URINE AND ITS CONSTITUENTS. 711 



largest quantity of all nitrogen ingested with the food. From 85 to 

 86 per cent, of the total nitrogen of the urine is found in urea, the 

 formation of which in the liver has been considered heretofore. Urea 

 has never yet been found as a product of vegetable life, but is found 

 as a normal constituent of the urine of the mammalia, and in smaller 

 quantity in the excrement of birds, fishes, and some reptiles. It 

 occurs in small quantities also in blood, muscular tissue, lymph, per- 

 spiration, and many other animal fluids. Pathologically urea may 

 appear in all fluids and tissues. 



When pure, urea crystallizes from an aqueous solution in colorless 

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

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

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

 giving off ammonia gas and water, while 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 4- 2H 2 = C0 2 + 2NH 3 + H 2 O = (NH 4 ) 2 CO 3 . 



The same decomposition takes place in urine under the influence 

 of a bacterial enzyme, 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, car- 

 bon dioxide, and nitrogen : 



CO(NH 2 ) 2 + 6C1 + H 2 = 6HC1 + 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 : 



4 3 + H 2 + O = CO(NH 2 ) 2 + 

 Uric acid. Urea. Alloxan. 



c. By the action of caustic alkalies upon creatine : 



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

 Creatine. Urea. Sarcosine. 



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

 place when its solution is evaporated and allowed to crystallize : 



NH 4 .CXO = CO(NH 2 ) 2 . 



