Having now found what saltpetre is and how it is formed, let us advance to the consideration of it as a source of power.

Through the exertion of chemical attraction the gaseous elements of the atmosphere have become solid in the saltpetre; and as we know the weight of this part in a cubic inch of saltpetre, the volume of the gases combined is easily ascertained to be about eight hundred times that of the saltpetre.  Hence, as every cubic inch of condensation represents an atmosphere as large as the cubic inch of saltpetre formed, we may roughly estimate that the condensing force arising from chemical attraction in this case is 800 times 15 lbs., or 12,000 lbs.!

Strictly speaking, only about four-tenths of a cubic inch of potash holds this enormous power in connection with it so as to form a cubic inch of saltpetre, which we may handle and bruise, may melt and cool, dissolve and crystallize, without explosion or change.  It contains conserved a force which represents the aggregate result of innumerable minute actions, taking place among portions of matter which escape our senses from their minuteness and excite our wonder by their transformation.  Closely similar are these actions to the agencies in vegetation which build up the wood of the tree or the material of 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 deg.  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.

We have used the well-known term Gunpowder to express the compound by which we easily obtain the power latent in saltpetre; and the use of the term suggests the employment of guns, which is secondary to the main point we are illustrating.  As the enormous consumption of power takes place during peaceful times, so the consumption of saltpetre during a state of war is much lessened, because the prosecution of public and private works is then nearly suspended.