﻿96 Messrs. H. T. Tizard and D. R. Pye on the 



law of mass action, even after taking into account the possible 

 effects of a different rate of loss of heat in the mixtures with 

 which they experimented ; the authors state, in fact, that they 

 "can offer no satisfactory explanation of the phenomena 

 observed." Recent experiments by one of us * have shown 

 that the ignition temperatures of hydrocarbon-air mixtures 

 are independent, within the errors of measurement, of the 

 proportion of the hydrocarbon within quite wide limits. For 

 instance, when the proportion of heptane to air was varied in 

 the ratio of 1 to 10, the ignition temperature was only lowered 

 apparently by some 8° C. from 293° C. to 285° C. As the 

 error of observation may certainly amount to 4° C. in such 

 experiments, there is here no evidence that the alteration of 

 the concentration of heptane has any effect on the ignition 

 temperature. If, as might appear reasonable on the law of 

 mass action, the rate of reaction depended directly on the 

 concentration of the heptane, we should expect it to be 10 

 times as great in one case as in the other. Now, it is shown 

 later in this paper that the temperature coefficient B, in the 

 case of heptane, is about 13,000, from which it follows from 

 equation (5) or (8) that if the initial rate of reaction on com- 

 pression were really 10 times as great in one case as in the 

 other, the ignition temperature of the richer mixture should 

 be over 50° lower than that of the weaker mixture. One 

 could not fail to detect with certainty a difference of this 

 order : hence from the experimental results we come to the 

 conclusion that the rate of combustion under these conditions 

 does not depend on the concentration of the heptane vapour 

 within wide limits. Experiments with other similar sub- 

 stances support this view. 



YI. If this be correct, the only probable alternative is that 

 the rate depends essentially on the concentration of the 

 oxygen. We have not yet attempted to put this to a direct 

 test in the apparatus used by us, since it would be necessary, 

 to do this completely, to explode detonating mixtures of 

 hydrocarbons and oxygen ; and we were anxious to avoid 

 the danger of breaking the apparatus before other important 

 experiments were carried out. We intend, however, to 

 examine this point in the near future. 



It remains to consider whether, if the rate of reaction is 

 directly proportional to the concentration of oxygen, the 

 effect of the automatic decrease in oxygen content on ignition 

 can be safely left out of account in calculating " delays " by 



* Tizard (he. oit.). 



