6 Combination of Hydrogen and Chlorine l>y Light. [Apr. 1, 



a cloud can condense when the gas is submitted to a certain expansion. 

 This cloud-forming substance appears before any hydrochloric acid is 

 formed, and thus appears to be due to a true intermediate body. 



The induction period, or period of acceleration of the action, is an 

 essential part of the combination, and is to be attributed to the forma- 

 tion of intermediate compounds from water vapour and the two 

 reacting gases. The nature of these compounds is not discovered, 

 but the view is taken that they are molecular aggregates in which the 

 individual atoms can come into each other's spheres of influence and so 

 make intra-aggregate systems without much action on the whole indivi- 

 dual, which can then break up, giving rise to the more stable systems, 

 water and hydrochloric acid. This view of the process of the action 

 can be shown to involve the qualitative characteristics of the initial 

 stages of this particular action, and can be extended to actions in 

 gaseous systems when the presence of a catalyser such as water vapour 

 is necessary for the progress of the action at a finite rate. In the case 

 of such actions a period of induction must be expected to occur, and 

 the application of the law of mass action must be made with reference 

 to the intermediate compounds formed with the catalyser. There is, 

 therefore, no reason to expect that agreement will be found between 

 the theory of mass action, as applied to the end-product equations, 

 and the actual experimental results. 



