ADDITIONS. 399 



yellow precipitate of oxide of mercury or of a basic 

 mercuric carbonate. 



Second estimation. In the preliminary estimation too 

 much mercuric nitrate will probably have been added ; 

 so the process should be repeated, adding at once the 

 amount of mercuric solution found in the preliminary 

 estimation less 1 c.c. After well mixing, a drop is 

 added to a drop of sodium carbonate; if a yellow 

 colour results, the process must be repeated adding 

 less mercuric nitrate; probably however the mixed 

 drops will be colourless, if so add *1 c.c. mercuric 

 nitrate, mix and test again, and so on adding ! c.c. at 

 a time until a yellow colour is obtained. 



Correction. Since the sodium chloride in the urine 

 prevents the precipitation of urea by mercuric nitrate 

 (cf. 5 e) more mercuric nitrate is added to the urine 

 than is required to precipitate the urea in it. The 

 excess thus added is found experimentally to be about 

 2 c.c. for 10 c.c. of urine. Hence 2 c.c. must be deducted 

 from the number of c.c. of the standard solution actually 

 used. 



If v be the corrected number of c.c. of mercuric nitrate 

 used, the 10 c.c. of urine taken, contain -Qlv grams of 

 urea, i.e. the percentage of urea in the urine is the Q'v. 



Note. The method as above given is very rough. For 

 an accurate estimation, the sodium chloride as well as 

 the phosphates must be separated from the urine, the 

 filtered mixture must be neutralized, and again neu- 

 tralized with sodium carbonate after each addition of 

 mercuric nitrate. It must also be remembered that 

 the amount of urea found by this method is too high, 



