256 PRACTICAL PHYSIOLOGY 



passing some of the gas liberated into another test tube containing 

 lime water or baryta water, when, on shaking, the latter will turn 

 milky. 



A very similar reaction is obtained by adding a hypobromite or 

 hypochlorite to urine. 



CO(NH 2 ) 2 + SNaBrO = C0 2 + N 2 + 2H 2 O + SNaBr. 



The carbon dioxide formed combines with excess of caustic soda 

 present in the hypobromite. This reaction is employed in the 

 quantitative estimation of urea (see below, p. 257). 



There are several reactions which are peculiarly interesting, since 

 they demonstrate the chemical relationships of urea to its probable 

 precursors in the tissues. Thus, if urea be hydrolysed (i.e. be 

 caused to take up water) it forms ammonium carbonate : 



HOH 

 HOH 



(Urea) (Water) (Ammonium 



carbonate) 



This process occurs in urine which has stood for some time, the 

 hydrolysis being effected by several kinds of microbes such as the 

 micrococcus ureae. It may also be produced by boiling urea with 

 strong acids or alkalis ; in both cases the ammonium carbonate is 

 further decomposed, liberating, in the case of alkalis, ammonia gas 

 (the carbon dioxide being absorbed by the alkali present), and in the 

 case of acids, carbon dioxide gas (the ammonia being absorbed by 

 the acid present). 



EXPERIMENT IV. Prepare a solution of pure urea, and divide it 

 into two portions, A and B. To A add about 10 drops of sulphuric 

 acid and boil, meanwhile collecting the vapour which comes off in a 

 second test tube containing lime or baryta water. By this becoming 

 milky, the presence of carbon dioxide gas is demonstrated. To B 

 add about 5 drops saturated caustic potash and boil. Ammonia 

 gas is evolved, so that a moistened strip of red litmus paper is turned 

 blue if held in the fumes, which smell strongly of ammonia. 



EXPERIMENT V. Dry heat splits urea into ammonia gas and a 

 body called biuret. Heat some urea crystals in a dry test tube. 

 Note that they melt and give off ammonia. Continue heating for a 

 few minutes, then cool the test tube and dissolve the residue in water, 

 and to this solution apply the biuret test. A rose pink colour 

 results (see Peptone, p. 203). 



Conversely, we can change cyanic acid into urea by evaporating an aqueous 

 solution of ammonium cyanate (NH 4 CNO) to dryness. This salt has the same 

 empirical formula as urea, but its structural formula is different : 



NH 2 



(Ammonium cyanate) (Urea) 



It was by this means that Wohler first showed that organic bodies of animal 

 origin could be formed from inorganic substances. 



