THE URINE. 181 



bcnate. If the urea does not satisfy these tests it must be 

 xecrystallized from absolute alcohol. 



Moreover, the urea must in any case be freed from the 

 adhering hygroscopic water or tested as to its complete dry- 

 ness. To do this, place about 2.5 g. of urea on a large watch- 

 glass, weigh this accurately with the watch-glass, put into 

 the desiccator, and weigh again after twenty-four hours. If 

 the weight remains constant (a difference of 0.5 mg. may be 

 neglected), the urea may be at once used; if not, put the 

 watch-glass with the urea back into the desiccator and weigh 

 again after twenty-four hours. Weigh off accurately on a 

 previously weighed watch-glass exactly 2 g. of this urea, 

 then shake it into a funnel placed in a 100-cc. measuring- 

 flask, avoiding any loss, and rinse off the watch-glass carefully 

 into the funnel with water from the wash-bottle. Wash the 

 urea in the funnel into the flask with distilled water, rinse the 

 funnel several times, dissolve by cautious shaking the urea 

 which still remains in the flask, fill the flask up to the mark 

 with distilled water, put in the stopper, and mix thoroughly. 



When the urea solution is prepared, place the mercury 

 solution in a burette (in case this is not dry it is washed out 

 several times with small quantities of the mercury solution), 

 and read the position of the fluid in the burette. Place 10 cc. 

 of the urea solution in a small beaker, run in at first 17-18 cc. 

 of the mercury solution in a continuous stream, and test the 

 mixture to determine if it already contains an excess of 

 mercury. For this purpose place a drop of the well-stirred 

 mixture in a watch-glass filled with sodium carbonate solu- 

 tion. The watch-glass should rest upon black paper, and the 

 drop should be allowed to flow in from the side. If a distinctly 

 yellow color is perceptible alongside of the white mercury- 

 urea compound, the end-point has been reached. If not, add 

 about 0.3 to 0.5 cc. more of the mercury solution and test 

 again, etc. 



