ON GASEOUS EXPLOSIONS. 327 



mean temperature of the exposed surface at points separated by an inch 

 from the cooling water cannot exceed quite a moderate value. Probably 

 about 200° C. is a superior limit for the cylinder liner. Similar calcula- 

 tion of a still rougher kind, but still sufficiently accurate to give the 

 order of magnitude of the quantity sought, shows that the fluctuation above 

 and below the mean in the course of a cycle is very unlikely to exceed 

 20° C. The latter conclusion has been confirmed by some experiments 

 made by Professor Coker with a preliminary account of which he has 

 favoured the Committee. Measuring the cyclical variation of temperature 

 of the inner surface of a 12-li.p. gas-engine cylinder by methods similar 

 to those adopted by Professors Callendar and Nicholson in their well- 

 known work on the steam-engine, he found that the maximum was only 

 /° F. ill excess of the mean. The direct measurements by Professor 

 Hopkinson of the temperature of the walls of an explosion vessel lined 

 with copper strip also lead to the conclusion that it is quite moderate. 

 This cold metal must obviously profoundly affect the combustion in its 

 neighbourhood. In a layer of gas of appreciable thickness the combus- 

 tion will be of a smouldering character, depending upon the velocity 

 with which the unburnt gas in contact with the walls can diffuse into 

 the hotter regions at a distance from them, and so be brought to the 

 ignition temperature. This layer being cold and highly compressed 

 might account for a considerable fraction of the heat, though its actual 

 thickness may be only a few tenths of a millimetre. It would appear 

 probable that the continued burning which undoubtedly goes on after 

 the time of maximum pressure in many explosions, and probably also 

 occurs during the first portion at least of the expansion stroke of a gas- 

 engine, is mainly of this character.' 



Motion of the Gas. — In many explosions intense ^ ibratorj' motions of 

 the gas are set up. The effect of these sometimes appears with a quick- 

 period indicator as a rapid variation of pressure. It is a question of 

 some importance how these motions aflfect the memi pressure shown by 

 ^ g^uge. The damping-down of the motion which occurs in consequence 

 of viscosity of course only means that the motion becomes distributed 

 among the molecules in a random way, instead of following a definite 

 arrangement. The total kinetic energy remains the same. But it is not 

 certain that the mean effect on a pressure gauge of the molecular impacts 

 will be the same. This is a question which might be considered by 

 someone to whom the methods of the kinetic theory of gases are familiar. 

 It is of course not a surface phenomenon. 



Results of Observatiotis. — The temperatures reached in these explosion 

 experiments range from about 1 300° up to 3000° C. Temperatures of below 

 1500° are, however, obtained by the use of weak mixtures, involving 

 slow burning and large cooling corrections, and but little reliance can be 

 placed on the results. Langen made very few observations on mixtures 

 giving a lower temperature" than 1500°, and takes that as the lower 

 limit of the range of temperature to which his observations apply. The 

 extreme upper limit of the constant pressure experiments is 1400°, 

 The temperature of 3000° C. is about that reached in the explosion of 

 hydrogen and oxygen in their combining proportions. This is much 

 above the mean temperature ordinarily reached in the gas-engine, the 



Professor Bone is doubtful whether ' smouldering combustion ' plays so con-, 

 sulerable a part in gaseous explosions as is here suggested. 



