, but there is always something
fatally defective about the tactics pursued in using them. It may be
said in general terms that in these days of extreme power in fighting
machines, the greater the efficiency the less the simplicity and the
more knowledge required in the care of the weapons. When powder was
merely powder the advice of the old adage to "trust in God and keep
your powder dry" was ample to maintain the efficiency of the powder
for all purposes; but nowadays if you keep your powder dry you will
burst your gun, and if you keep your gun-cotton dry you are liable to
blow up your ship.

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.

The power exerted by these nitro-substitution products is due to the
fact that they detonate, i.e., they are instantaneously converted into
colorless gas at a very high temperature, and in addition they have
almost no solid residue. Nitro-glycerine actually leaves none at all,
while gunpowder leaves sixty-eight per cent. The first departure in
gunpowder from the old-time constituents of black powder just
mentioned was for the purpose of obtaining less pressure and slower
combustion than could be produced by mere granulating or