NITRIC ACID ON VARIOUS VEGETABLES. 
415 
cric acid. If nitropopulic acid is boiled with a mixture of hydrochloric acid and 
chlorate of potash, it is also decomposed, being changed into chloranil. When nitro- 
populic acid is treated with a cold solution of hypochlorite of lime, no effect is pro- 
duced ; and even when the mixture is treated to nearly the boiling-point, no chloro- 
picrine is evolved ; but when it has been boiled for a few minutes, a sudden efferves- 
cence takes place, and the nitropopulic acid gives off abundance of chloropicrine. 
This reaction is highly characteristic of nitropopulic, indigotic and chrysammic acids, 
and furnishes an easy test by which any of these acids may be immediately distin- 
guished from either nitropicric or oxypicric acids, both of which evolve chloropicrine, 
either in the cold, or when very gently heated with hypochlorite of lime. 
I must here rectify a slight error which I committed, in a short notice upon indi- 
gotic acid, which I published about two years ago. I then stated that indigotic acid 
might be heated with a solution of hypochlorite of lime without the production of 
chloropicrine. This is so far quite true ; but if, as has just been stated, the mixture is 
boiled for some minutes, effervescence ensues and chloropicrine is abundantly evolved, 
Nitropopulic acid may also be easily distinguished from the above-mentioned acids 
by the peculiar character of its potash salt. For though, as already stated, nitropo- 
pulate of potash is very slightly soluble in pure water, it dissolves very readily in an 
alkaline solution, and is precipitated again when the solution is neutralized with 
an acid. The potash salts of nitropicric, chrysammic and oxypicric acids, on the 
contrary, are less soluble in alkaline lyes than they are in pure water. And indigo- 
tate of potash, though it resembles the nitropopulate of that base, in being v^ery 
soluble in alkaline solutions, differs from it in being much more soluble in cold water, 
and in being much more easily decomposed by any of the stronger acids. Thus, when 
a concentrated solution of indigotate of potash is treated in the cold with a slight 
excess of muriatic acid, the salt is decomposed and a considerable portion of the in- 
digotic acid is precipitated. Nitropopulate of potash, on the contrary, can only be 
decomposed when it is boiled with a great excess of muriatic acid. The small amount 
of solubility in cold water which the neutral potash, soda and ammonia salts of nitro- 
populic acid exhibit, is quite characteristic, and distinguishes them from the cor- 
responding salts of indigotic acid. The taste of nitropopulic acid is very peculiar, 
being at first very strongly acid like oxalic acid, then astringent, and finally exceed- 
ingly bitter. Nitropopulic acid stains the skin permanently yellow. When gently 
heated in a retort, it sublimes and crystallizes on cooling. When strongly heated in 
the open air, it burns with a bright yellow flame, and when any of its salts are heated 
on platinum foil, they explode with considerable violence. Nitropopulic acid strikes 
a deep red colour with a solution of perchloride of iron. With the protosulphate of 
iron it merely yields a deep yellow. 
The chief points which require to be attended to in preparing nitropopulic acid, may 
be shortly stated as follows: — The mixture of dilute nitric acid and the extract of 
the Populus halsamifera, must never be so highly heated as to boil ; for if the heat is 
allowed to rise very high and the acid is concentrated, almost the whole of the nitro- 
3 H 2 
