CHEMICAL COMPOSITION OF THE HUMAN BODY 43 



is present in small quantities in the urine (in the form of calcium 

 oxalate, yielding the * 'envelope crystals "). 



While the combustion of fats and carbohydrates yields 

 only CO 2 and rf 2 O, combustion of proteids furnishes these 

 and a series of other products which contain the nitrogen, 

 sulphur, and phosphorus of the proteid. These products are 

 of importance in estimating the extent of proteid metabolism. 



By the combustion of the proteid, its sulphur and phos- 

 phorus form sulphuric and phosphoric acids, which are 

 excreted in the form of salts. 



The nitrogenous end-products of metabolism can unite 

 with still more oxygen ; hence, in the physiological combus- 

 tion of proteids, the proteids are not fully oxidized, but an 

 oxidizable residue is left which cannot be used in the body. 

 The nitrogeneous end-products are: 



1. Ammonia, excreted in small quantities as ammonia 

 salts. 



2. Urea, CO(NH 2 ) 2 , the diamid of carbonic acid or 

 carbamid. 



Urea crystallizes in colorless needles or long, rhombic 

 prisms ; it is neutral and has a cool saltpetre taste. It melts 

 at 130-132 C., but in solutions undergoes decomposition 

 at 60-70 C. It is soluble in water and in alcohol, but not 

 in ether. 



Decomposition of urea. On heating, dry urea forms am- 

 monia and biuret. Two molecules of urea form one mole- 

 cule of biuret and one of ammonia. 



2CO(NH 2 ) 2 = NH 2 .CO.NH.CO.NH 2 + NH 3 . 



Biuret gives a reddish-violet color with copper sulphate 

 and potassium hydrate (origin of the term biuret reaction). 



By heating with baryta-water, alkalies, and by the action 

 of certain micro-organisms (during alkaline urine fermenta- 

 tion) urea takes up water and forms ammonium carbonate. 



An alkaline solution of sodium hypobromite decomposes 

 urea into carbon dioxide, water and nitrogen : 



CO(NH 2 ) 2 + 3 NaOBr = 3 NaBr + CO 2 + 2 H 2 O + N 3 . 



