the coal destined to serve for the production of fire in all the
applications of steam which we have briefly noticed in illustration.

In availing ourselves of the concentrated power accumulated in
saltpetre, we resort to bodies which easily kindle when fire is applied,
such as sulphur and finely powdered charcoal: these substances are
most intimately mixed with the saltpetre in a powdered state, and the
dampened mass subjected to great pressure is afterwards broken into
grains of varied size, constituting gunpowder.

The substances thus added to the saltpetre have both the disposition and
the power of burning with and decomposing the nitrous element of the
saltpetre, and in so doing they do not simply open the way for the
energetic action of the gases escaping, but, owing to the high
temperature produced, a new force is added.

If the gases escaped from combination simply, they would exert for every
cubic inch of saltpetre, as we have here considered it, the direct power
of 12,000 lbs.; but under the new conditions, the volume of escaping gas
has a temperature above 2,000° Fahrenheit, and consequently its force
in overcoming resistance is more than four times as great, or at least
48,000 lbs.

Such, then, is the power which can be obtained from a cubic inch of
saltpetre, when it is so compounded as to form some of the kinds of
gunpowder; and the fact of greatest importance in this connection is the
control we have over the amount of the force exerted and the time in
which the energy can be expended, by variations in the proportions of
the eliminating agents employed.