A large charge of gunpowder fired in the ordinary way under water requires a strong case to retain the gases until the action has become general, or, owing to its slow rate of burning, the case would be broken before the whole of the charge had been ignited, and part of the charge drowned.
This is often to be noticed when firing fine-grained powder in heavy guns.
Igniting the charge at several points diminishes the confinement needed.
Mode of Firing.—The application of heat, directly or indirectly, is the principal means of causing an explosion.
The flame from a percussion cap or primer, or a platinum wire heated to incandescence by an electric current, will directly ignite a charge. Friction, concussion, &c., will indirectly ignite a charge due to the conversion of mechanical energy into heat.
It would appear that when one explosive body is used as a means of firing another, the resultant explosion is due to the blow suddenly formed by the gas of the firing charge acting percussively upon the mass to be exploded. If such were the case, then the most powerful explosive would be the best agent for causing an explosion. But it is not so.
For example, nitro-glycerine, which is far more powerful than fulminate of mercury, requires more than 1000 grains to explode gun-cotton, while only 15 grains of the latter is needful for the same work, &c.
A small quantity of an explosive substance which is sensitive to friction or percussion is often used to ignite the original charge.
Detonation.—The instantaneous explosion of the whole mass of a body is defined as "detonation."