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 a little bit of lime or plaster become accidentally mixed with it, it may become highly dangerous.  A local explosion may occur which might have the effect of causing the explosion of the whole mass.  Picric acid can be fired by a detonator, 5-grain fulminate, and M. Turpin patented the use of picric acid, unmixed with any other substance, in 1885.  The detonation of a small quantity of dry picric acid is sufficient to detonate a much larger quantity containing as much as 17 per cent. of water.

It is chiefly due to French chemists (and to Dr Sprengel) that picric acid has come to the front as an explosive.  Melinite,[A] a substance used by the French Government for filling shells, was due to M. Turpin, and is supposed to be little else than fused picric acid mixed with gun-cotton dissolved in some solvent (acetone or ether-alcohol).  Sir F.A.  Abel has also proposed to use picric acid, mixed with nitrate of potash (3 parts) and picrate of ammonia (2 parts) as a filling for shells.  This substance requires a violent blow and strong confinement to explode it.  I am not aware, however, that it has ever been officially adopted in this country.  Messrs Designolles and Brugere have introduced military powders, consisting of mixtures of potassium and ammonium picrates with nitrate of potassium.  M. Designolles introduced three kinds of picrate powders, composed as follows:—­