Reduced Pressures by Impulsire Electric Sparks. 353 



There is here an agreement between the possible ratios and 

 the pressures at which steps were observed which is on the 

 whole too close to be accidental. It is possible that the 

 slower rate of initial combination ofc* methane and oxygen is 

 favourable to the production of these well-marked stages. 

 Similar steps have been shown to coincide when ignition is 

 by condenser discharge. 



An action of this kind is not to be expected, though it 

 cannot be ruled out as impossible, when ignition proceeds by 

 general bombardment over the wave-front as in an estab- 

 lished travelling flame. It is most likely to occur when in 

 addition to the normal time of combination of the two gases 

 another element is introduced, in this case the time of 

 duration of the spark and of the ionization that leads up 

 to it. 



The explanation now offered why the electrical spark 

 impulse necessary for igniting gas changes suddenly when 

 the proportion of gas to oxygen goes through certain values, 

 is that in the passage of a spark ions necessary for chemical 

 combination are formed, and that before these have recoin- 

 bined electrically and become inert, combustion lias begun. 

 The proportions of gas and oxygen in the upper and lower 

 limit mixtures show* that in these mixtures there is a sudden 

 change of inflammability determined solely by the ratios 

 of the numbers of oxygen atoms to one of combustible 

 .gas. What is now found is that when the source of 

 ignition has a very short duration these steps occur whenever 

 the proportion of oxygen atoms to combustible gas passes 

 through integral values such as the first few natural 

 numbers. 



Change of gas pressure has the same effect on the rate of 

 group formation as change of proportion of the mixture. 

 From a purely electric point of view steps might occur when 

 the conductivity of the sparking path changed suddenly. 

 It is only possible to imagine such a change taking place 

 after chemical combination has begun, but if the rate of 

 development of the latter is so slow that a definite time of 

 duration of spark is necessary to allow it to work up to a 

 state of self-ignition, it might appear to have an electrical 

 origin and to explain Paterson and Campbell's stepped 

 ignition curves as depending on an instantaneous change of 

 •capacity, but the origin of their steps and those now observed 

 would be the same. 



* "The Limits of Inflammability of Gaseous Mixtures," Phil. Mag. 

 toL xxxiii. February 1917, p. 190. 



