EXCRETIOK. 



from the body. For its removal, the secretion of urine seems especially 

 provided, though urea itself is not toxic. 



Properties. Urea, like the other solid constituents of the urine, 

 exists in a state of solution. When in the solid state, it appears in the 



Fig. 298. Crystals of Urea, 



form of delicate silvery acicular crystals, which, under the microscope, 

 appear as four-sided prisms (fig. 298). It may be 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 to dryness, and purifying the residue by repeated 

 solution in water or in alcohol, and finally allowing it to crystallize. It 

 readily combines with some acids, like a weak base: and may thus be 

 conveniently procured in the form of crystals of nitrate or oxalate of 

 urea (figs. 299 and 300). 



Urea is colorless when pure; when impure it may be yellow or 



Fig. 299. Crystals of Urea nitrate. 



Fig. 300. Crystals of Urea oxalate. 



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

 neither an acid nor an alkaline reaction, and deliquesces in a moist and 

 warm atmosphere. At 15 C. (59 F.) it requires for its solution less 

 than its own weight of water ; it is dissolved in all proportions by boil- 

 ing water; but it requires five times its weight of cold alcohol for its 

 solution. It is insoluble in ether. At 120 C. (248 F.) it melts with- 



