It is rather difficult to-day to define what high explosives are, in
contradistinction to gunpowder. Thirty years ago we could say that
powder was a mechanical mixture and the others were chemical
compounds; but of late years this difference has disappeared.
The dynamical difference, however, still remains. Gunpowder in its
most efficient form is a slow-burning composition, which exerts a
relatively low pressure and continues it for a long time and to a
great distance. High explosives, on the contrary, in their most
efficient form, are extremely quick-burning substances, which exert an
enormous pressure within a limited radius. Ordinary black gunpowder
consists of a mechanical mixture of seventy-five per cent. of
saltpeter, fifteen per cent of charcoal, and ten per cent. of sulphur.
The most important of the high explosives are formed by the action of
nitric acid upon organic substances or other hydrocarbons, the
compound radical NO2 being substituted for a portion of the
hydrogen in the substance. The bodies thus formed are in a condition
of unstable equilibrium; but if well made from good material, they
become stable in their instability, very much like Prince Rupert's
drops, those little glass pellets which endure almost any amount of
rough usage; but once cracked, fly into infinitesimal fragments.
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