combination, but also to the foreign substance mixed with the detonating gas.36 The necessity of a definite temperature for the ignition of detonating gas is also seen from the fact that pure detonating gas explodes in the presence of a red-hot iron wire, or of charcoal heated to 275°, but with a lower degree of incandescence there is not any explosion. It may also be brought about by rapid compression, when, as is known, heat is evolved.37 Experiments made in the eudiometer showed that the ignition of detonating gas takes place at a temperature between 450° and 560°.38
The combination of hydrogen with oxygen is accompanied by the evolution of a very considerable amount of heat; according to the determinations of Favre and Silbermann,39 1 part by weight of hydrogen in forming water evolves 34,462 units of heat. Many of the most recent determinations are very close to this figure, so that it may be taken that in the formation of 18 parts of water (H2O) there are evolved 69 major calories, or 69,000 units of heat.40 If the specific heat of aqueous vapour (0·48) remained constant from the ordinary temperature to that at which the combustion of detonating gas takes place (but there is now no doubt that it increases), were the combustion concentrated at one point41 (but it occurs in the whole region of a flame), were there no loss from radiation and heat conduction, and did dissociation not take place—that is, did not a state of equilibrium between the hydrogen, oxygen, and water come about—then it would be possible to calculate the temperature of the flame of detonating gas. It would then be 8,000°.42 In reality it is very much lower, but it is nevertheless higher than the temperature attained in furnaces and flames, and is as high as 2,000°. The explosion of detonating gas is explained by this high temperature, because the aqueous vapour formed must occupy a volume at least 5 times greater than that occupied by the detonating gas at the ordinary temperature. Detonating gas emits a sound, not only as a consequence of the commotion which occurs from the rapid expansion of the heated vapour, but also because it is immediately followed by a cooling effect, the conversion of the vapour into water, and a rapid contraction.43
