992 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



(2) Through the union of ammonia with carbonyl chloride : 



OCC1 2 + 2NH 3 = OC(NH 2 ) 2 + 2HC1. 



(3) By evaporating an aqueous solution of ammonium cyanate : 



0:C:N.NH 4 == OC(NH 2 ) 3 . 



This was Wohler's notable preparation in 1828 of an "organic" compound, 

 a product of life, without the aid of a " vital force." 



(4) As a decomposition product of guanin, xanthin, creatin, uric acid, etc. 



(5) From proteid, through hydrolytic cleavage l (see p. 994). This origin 

 has not as yet been confirmed. 



Properties. Urea is a weak base, of great stability when within the alka- 

 line fluids and tissues of the body. It is soluble in water in all proportions, 

 very soluble in hot, less so in cold alcohol, whence it crystallizes in needle-like 

 forms. It melts at 132 and recrystallizes on cooling. Heated higher it is 

 converted into biuret, a substance which gives a violet color with dilute cupric 

 sulphate in a sodium-hydrate solution (called the biuret reaction) : 



C< NH NH 

 ~ C< NH, + " 



Heating urea with water over 100 in sealed tubes, boiling it with alkalies 

 or acids, bacterial action (see p. 956), all convert it through hydrolysis into 

 carbonic oxide and ammonia. Such decomposition may take place in the 

 stomach in uraemia. 2 Nitrous oxide breaks up urea, thus: 



OC(NH 2 ) 2 + 2HN0 2 == C0 2 + 3H 2 O + 4N, 



and hypobromite of soda acts in like manner in the presence of alkali : 

 OC(NH 2 ) 2 + 3NaBrO = CO 2 + 2H 2 O + 2N + 3NaBr. 



The alkali present absorbs the C0 2 , and the volumes of N afford a measure for the 

 amount of urea present (method of Hiifner, apparatus by Doremus). 



Urea combines with nitric acid to form urea nitrate, OC(NH 2 ) 2 .HNO 3 , 

 which is insoluble in nitric acid. Urea oxalate, which is formed in similar 

 manner by the combination of urea with oxalic acid, is insoluble in water. 

 Many combinations with metallic salts have been prepared, of which one with 

 mercuric nitrate, of uncertain formula, is the basis of Liebig's method of titra- 

 tion for urea. 



UREA IN THE BODY. This subject has been discussed under Nutrition. 

 It can only be briefly considered here. When urea is fed it is rapidly excreted 

 in the urine. The excreted nitrogen of proteid appears in mammalia in greater 

 part as urea. Amido- products of proteid decomposition, glycocoll, leucin, 

 aspartic acid, uric acid, when fed are converted by the body into urea. So like- 

 wise are ammonium carbonate, lactate, and tartrate. Ammonium chloride, on 



1 Drechsel: Archiv fiir Physiologic, 1891, p. 261. 



2 Voit: Zeiischrift fiir Biologie, 1868, Bd. 4, p. 150. 



