Upon the quickness of combustion mainly depends the applicability of gunpowder for Military purposes.
Ignition by heat.
Gunpowder may be inflamed in a variety of ways, but whatever be the method, one portion of the substance must in the first instance be raised to a temperature a little above that necessary to sublime the sulphur, which can be removed from the other ingredients, by gradually raising the compound to a heat sufficient to drive it off in a state of vapour. The heat required for this purpose is between 600° and 680° Fahrenheit.
Progressive combustion.
When a charge of powder is exploded in the bore of a gun, to all appearance there would seem to be an instantaneous generation of the whole force. But in fact it is not so, a certain time being necessary to the complete combustion of the substance. This gradual firing is of the utmost importance, for were it otherwise, the gun, unless of enormous strength, must be shattered in pieces, as well as the projectile; for in such a case, this great force being suddenly exerted upon one part only of the material, there would not be time for the action to be distributed over the particles, at any great distance, before those in the immediate vicinity of the explosion, were forced out of the sphere of action of the cohesive force, and consequently rupture must take place.
Substances which have a more violent action than powder.
The effect of such an action may be observed by exploding detonating powders, in which are contained chlorate of potash or fulminating mercury. The action of that peculiar substance the chlorite of nitrogen is still more remarkable. There is also another compound, containing three parts of saltpetre, one part of carbonate of potash and one part of sulphur, which when brought to a certain heat will explode with great violence, its destructive force being very considerable; and this is principally due to the rapidity of the evolution of the gas, for its amount is less than that produced from gunpowder, but the complete decomposition occurs in a much shorter time.
In a damp state less quickly fired, and why.
If gunpowder be in a damp state, the velocity of combustion will be less than when dry, and also a longer time will be necessary to ignite it, since the moisture upon its conversion into vapour, absorbs a certain amount of heat which remains latent, and of which the useful effects so far as igniting the powder is concerned, is entirely lost.
Ignition by percussion.