CHEMICAL BASIS OF THE ANIMAL BODY. 1257 



for a given weight of urea, this latter reaction forms the 

 basis of a method for the quantitative determination of 

 urea. 



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

 freshly prepared aqueous solution of f urfuraldehyde — C 5 H 4 2 

 (aldehyde of pyromucic acid) and then immediately with a drop 

 of hydrochloric acid (sp. gr. =1*10) a play of colours is ob- 

 served 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 c.c. of 

 aqueous furfuraldehyde solution. 



Detection in solutions. In addition to the microscopic 

 appearance of the crystals obtained on evaporation, the nitrate 

 and oxalate should be formed and examined. Another part 

 should give a precipitate 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 carbon dioxide will be 

 obtained. To a fourth part pure nitric acid in excess and a 

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

 ence of urea a colourless mixture of gases (N and C0 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. 



Quantitative determination. For this some special manual 

 should be consulted. 



The determination of the total nitrogen in urine is also of 

 great importance and is now usually carried out by converting 

 all the nitrogen of a measured portion of urine into ammonia 

 by boiling with fuming sulphuric acid and the subsequent 

 addition of potassium permanganate. The ammonia is then 

 expelled from the acid solution by distillation with an excess 

 of caustic soda or potash, the ammonia being received into a 

 measured volume of standardized acid, whose diminution of 

 acidity due to the absorption of ammonia is finally determined 

 by titration with standard alkali. 



Substituted ureas. The hydrogen atoms of urea can be replaced 

 by alcohol- and acid-radicles. The results are substituted ureas in 

 the first case, or ureides as they are called in the second, when the 

 hydrogen is replaced by the radicle of an acid. Many of them are 

 called acids, since the hydrogen from the amido group, if not all 

 replaced as above, can be replaced by a metal. Thus the substitu- 



