XXIII.] URINE. 239 



somewhat those of nitrate of urea, may also 

 be seen. 



d. Dilute considerably the remaining solution, 

 and add to a part of it a solution of mercuric 

 nitrate. A white precipitate of mercuric 

 oxide combined with urea will at once take 

 place. 



c. To the rest of the urea solution add a little 

 sodium chloride and then mercuric nitrate 

 drop by drop. A certain quantity of the 

 mercuric nitrate must be added before any 

 precipitate is formed; a double decomposi- 

 tion takes place between the mercuric and 

 sodium salts so that as long as any sodium 

 chloride is present, the mercuric nitrate is 

 not available for precipitating the urea. 



/. Place a little urea in a test-tube and heat it 

 carefully over a flame, when the temperature 

 is somewhat over 100C. it melts 'and am- 

 monia is given off (recognized by its odour), 

 heat it a little longer then let cool; add a 

 small quantity of water to dissolve the resi- 

 due and pour the fluid into another test- 

 tube containing a strong solution of sodium 

 hydrate and a drop or two of cupric sulphate. 

 A violet colour will be produced (biurct 

 reaction). 



6. Evaporate 20 c.c. of urine in a water-bath till its 

 volume is reduced to about 2 c.c. Filter, and 

 examine the filtrate for urea ( 5, 6. c). 



