2/O 



PHYSIOLOGICAL CHEMISTRY. 



132 C. and liberates ammonia. Continue heating until the fused 

 mass begins to solidify. Cool the tube, dissolve the residue in 

 dilute potassium hydroxide solution and add very dilute cupric 

 sulphate solution (see p. 92). The purplish-violet color is due to 

 the presence of biuret which has been formed from the urea through 

 the application of heat as indicated. This is the reaction : 



NH 2 



NH 2 C = 



\ 

 NH 4- NH. 



J-0- 



NH 2 



Urea* 



= 

 NH 2 



Biuret. 



6. Urea Nitrate. Prepare a concentrated solution of urea by 

 dissolving a little of the substance in a few drops of water. Place 

 a drop of this solution on a microscopic slide, add a drop of con- 



FIG. 88. 



UREA OXALATE. 



centrated nitric acid and examine under the microscope. Compare 

 the crystals with those reproduced in Fig. 86, p. 268. 



7. Urea Oxalate. To a drop of a concentrated solution of urea, 

 prepared as described in the last experiment (6), add a drop of a 

 saturated solution of oxalic acid. Examine under the microscope 

 and compare the crystals with those shown in Fig. 88, above. 



