248 



PRACTICAL PHYSIOLOGY 



EXPERIMENT I. To some urine, which has been evaporated to 

 small bulk on a water -bath, add some pure (not fuming) 

 HN0 3 , and cool the mixture by holding the test tube under the 

 tap. Crystals of urea nitrate separate out. Examine these with 

 the microscope, and note that they are either rhombic tables or six- 

 sided plates, which overlap each other like the tiles of a roof (see 

 Fig. 151). 



EXPERIMENT II. Repeat Experiment I. with oxalic acid, and 

 note that the crystals are not unlike those of the nitrate, being 

 elongated plates with bevelled pointed ends (Fig. 152). 



FIG. 151. Urea nitrate. 



FIG. 152. Urea oxalate. 



Like all other substituted ammonias urea is decomposed by nitrous 

 acid HN0 2 carbonic acid gas and nitrogen being evolved : 



EXPERIMENT III. Add some fuming nitric acid (i.e. containing 

 HN0 2 ) to urine and note the effervescence which results. That one 

 of the gases evolved is C0 2 can be proved by holding the mouth of the 

 test-tube over another one containing lime water, when, on shaking, 

 the latter will turn milky. A very similar reaction is obtained by 

 adding a hypobromite or hypochlorite : 



2 + 3NaBr = C 



2H + 3NaBr - 



This reaction is taken advantage of in its quantitative estimation (see 

 below, p. 250). 



There are several reactions which are peculiarly interesting, since 

 they demonstrate the chemical relationships of urea to its probable pre 



