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 wUl at once take 

 place. 



e. 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. 



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

 carefully over a flame, when the temperature 

 is somewhat over 100° C. 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 (biuret 

 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, b. c). 



