30 Larmor, Physical Aspect of the Atomic Theory. 



reactions where more than two molecules are concerned, 

 such as 2H2 + O2 and 2CO + O2 the chance of them 

 all being in contiguity at the same instant is extremely 

 small compared with that for two, and the reaction can 

 therefore only proceed very slowly, unless it proceeds by 

 intermediate binary reactions such as the formation of 

 H2O2 in the first case or reaction with contained water 

 vapour in the second case. This superior velocity of 

 binary combination has possibly a bearing on the specific 

 catalytic action of traces of certain foreign vapours in 

 facilitating explosive combination, as determined by 

 Dixon." I see no reason to abandon the conclusion thus ex- 

 pressed, t is quite sound to reason, in the statistical man- 

 ner introduced by Guldberg and Waage, that the relative 

 number of direct diad combinations between molecules of 

 types A and B is k^j^.nj^.ni^, where the factors of type 

 n are the respective numbers of these molecules, and that 

 the number of direct triad combinations between types 

 A, B and C is k ^Ji(,. Hj^.u . u^ ; but it would appear that the 

 coefficient ^^ec of the latter combination must be almost 

 infinitesimally small compared with Z'^^. Thus if we 

 imagine the scale of magnitude of a gas at a pressure of one 

 atmosphere to be magnified so that the diameter of each 

 moving molecule becomes about one inch, there will be in the 

 model roughly about one molecule in each cubic foot, and 

 a molecule will have to travel about a hundred feet before 

 it encounters another one. Such binary encounters will 

 thus happen with some frequency, and from some of them 

 combination may ensue. But the chance of three molecules 

 coming together simultaneously is negligible : the only way 

 in which a trimolecular combination can arise is by one of 

 the molecules attaching to itself another, in a manner 

 perhaps relatively transient, and this pair going off 

 together to meet the third, each acting so to speak as a 



