Reduced Pressures by Impulsive Electric Sparks. 351 



between the numbers of molecules in such groups that the 

 observed sudden stages in ignition are here considered to 

 arise. 



There are in hydrogen-air mixtures two such possibilities, 

 the clusters may be of hydrogen molecules around an 

 activated oxygen molecule, or the reverse. The molecules 

 through which the spark immediately passes may even bo 

 dissociated by its extremely high temperature so that, as in 

 arcs, transient groups in which there are atoms may be 

 formed. The ratios of collision frequency between hydrogen 

 molecules in a mixture can be varied in two ways, by 

 changing the proportion of gas to air, or by changing the 

 pressure of a mixture of fixed composition. A variation of 

 action which takes place as a consequence of change of 

 pressure can be expressed in terms of the corresponding- 

 variation of composition. There was no a priori reason to 

 expect that such a mode of interpretation would explain the 

 salient points of the ignition curves, but ic appears on 

 making the comparison that there is a suggestive coincidence 

 of this nature. 



The composition of the mixture was 29'2 per cent, of 

 hydrogen by volume in air, giving the proportion H 2 -j-0. 

 Bv raising the proportion of hydrogen a series of mixtures is 

 obtained in which the ratio of combustible gas to oxygen has 

 the values of the natural numbers in succession. Taking the 

 collision frequency in the H 2 4-0 mixture at atmospheric 

 pressure as unity .that in the other mixtures is given below. 



Table I. 



Mixture. 



H 2 + 



2H 2 +0 



3H 2 +0 



4H 2 -t-0 



5H 2 +o .; 



6H 2 +0 



8H 2 +0 



10H 2 +O 



The third column is the pressure in atmospheres which 

 has the same ratio of collision frequency or partial pressure 

 as the mixtures in the second column at atmospheric 

 pressures. 





Ratio of 





Per cent. 



collision 



Observed 



in Air. 



frequencies. 



steps at 



292 



10 



1-0 



45-2 



064 



•6 



552 



0-53 



— 



622 



0-47 



•48 



67-4 



0-43 



— 



70-2 



0-41 



•4 



70-8 



0-38 



'38 



80-7 



0-3(5 



— 



