APP.] CHEMICAL BASIS OF THE ANIMAL BODY. 747 



Oxalate of urea. [(NH 2 ) 2 CO] 2 . H 2 C 2 4 + H 3 O. 



Often crystallises in long thin prisms, but under the microscope is 

 obtained in a form closely resembling the nitrate; it is slightly soluble 

 in water, less so in alcohol. 



With mercuric nitrate urea yields three salts, containing respectively, 

 4, 3 and 2 equivalents of mercuric oxide to one of urea. The first is the 

 precipitate formed in Liebig's quantitative determination of urea and 

 may be represented by the formula 2N 2 H 4 CO . Hg (NO 3 ) 2 3HgO. The 

 exact constitution of these salts has not yet been determined. 



Preparation. Ammonic sulphate and potassic cyanate are mixed to- 

 gether in aqueous solution, and the mixture is evaporated to dry ness. 

 The residue when extracted with absolute alcohol yields urea. From 

 urine, either by evaporating to dryness, having previously precipitated 

 the urine with normal and basic lead acetate in succession and removed 

 the lead by sulphuretted hydrogen, and then extracting with alcohol; or 

 concentrating only to a syrup, and then forming the nitrate of urea; 

 this is washed with pure nitric acid and decomposed with baric 

 carbonate. 



Detection in Solutions. In addition to the microscopic appear- 

 ance of the crystals obtained on evaporation, the nitrate and oxalate 

 should be formed and examined. Another part should give a precipitate 

 with mercuric nitrate, in the absence of sodic chloride, but not in the 

 presence of this last salt in excess. A third portion is treated with 

 nitric acid containing nitrous fumes ; if urea is present, nitrogen and 

 carbonic anhydride will be obtained. To a fourth part nitric acid in 

 excess and a little mercury are added, and the mixture is warmed. In 

 presence of urea a colourless mixture of gases (N and C0 2 ) is given off. 

 A fifth portion is kept melted for some time, dissolved in water, and 

 cupric sulphate and caustic soda are added; a red or violet colour, due 

 to biuret, is developed. 



Quantitative determination. For this some special manual 

 must be consulted '. It will suffice here to point out that the determina- 

 tion is made either with a solution of mercuric nitrate of known strength 

 (Liebig) ; by decomposing the urea by means of sodic hypobromite into 

 nitrogen, carbonic anhydride and water and measuring the nitrogen 

 (Knop) [N 2 H 4 CO + SNaBrO = SNaBr + CO 2 + 2H 2 O + NJ or by heating 

 the urea with caustic baryta in a sealed tube, the urea being determined 

 by the weight of baric carbonate formed (Bun sen). 



Urea is generally considered to be an amide of carbonic acid i.e. car- 

 bamide. The amide of an acid is formed when water is removed from the 

 ammonium salt of the acid ; if the acid be dibasic and two molecules of 

 1 Neubauer and Vogel, Analyse des Harris, vin. Aufl. 1881. S. 264. 



