f a small quantity of
picric acid be heated in a moderate fire, in a crucible, or even in an
open test tube, it will melt (at 120 deg. C. commercial acid), then give off
vapours which catch fire upon contact with air, and burn with a sooty
flame, without exploding. If the burning liquid be poured out upon a cold
slab, it will soon go out. A small quantity carefully heated in a tube,
closed at one end, can even be completely volatilised without apparent
decomposition. It is thus obvious that picric acid is much less explosive
than the nitric ethers, such as nitro-glycerol and nitro-cellulose, and
very considerably less explosive than the nitrogen compounds and
fulminates.

It would, however, be quite erroneous to assume that picric acid cannot
explode when simply heated. On the contrary, Berthelot has proved that
this is not the case. If a glass tube be heated to redness, and a minute
quantity of picric acid crystals be then thrown in, it will explode with a
curious characteristic noise. If the quantity be increased so that the
temperature of the tube is materially reduced, no explosion will take
place at once, but the substance will volatilise and then explode, though
with much less violence than before, in the upper part of the tube.
Finally, if the amount of picric acid be still further increased under
these conditions, it will undergo partial decomposition and volatilise,
but will not even deflagrate. Nitro-benzene, di-nitrobenzene, and mono-,
di-, and tri-nitro-naphthalenes behave similarly.

The manner in which picric acid will decompose is thus dependent upon the
initial temperature of the decomposition, and if the surrounding material
absorb heat as fast as it is produced by the decomposition, there will be
no explosion and no deflagration. If, however, the absorption is not
sufficient to prevent deflagration, this may so increase the temperature
of the surrounding materials that the deflagration will then end in
explosion. Thus, if an explosion were started in an isolated spot, it
would extend throughout the mass, and give rise to a general explosion.

In the manufacture of picric acid the first obvious and most necessary
precaution is to isolate the substance from other chemicals with which it
might accidentally come into contact. If pure materials only are used, the
manufacture presents no danger. The finished material, however, must be
carefully kept from contact with nitrates, chlorates, or oxides. If only