Explosion

Explosion. In pure chemistry the term explosion is given to a change of condition of a chemical system, which, once initiated, is quickly propagated through the system with the evolution of energy, and without the supply of external energy. The term is, however, generally applied to any sudden bursting of a body, usually owing to the rapid generation of large quantities of gas, though in some cases, as in boiler explosions, owing to gradually increasing pressure from the interior.

Explosions are usually initiated by it flame or sudden concussion. In the latter case rapid impact of a small mass is more efficacious than that of a more slowly moving larger body. In some cases a particular form of vibration can cause the explosion; thus iodide of nitrogen placed on a vibrating string explodes when a certain rate of vibration is reached. Many substances are dependent on some resistance for their explosion, merely burning quietly if the gases can pass freely away. In such cases the slightest possible resistance, as a sheet of gaper, may be sufficient to convert the harmless combustion into a dangerous detonation. Explosions in coal mines have long attracted much attention, but some doubts still exist as to the chief cause. They are usually ascribed to the marsh gas (fire-damp) CH4 in the mine, but it has been shown that the ignition of fine dry coal dust alone is sufficient to cause an explosion, just as flour dust has given rise to explosions in flour mills. The cause of the work done by the explosion of a substance, such as gunpowder, is the very rapid evolution of large quantities of gases, which would occupy in the substance mentioned, under ordinary conditions, about 300 times the space taken up by the powder itself, while owing to the high temperature it has been estimated that the pressure, when the powder entirely fills the space in which it is fixed, rises to 6 or 7 thousand atmospheres. The velocity of explosion - i.e. the rate at which the changes are propagated through the system, and its dependence upon the external physical conditions - has been the subject of much experimental work, though owing to its nature it offers great difficulty to accurate experiment.