ON GASEOUS EXPLOSIONS. 825 



these temperature differences by convection and conduction, could it take 

 place -without loss of heat, would cause no change of pressure. The 

 volumetric heat is, however, not constant, but may quite possibly be 50 

 per cent, greater in the hottest than in the coldest part of the mass. The 

 attainment of thermal equilibrium must, in fact, cause a change of pres- 

 sure, and would contribute to the correction which has been designated ^-> 

 (see fig. 3). The amount of the change might be the subject of rough calcu- 

 lation, taking an assumed distribution of temperature and assuming values 

 for the volumetric heat. Such a calculation in the present state of know- 

 ledge would only be of value as showing the possible order of magnitude 

 of the quantity sought, and the assumptions made could therefore be 

 of a character to make the calculation fairly simple. More accurate 

 knowledge both of temperature distribution and of thermal capacity will 

 enable greater accuracy to be attained in the estimation of this coi-rection, 

 which will be of such a kind that a method of successive approximation 

 can be pursued, the revised values of thermal capacity resulting from its 

 application being applied to a more accurate calculation of the correction 

 if necessary. 



The temperature variation set up by the cooling action of the walls is 

 a surface phenomenon, and as such the correction which it necessitates 

 can probably be determined and eliminated by experiments with vessels 

 of different sizes. The variation caused by the change of pressure during 

 the period of inflammation is not of this character ; and the necessity for 

 a large correction on this account is quite consistent with the observations 

 of Berthelot, or of Mallard and Le Chatelier and of Langen. In these experi- 

 ments the maximum pressure reached in the explosion was measured, and 

 at the time of maximum pressure very large differences of temperature 

 are known to exist at a distance from and quite independent of the 

 walls. 



Soon after maximum pressure, however, the temperatures at points 

 remote from the walls are equalised to a large extent by convection 

 currents. There then remains only the layer of gas near the walls to be 

 considered in this connection. If, therefore, the measurements be post- 

 poned until a long enough time has elapsed to admit of this internal 

 equalisation, the correction becomes of the surface kind, and can be dealt 

 with by the method ai)propriate to corrections of that type. But in that 

 case the heat lost will be too large a quantity to admit of rough estima- 

 tion ; it must be directly measured. 



Chemical Equilibrium. — ^The view that chemical equilibrium is not 

 attained until some time after the moment of maximum pressure was 

 first put forward by Clerk in 1885, who then expressed the opinion that 

 the greater part of the so called ' suppression of heat ' in explosions was 

 to be ascribed to this cause. On the other hand, Continental writers have 

 almost completely ignored it. For example, Langen makes practically no 

 reference to this in his paper. It can hardly be doubted, however, that 

 in many explosions, especially of weak mixtures, a considerable amount 

 of the energy is in the chemical form at the moment of maximum 

 pressure. On the other hand it seems probable to the Committee that 

 the amount of unburnt gas at this moment in such experiments as those 

 of Langen was not such as to very greatly affect the results. This belief 

 is based on the supposition that the incomplete combustion is due to the 

 cooling action of the walls. It seems probable that very shortly after the 

 attainment of maximum pressure — that is, within a time small compared 



