416 
DR. JOHN STENHOUSE ON THE ACTION OF 
populic acid will be decomposed, being changed into nitropicric acid. In this respect 
nitropopulic acid resembles indigotic acid, which, by digestion with strong nitric acid, 
is also converted into nitropicric acid. When the mixture of the crude acids is 
dissolved in water, it must be filtered only when perfectly cold, so as to separate the 
resinous matter of the extract and retain it upon the filter. This resinous matter, 
which has usually a deep green colour when it is treated a second time with nitric acid, 
yields a second quantity of the mixed nitrogeiiated acids. It is also necessary to 
precipitate the mixture of the nitropicric and nitropopulic acids both together by 
exactly saturating them with potash, and to withdraw them from the mother-liquor 
before we attempt to separate the mixture of the two salts from each other ; for if 
much resinous matter and any considerable amount of inorganic salts are contained 
in the solution, neither the nitropicrate nor the nitropopulate of potash readily crystal- 
lizes out of it. The mother-liquor always contains a large quantity of quadroxalate 
of potash and free oxalic acid, the removal of which latter substance cannot be 
effected by means of lime, as the basic lime-salt of nitropopulic acid is scarcely more 
soluble in water than the neutral oxalate of lime. It is likewise very necessary to 
exclude the air from nitropopulic acid as much as possible during its purification ; 
and the dried acid should be preserved in well-stoppered bottles, as it rapidly becomes 
yellow on exposure to the air. 
Nitropopulic acid, when dried first in the air and then in vacuo, loses /•21 per 
cent, water. Thus 0’957 grm. lost in vacuo 0‘069=7'21 per cent. 
The nitropopulic acid dried in vacuo, when burned with oxide of copper and 
copper turnings, gave the following results : — 
I. 0*323 grm. of acid gave 0*4355 CO 2 and 0 05 water. 
II. 0*339 grm. of acid gave 0*455 carbonic acid and 0*058 water. 
The nitrogen was determined qualitatively by Liebig’s method. 
Tubes. 
Vol. mixed gases. 
Vol. after absorption, or N. 
Vol. of carbonic acid. 
1 
33 
4 
29 
0 
25 
3 
22 
3 
32 
4 
28 
4 
39 
5 
34 
0 
24 
3 
21 
6 
27 
3i 
23i 
7 
33 
4 
29 
8 
35 
4i- 
301 
9 
29 
25i 
10 
33 
4 
29 
11 
32 
4 
28 
12 
43 
^2 
37i 
13 
37 
32i 
14 
28 
24i 
15 
36 
4^ 
^2 
31i 
16 
33 
4 
29 
17 
27 
3i 
23i 
18 
32 
4 
28 
578 
72 
506 
