UREA. 



453 



exists in a state of solution. But it may be procured in , 

 the solid state, and then appears in the form of delicate 

 silvery acicular crystals, which, under the microscope, 

 appear as four-sided prisms (fig. 126). It is obtained in 

 this state by evaporating urine carefully to the consistence 

 of honey, acting on the inspissated mass with four parts 

 of alcohol, then evaporating the alcoholic solution, and 

 purifying the residue by repeated solution in water or 

 alcohol, and finally allowing it to crystallise. It readily 

 combines with an acid, like a weak base ; and may thus be 

 conveniently procured in the form of a nitrate, by adding 

 about half a drachm of pure nitric acid to double that 

 quantity of urine in a watch glass. The crystals of nitrate 

 of urea are formed more rapidly if the urine have been 

 previously concentrated by evaporation. 



Urea is colourless when pure ; when impure, yellow or 

 brown : without smell, and of a cooling, nitre-like taste ; 

 has neither an acid nor an alkaline re- action, and deli- 

 quesces in a moist and warm atmosphere. At 59 F. it 

 requires for its solution less than its weight of water ; it is 

 dissolved in all proportions by boiling water ; but it re- 

 quires five times its weight of cold alcohol for its solution. 

 At 248 F. it melts without undergoing decomposition; at 

 a still higher temperature ebullition takes place, and car- 

 bonate of ammonia sublimes ; the melting mass gradually 

 acquires a pulpy consistence ; and, if the heat is carefully 

 regulated, leaves a grey- white powder, cyanic acid. 



Urea is identical in composition with cyanate of ammo- 

 nia, and was first artificially produced by Wohler from this 

 substance. Thus : 



Cyanate of Ammonia. Urea. 



CHNO. H 3 N = CH 4 N,0. 



The action of heat upon urea in evolving carbonate of 

 ammonia, and leaving cyanic acid, is thus explained. A 

 similar decomposition of the urea with development of 

 carbonate of ammonia ensues spontaneously when urine is 



