of Hydrogen and Chlorine tinder the influence of Light. 405 

 hies of k we find for B using the equ; 



d _ (m , djog&i _ T 2 dlog ^\ 3_ 



r 1 e^ 2 dT 2 JTf-Tf 



from the values of & we find for 5 using the equation in the 

 form 



T B T, = 10° 



20° -410 



30 -392 



40 -372 



50 -391 



60 -403 



And using the mean value of B, '398, we obtain for A 



10° -488 



20 - 483 



30 - 480 



40 - 483 



50 - 485 



60 - 484 



The constancy of these values is more accurate than we could 

 have expected. It seems, however, to shew that the assumption 

 of special molecules taking part in the first stages of the action 

 or some equivalent supposition will lead us to an understanding 

 of the mechanism of the action. As we should expect the constant 

 A is negative, which indicates that the higher the temperature 

 the smaller the number of special molecules in the water vapour. 

 B is positive, which to some extent indicates that the action 

 between the chlorine molecules and the special water molecules 

 is one which increases with temperature. Aggregates of water 

 molecules as a step in the action would require higher powers 

 of the vapour pressure in the expression for the velocity, and 

 this would lead to results inconsistent with the experiments, 

 unless in some way aggregates occurred whose number was pro- 

 portional to the first power of the pressure, that is to the first 

 power of the concentration of the water vapour. At present it 

 is difficult to see how this could occur, so we assume the existence 

 of the special kind of molecules. We have as yet no evidence to 

 throw light on the question as to how these differ from ordinary 

 molecules. 



