414 
DR, JOHN STENHOUSE ON THE ACTION OF 
acid, when the nitropopulate of potash precipitated as a compact crystalline powder, 
which was collected on a filter and washed with a little cold water. It was dissolved 
a second time in a dilute solution of carbonate of potash, and again precipitated by 
muriatic acid. The object of this second treatment with an alkaline carbonate, was 
to remove any trace of nitropicrate of potash which might adhere to it. The nitro- 
populate of potash was still further purified by being digested with animal charcoal, 
and repeatedly crystallized out of hot water. No potash was added in these latter 
crystallizations, because an excess of potash, when assisted by heat, has the effect of 
decomposing nitropopulic acid. When, through these operations, the nitropopulate 
of potash has been sufficiently purified, it has merely a pale lemon-yellow colour, and 
may be employed for preparing nitropopulic acid. It is therefore boiled with a very 
considerable excess of muriatic acid, and on the cooling of the solution, the nitropo- 
pulic acid is deposited in silky needles, forming concentric groups of a pale yellow 
colour. Nitric acid cannot be advantageously employed for this purpose instead of 
muriatic acid, for, as will subsequently appear, nitropopulic acid is destroyed when 
it is boiled with strong nitric acid. The nitropopulic acid in this stage of its prepa- 
ration is not altogether free from a small quantity of a yellowish colouring matter, 
which adheres to it with great tenacity. Its crystals require, therefore, to be again 
dissolved in dilute muriatic acid ; and they are subsequently still further purified by 
being repeatedly digested with animal charcoal, and crystallized out of hot aqueous 
solutions. The crystals of nitropopulic acid, when quite pure, are perfectly colour- 
less, and the solution from which they have crystallized has only a slightly yellowish 
shade. The pure nitropopulic acid requires to be pretty quickly separated from the 
mother-liquor by being pressed between folds of blotting-paper and immediately 
dried in vacuo, as when it is exposed to the air, even for a short time, it is very apt 
to become yellow. When it is deposited by slow cooling out of dilute aqueous solu- 
tions, it forms hard prismatic crystals arranged in concentric groups ; but when a 
hot concentrated solution of the acid is rapidly cooled, it becomes filled with a mass 
of long slender silky needles. Muriatic acid has the effect of preventing the oxida- 
tion of nitropopulic acid, which may be readily obtained from a dilute solution of 
that acid in colourless crystals. 
Nitropopulic acid is very soluble in water, and still more so in weak and strong- 
spirits of wine. An addition of sulphuric or muriatic acid to its aqueous solution 
has the effect of considerably diminishing its solubility, and the colour of the solu- 
tion becomes fainter in proportion as the amount of the mineral acid is increased. 
But when the acid solution is diluted with water, the yellow colour again reappears. 
When an excess of nitropopulic acid is boiled with dilute muriatic acid part of it 
dissolves, Mdiile the portion which remains undissolved melts and forms a yellowish 
oil, which solidifies and crystallizes when the liquid cools. Nitropopulic acid may 
likewise be gently heated in sulphuric acid without being decomposed, and it crystal- 
lizes again when the solution cools. When nitropopulic acid, however, is digested 
with pretty concentrated nitric acid, it is decomposed, being converted into nitropi- 
