CHEMICAL BASIS OF THE ANIMAL BODY. ir.7 



The crystals are but slightly soluble in nitric acid, or alcohol, 

 more soluble in cold water, and much more so in hot water. They 

 are insuluhle in ether. 



Oxalate of urea. [(NH t ) a CO],.H 2 C 8 4 + H 2 O. 



Obtained by the addition of concentrated aqueous solution of 

 oxalic acid to a concentrated aqueous solution of urea. This salt 



Fio. 16. CRYSTALS OF OXALATE OF UREA. (Krukenberg after Kiihue.) 



crystallises out in rhombic tables closely resembling those of the 

 nitrate, but they are frequently aggregated into a characteristic 

 prismatic form. As in the case of the nitrate some care is required 

 with respect to the concentration of the respective solutions during 

 its preparation. 



The crystals are less soluble in oxalic acid than in water, but 

 may in other respects be taken as resembling those of the nitrate 

 in respect of their solubilities. 



Of the many salts which urea forms with other bases and salts 

 those which it yields with mercuric oxide and nitric acid are of 

 most importance. When a solution of mercuric nitrate is added 

 to one of urea a precipitate is formed which, dependently upon tin- 

 concentration and relative amounts of the two solutions, may con- 

 tain some one of three possible salts, consisting of [(NH a ) s CO]j. 

 \'O 3 ) 2 united with 1,2, or 3 molecules of mercuric oxide (HgO). 

 When tin- solutions are fairly neutral and itifntr, the salt with 

 :) molecules of HgO is formed [(NH,), C0] 2 . Hg(NO.), . 3 HgO. 

 This is the salt formed in the reactions on which Liebig's vol- 

 umetric method for the determination of urea is based. 



The other more important reactions of Urea. 



1. Urea may be heated dry in a tube to 120 without brin-_: 

 decomposed ; on further raising the temperature it melts at 132 - 6 01 



1 Reissert, Ber. d. d. chem. Getell. Ed. xxm. (1890), S. 2244. 



