454 PHYSIOLOGICAL CHEMISTRY. 



urine of the mammalia, and in smaller quantity in the excrement of 

 birds, fishes, and some reptiles. It occurs also in blood, muscular 

 tissue, chyle, lymph, bile, perspiration, and many other animal fluids. 

 When pure, urea* crystallizes from an aqueous solution in colorless 

 prisms ; it is odorless, and has a cooling, bitter taste ; it easily dis- 

 solves in water, the solution having a neutral reaction ; it fuses when 

 heated to 130 C. (266 F.), but decomposes at a higher temperature, 

 giving oif ammonia gas and water, whilst a number of other sub- 

 stances are formed at the same time. A pure solution of urea does 

 not decompose at ordinary temperature, but on boiling, and especially 

 under pressure, it takes up water, and is decomposed into ammonia 

 and carbon dioxide, or into ammonium carbonate : 



CO(NH 2 ) 2 + 2H 2 O = C0 2 -f 2NH 3 + H 2 O = (NH 4 ) 2 CO 3 . 



The same decomposition takes place in urine under the influence 

 of a ferment (most likely present in urine, or perhaps derived from 

 the air), if the temperature be not too low. 



A solution of urea is decomposed by the action of chlorine or 

 bromine with generation of hydrochloric (or hydrobromic) acid, 

 carbon dioxide, and nitrogen : 



CO(NH 2 ) 2 + 6C1 + H 2 O = 6HC1 -f CO 2 + 2N. 



Alkali hypochlorites or hypobromites cause a similar decomposi- 

 tion, upon which is based the quantitative estimation of urea. 



Urea forms with acids definite salts, and with certain oxides and 

 salts definite compounds. 



Urea is formed artificially by numerous decompositions, as, for instance : 



a. By a process similar to the one taking place in the animal system, viz.* 

 by limited oxidation of albuminous substances by potassium permanganate. 



b. By oxidation of uric acid in the presence of water : 



C 5 H 4 N 4 3 + H a O + O = CO(NH 8 ) 2 + C 4 H 2 N 2 O 4 . 

 Uric acid. Urea. Alloxan. 



c. By the action of caustic alkalies upon kreatin : 



C 4 H 9 N 3 2 + H 2 O = CO(NH 2 ) 2 + C 3 H 7 NO 2 . 

 Kreatin. Urea. Sarcosine. 



d. By the molecular transformation of ammonium cyanate, which takes 

 place when its solution is evaporated and allowed to crystallize : 



NH 4 .CNO : : CO(NH 2 ) 2 . 



e. By the action of carbonyl chloride, COC1 2 , on ammonia : 



COC1 2 + 2NH 3 = 2HC1 + CO(NH 2 ) 2 . 

 /. By the action of ammonia on ethyl carbonate : 



(C 2 H 5 ) 2 C0 3 + 2NH 3 = 2(C 2 H 5 OH) -f CO(NH 2 ) 2 . 



