ELEMENTS OF COMPOSITION. 



.41 



Urea combines with oxygen acids, forming salt-like combinations, 

 in which, one molecule of water is 

 always present ; thus it is with nitric 

 and oxalic acid. 



These two combinations are of par- 

 ticular importance in the recognition 

 of urea, owing to their characteristic 

 crystalline form. 



Nitrate of urea, CO^H^H^O, 

 (fig. 21, a a), crystallizes in pearly 

 scales or glittering white leaves, which 

 appear under the microscope in the 

 form of rhombic or hexagonal tables. 



Oxcdate of urea, 2CO(NH 2 ),, 

 H. 2 C. 2 4 + 2H 2 (fig. 21,6 6,) appears 



to the naked eye in the form of long 20 _ Crystals of urea . four . 8ide ; pillars; 

 thin leaves or prisms, found under 

 the microscope to be made up of 



hexagonal tables as a rule, but also of four-sided prisms, 

 belong to the monoclinic system. 



Urea combines also with metallic oxides and salts, as, for instance, 

 with chloride of sodium. 



6, indefinite crystals, such as are usually 

 formed in alcoholic solutions. 



Both salts 



Fig. 21. Crystals of combinations of urea with nitric and oxalic acid. 

 a a, nitrate of urea ; b 6, oxalate of the same. 



As regards its decomposition, urea may be artificially split up on 

 absorbing water into carbonic acid and ammonia. 



The same change is brought about by contact with animal matters 

 undergoing putrefaction, such as protein compounds, or mucus, &c. It 

 is owing to this fermentation that urine becomes after a time alkaline 

 on being exposed to the air. 



Urea may be obtained from other alkaloids, such as kreatin and 

 allantoin, by treatment of the latter with alkalies ; further, by subjecting 

 uric acid to the action of oxidising acids and caustic potash. Urea may 

 be produced artificially in many ways besides. 



Carbamide appears in human urine as the most important of all its solid 

 constituents. It amounts to from about two and a half to three per cent. 



