_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." The essential difference between an explosion and a detonation is the comparative suddenness of the transformation of the solid or liquid explosive substance into gas and vapour. Some explosive bodies, such as the fulminates, &c., always detonate, while the detonation of others depends on the mode of firing. Nitro-glycerine always explodes violently, but when fired with an initiatory charge of fulminate of mercury it is much more powerful than when fired with gunpowder. Compressed gun-cotton in the air-dry state can be detonated by 2 grains of fulminate of mercury embedded in the material, but when it contains 3 per cent. of water over and above the 2 per cent. which exists normally in the air-dry substance, 15 grains of the fulminate will not always do so. _Theory of Detonation._--The theory of detonation is not yet thoroughly understood. That it is not alone due to the heat caused by the impact of the mechanical energy of the particles of gas, set free from the initiatory charge on the principal mass, is proved by the fact of its being possible to detonate wet gun-cotton. Professor Bloxam terms detonation to be "sympathetic" explosion. Experiments carried on in England by Professor Abel, and in France by MM. Champion and Pellet, tend to show that it is due to the vibratory action of the detonating agent. Thus a glass may withstand a strong blow, though a particular note or vibration will smash it. All explosive compounds and mixtures, including gunpowder, are susceptible of violent explosion through the agency of a detonation. _Roux and Sarrau._--Roux and Sarrau divide explosions into two orders:-- 1st order.--Detonations. 2nd order.--Simple explosions. Simple explosions are produced by direct inflammation, or by a small charge of gunpowder.