CHEMICAL BASIS OF THE ANIMAL BODY. 159 



Micrococcus ureae 1 , from which a soluble hydrolytic enzyme may be 

 extracted 2 . (See above, p. 70.) 



2. When treated with nitrous acid, e.g. impure yellow nitric acid, 

 it is decomposed finally into carbonic anhydride, nitrogen and water : 

 (NH 2 ) 2 CO + 2HNO 2 = CO 2 + 2N 2 + 3H 2 0. A similar decomposition 

 is obtained by the action of sodium hypochlorite or hypobromite : 

 (NH 2 ) 2 CO + 3 NaBrO = 3 NaBr + C0 2 + N 2 + 2 H 2 O. Since the volume 

 of nitrogen evolved is constant for a given weight of urea, this latter 

 reaction forms the basis of a method for the quantitative determination 

 of urea. (Knop-Hiifner.) 



3. When a crystal of urea is treated with a drop of concentrated 

 freshly prepared aqueous solution of furfurol C 5 H 4 O 2 (aldehyde of 

 pyroinucic acid) and then immediately with a drop of hydrochloric acid 

 (sp. gr. = I'lO) a play of colours is observed which passes rapidly from 

 yellow through green blue and violet to a final brilliant purple. The 

 test may be also applied by the addition of three drops of the acid to a 

 mixture of one drop of 1 p.c. aqueous urea solution and *5 cc. of 

 aqueous furfurol solution 3 . 



Detection in Solutions. In addition to the microscopic appear- 

 ance of the crystals obtained on evaporation, the nitrate and oxalate 

 should be formed and examined. Another part should give a precipi- 

 tate with mercuric nitrate, in the absence of sodium chloride, but not 

 in the presence of this last salt if in excess ; in presence of sodium 

 chloride the mercuric nitrate reacts first with the sodium salt in 

 preference to the urea. A third portion is treated with nitric acid 

 containing nitrous fumes ; if urea is present, nitrogen and carbonic 

 acid will be obtained. To a fourth part pure nitric acid in excess 

 and a little mercury are added, and the mixture is warmed. In 

 presence of urea a colourless mixture of gases (N and CO 2 ) is given off. 

 A fifth portion is treated, after evaporation to dryness, in the way above 

 described for the application of the biuret reaction, and a sixth part is 

 tested with furfurol. 



Quantitative determination. The methods are based on some 

 of the reactions above described. They consist of (i) Precipitation by a 

 standardised solution of mercuric nitrate (Liebig). (ii) Decomposition 

 into carbonic acid and nitrogen by means of sodium hypobromite, 

 and measurement of the volume of nitrogen (Knop-Hiifner). (iii) 



1 Pasteur, Compt. Rend. T. L. (1860), p. 869. Van Tieghem, Ibid. T. LVIII. (1864), 

 p. 210. Jaksch, Zt.f. physiol Chem. Ed. v. (1881), S. 395. 



2 Musculus, Pfliiger's Arch. Bd. xn. (1876), S. 214. Lea, Jl. of Physiol. Vol. vi. 

 (1885), S. 136. 



3 Schiff, Ber. d. d. chem. Gesell. 1877, S. 773. 



