52 ORGANIC CONSTITUENTS. 



With most concentrated acids it gives crystalline salts, espe- 

 cialty with nitric and oxalic acids. It is not precipitated from 

 its aqueous solution by metallic salts, ferrocyanide of potassium, 

 or tannic acid. With hyponitrous acid it is instantly decom- 

 posed into nitrogen and carbonic acid gases, which are evolved 

 in equal volumes ; with chlorine it forms hydrochloric acid, ni- 

 trogen, and carbonic acid. These decompositions are rendered 

 obvious by the formulae 



C, H4N2O2 + 2NO3 =4N + 2C02-l-4HO 



Cj H, N2 O2 + 2H0 + 6C1:=2N -f 200^ -|- 6HC1. 



Compounds of urea. Nitrate of urea is obtained by the 

 direct addition of nitric acid in excess, to a concentrated solu- 

 tion of urea. Its formula is C^ H^ N„ 0,^-FNO^ + HO. It 

 most commonly crystallizes in large colourless leaves, but some- 

 times in small solid prisms. It dissolves in eight parts of cold, 

 but more freely in hot water. It is sparingly soluble in nitric 

 acid, wdth which it may be boiled without decomposition. This 

 salt effloresces with great rapidity.^ 100 parts of nitrate of 

 urea correspond to 48-945 of urea. (Regnault and Percy.) 



Oxalate of urea is obtained by the mixture of concen- 

 trated hot solutions of urea and oxabc acid. Its formula is 

 C„ H^ N„0., + C,, O3 + IIO. It crystallizes in long slender plates 

 or prisms, as the fluid cools, since it is much less soluble in cold 

 than in hot water. 



At a temperature of 61° water dissolves only 4-37{5, and 

 alcohol VQl, of the oxalate of urea. Oxalic acid displaces 

 nitric acid from its combination with lu'ea. 100 parts of oxalate 

 of urea correspond to 62'564 of urea. (Berzelius.) 



Sulphate of urea may be obtained by the double decomposition 

 of oxalate of urea and sulphate of lime. 



' Nitrate of urea, when heated to about 316°, decomposes, and disengages a con- 

 siderable quantity of carhonic acid and nitrous oxide, in tlie exact proportion of two 

 volumes of the first to one of the latter; the residue consists of free urea and of 

 nitrate of ammonia. Nitrate of ammonia and urea crystallize successively out of an 

 aqueous solution of the residue. Tliese changes are shown by the formula 

 4 (Cj H, Ng O2, NO5, IIO) = 4 CO + 2 NO + 2 (C^ H^ N^ Oj + S (NH3, NO5, HO). 



The nitrate of ammonia subsequently changes into water and nitrous oxide, and 

 the urea into carbonic acid and ammonia. 



During the decomposition of tlie nitrate of urea a new acid is formed in extremely 

 minute quantities. It crystallizes in grayish white brilliant lamellae, reddens litmus 

 paper, and is very slightly soluble in water, which allows of its being separated from 

 urea and nitrate of ammonia. Pelouze has assigned it the formula Cj H3 Nj O4. 



