302 Mr. T. Carlton Sutton on the 



The existence of this exponential indicates that either o£ 

 the following mechanisms is possible : — 



(1) For molecules of P l5 P 2 , &c. to react in accordance 

 with the forward action, they must collide with a relative 

 velocity of approach greater than some particular value v. 



Langevin * has shown that the number of such collisions 

 varies as 



-(-) 



where a is a positive constant, and the complete expression 

 for the amount of the forward action is therefore 



*n[Pj Xi « ° s/e. 



Hence, from the final result in Part I. of this paper, the 

 complete expression for the amount of the back action is 



MICQi]** *•/•*, 



where v' is such that Planck's b is equal to a(v 2 — v' 2 ). 



This result merely requires that for molecules of Q l5 Q 2 , &c. 

 to react in accordance with the back action, they must collide 

 with a relative velocity greater than some value v' . 



(2) A consistent result is also given by the assumption 

 that for molecules of P 1? P 2 , &c. to react in accordance with 

 the forward action, they must collide with a relative velocity 

 of approach lying between v and (v + 8v). 



The number of such collisions has been shown by Maxwell, 

 Clausius, and others to varv as 



s/e 



(- a ?) 



&, 



where a' is a positive constant, and the complete expression 

 for the amount of forward action is 



kUlP^e'^hv. 



Hence, the complete expression for the back action may be 

 written in accordance with the result of Part L as 



where Planck's b is a'(v 2 — v' 2 ) and Planck's a is kSv/k'Sv', 

 * Le Radium, April 1913, Langevin and Rey. 



