V UREA 185 



Urea is a white crystalline solid, very "soluble in water, 

 and composed of carbon, oxygen, hydi-ogen and nitrogen. 

 Its chemical formula is (NH,,)oCO, from which it is seen 

 to contain rather more than 46 per cent, of nitrogen. It 

 forms characteristic crystalline compounds with nitric 

 acid and oxalic acid, which serve for its qualitative identi- 

 fication. When acted on by sodium hypobromite, urea is 

 decomposed in such a way that aU the carbon becomes 

 carbon dioxide (carbonic acid gas) and the nitrogen is 

 given off as a gas : 



(XH,).,CO + SNaBrO = Ng + CO., + 2H,0 + 3XaBr. 



This is an important reaction, since by measuring the 

 nitrogen evolved the urea may be estimated quantita- 

 tively ; a method now very generally employed. When 

 in solution, under the influence of a ferment sometimes 

 secreted by the mucous membrane of the bladder, or of 

 organisms from the air, urea takes up water and becomes 

 ammonium carbonate ; 



(NH.,).,CO + 2H.0 = (NH^),C03. 



This accounts for the ammoniacal odour of stale urine. 



Historically urea is interesting as being the first 

 organic animal product prepared (synthetically) from 

 inorganic sources. 



5. The Secretion of Urine.— Many of the constituents 

 of urine are present in blood. These appear in the urine 

 dissolved in a large quantity of water, whereas many 

 other substances also present in the blood do not,. in a 

 state of health, make their way into the urine. This 

 suggests the idea that the kidney is a peculiar and delicate 

 kind of filter which allows certain substances together 

 with a large quantity of water to pass through it, but 

 refuses to allow other substances to pass through. And 

 when we come to study the minute structure of the kidney 

 we find much to support this idea. Thus we saw that the 

 surface of the glomerulus is, practically, free, or in direct 



