ELEMENTARY PHYSIOLOGICAL CHEMISTRY 



249 



cursors in the tissues (see below). Thus if urea be hydrolysed (i.e. 

 be caused to take up a molecule of water) it forms ammonia carbonate : 



Urea. 



HOH 

 "HOH 



Water. 



Ammonia carbonate. 



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

 lysis being effected by several microbes. It may also be produced by 

 boiling urea with weak acids or alkalies, in both of which cases the 

 ammonium carbonate is further decomposed, liberating in the case of 

 alkalies ammonia gas (the C0 2 being absorbed by the alkali present), 

 and in the case of acids carbonic acid gas (the NH 3 being absorbed by 

 the acid present). 



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

 two portions A and B. To A add about 10 drops 10 per cent. HC1 

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

 test-tube containing lime water. By this becoming milky, the 

 presence of C0 2 gas is demonstrated. To B add about 5 drops 

 saturated KOH and boil. NH 3 gas is evolved so that a moistened 

 strip of red litmus paper is turned blue if held in the fumes, which also 

 smell strongly of ammonia. 



A substance intermediate between urea and ammonium carbonate, 



and having therefore the formula CO<^TT 4 , can be formed by allow- 



iNllg 



ing dry C0 2 gas to act on dry NH 3 . This is called ammonium carbamate, 

 and if heated to 135 C. it splits up into urea and water. 



Dry heat splits urea into ammonia gas and a body called biuret, 

 which, by further heating, changes into cyanuric acid (HCNO) 3 , which 

 is isomeric with cyanic acid, HCNO. 



EXPERIMENT Y. Heat some urea crystals in a dry test-tube. Note that 

 they melt and give off NH 3 . Continue heating for a few minutes then 

 cool the test-tube and dissolve the residue in H 2 0, and to this solution 

 apply the biuret test. A rose pink colour results (see Peptone, p. 224). 

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

 aqueous solution of ammonium cyanate (NH 4 CNO) to dryness. This 

 has the same empirical formula as urea, but its structural formula is 

 different : 



/ONH, /NH 2 



C ' C 



^N \NH 2 



Ammonium cyanate. Urea. 



It was by this means that Wohler first showed that inorganic could 

 be changed into organic bodies. 



