The electrical ignition of gaseous mixtures

The ignition of explosive gaseous mixtures for experimental purposes is generally made by an electric spark or train of sparks between fixed terminals. The temperature of inflammation cannot in this case be measured, and it is determined by that of a hot surface in contact with the gas, or by calculation from its adiabatic compression. The fact that there is a critical temperature of ignition and that the velocity of an explosion wave can be calculated from the thermal constants of the gas and air, has led to the view that the process is a thermal one throughout, with in general two stages, a period of slow combustion and rise of temperature, and the true explosion on this reaching a certain limiting value. There is, however, a more intimate possible cause of the division of the molecule of combustible gas which precedes explosive combination. Recent work on the ionisation of gases has made familiar the view that a molecule can be ionised by corpuscular radiation, and that by the gain or loss of such corpuscles the nature of the molecule can be profoundly modified. The present paper is an examination of certain typical gases and vapours for the purpose of finding evidence of the mechanism of the process by which the energy of the source is transferred to the gas at the moment of ignition. A very full report on gaseous combustion was given by Prof. W. A. Bone at the Sheffield meeting of the British Association, in the discussion upon which Sir J. J. Thomson called attention to the possible influence of electrons in preparing the way for an explosion wave by ionising the gas. Following this suggestion an important series of observations on gaseous ignitions has been recently made by my colleague, Mr. J. R. Thompson. He found that it is possible to ignite a cold explosive mixture by the incidence of X-rays on a platinum surface in it, and that when the source of ignition is a hot platinum wire an explosion is started at that temperature at which ions are discharges from the metal. These observations, if they do not decide the ionic origin of gaseous explosions in general, prove that ionisation and explosion are intimately connected.