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

 potash, and a little very dilute solution of cupric sulphate 

 = a violet colour (biuret reaction). 



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

 with a saturated watery solution of furfurol, and at once 

 add a drop of 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. Occurrence. Urea occurs in the blood, lymph, chyle, liver, 

 lymph glands, spleen, lungs, brain, saliva, amniotic fluid. The 

 chief seat of its formation is very probably the liver. It also 



