416 



PEACTICAL PHYSIOLOGY 



nitric acid, 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. 240). 



EXPERIMENT II. Repeat experiment with a saturated alcoholic 

 solution of oxalic acid, and note that the crystals are not unlike those 

 of the nitrate, being elongated plates with bevelled pointed ends 

 (Fig. 241). 



o 



FIG. 240. Urea nitrate. 



FIG. 241. Urea oxalate. 



Urea is decomposed by nitrous acid HN0 2 carbonic acid gas and 

 nitrogen being evolved : 



= N ~- OH - C 



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

 nitrous acid) to urine, and note the effervescence which results. That 

 one of the gases evolved is carbon dioxide can be proved by holding 

 the mouth of the test tube over another one 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. 





+ 3NaBr = C 



+ 3NaBr - 



The carbon dioxide formed combines with excess of caustic soda 



