994 CYANOGEN COMPOUNDS. 



burns without sensible ash, and its solution is not troubled by 

 a salt of lime. In the latter part of the evaporation of the first 

 impure solution of urea filtered from the oxalate of lime, inso- 

 luble oxalate of lime separates in crusts upon the surface, which 

 must be removed by filtration. 



Or, nitrate of urea may be formed by adding to the concen- 

 trated urine in a shallow bason an equal bulk of nitric acid of 

 1.42, taking care by the gradual mixing of the acid and placing 

 the bason in cold water, to prevent any considerable elevation 

 of temperature. This precaution is necessary, for the urine 

 contains chlorides, which when decomposed by the action of 

 nitric acid and heat, produce chlorine and nitrous acid, bodies 

 which re-act injuriously upon urea. The impure nitrate of urea 

 which crystallizes is washed several times with dilute nitric acid, 

 arid then dried by placing it upon a clean porous brick or tile, 

 which imbibes the acid liquor. It is redissolved and discoloured 

 by means of animal charcoal, crystallized again, and the solu- 

 tion of colourless nitrate of urea treated with carbonate of 

 barytes or carbonate of potash till perfectly neutralised. The 

 nitrate of barytes or nitrate of potash crystallizes first from the 

 concentrated solution at a low temperature, and the urea after- 

 wards from the mother liquor of those crystals. The urea may 

 be purified from any adhering salt by solution in alcohol and 

 crystallization from that liquid. 



Urea may also be prepared in large quantity by decomposing 

 the insoluble cyanate of lead with ammonia or its carbonate. 



Urea crystallizes in colourless, flattened, four- sided prisms, 

 is soluble in its own weight of cold water, in 4 or 5 parts of 

 cold alcohol, and in 2 parts of boiling alcohol; it fuses at 248. 



The taste of its aqueous solution is cooling, like that of nitre, 

 acrid and bitter. It is persistent in dry air, but deliquesces in 

 damp air. At a high temperature it undergoes decompo- 

 sition and produces ammonia, cyanate of ammonia, and 

 solid cyanuric acid. Alkalies give no indication of am- 

 monia in a cold solution of urea, nor is cyanic acid precipi- 

 tated from it by the metallic salts. The latter acid, however, 

 is revived when a solution of urea is evaporated with nitrate of 

 silver, cyanate of silver being deposited in a crystalline state, 

 and nitrate of ammonia remains in solution. When urea is dis- 

 solved in fused potash or in concentrated and boiling sulphuric 



