IGNITION

—This term is applied to combustions in which the fuel (material to be burned), unites with oxygen at such a rapid rate, that the reaction is accompanied by flame. All combustible material, considered chemically, unites with oxygen at any temperature, although in most instances the reaction at ordinary temperatures is so slow that it cannot be detected without accurate scientific apparatus. Like all chemical reactions this union may be increased by the applicanian of heat. In general their speed is doubled by a rise in temperature of 10° C. (18° F.). Eventually a point is reached at which the union proceeds so rapidly that the material bursts into flame, 7.e,, the reaction itself generates sufficient heat to heat particles of unburned material to incandescence, For all combustible substances there is a characteristic temperature called the ‘ignition point.’ If a very small portion be heated to this point, the entire ma quickly ignites, owing to the heat which is generated by the local reaction. When the speed of such ignition exceeds moderate limits, a so-called ‘explosion’ occurs. Waves of explosion may travel as fast as 5 miles per second. The application of this phenomenon is very extensive. The striking of a match consists in locally heating its surface by friction to the ignition point. Similarly the match applied to paper heats a small section. The ignition point or ‘flame point’ of gasoline and kerosene is regulated by law, and oils which catch fire below this temperature cannot be sold. Such laws have minimized the dangers of explosion and fire in the domestic use of i mineral oils. All gasoline engines are supplied with ‘ignition systems,’ which are a means of locally heating the explosive mixture of gasoline and air, b y an electric spark, this constituting the most convenient and easily controlled method of ignition.