CONDITIONS OF CHEMICAL CHANGE 



II. PHOTOCHEMICAL CHANGE IN GASES 

 Part II. Experimental 1 



By D. L. CHAPMAN, M.A. 



In the preceding article, in which the views held by different 

 investigators on the mechanism of the interaction of chlorine 

 and hydrogen were explained and discussed more or less in 

 detail, it was indicated that the most favoured hypothesis 

 during several years preceding 1905 involved the assumption 

 that an intermediate complex was formed, containing chlorine, 

 hydrogen and water, of the type represented by the formula 

 [Cl 2 ] x , [H 2 0] y , [H 2 ] z . It was supposed that this did not exist 

 in a freshly prepared mixture of chlorine and hydrogen but 

 was gradually produced when the mixture was exposed to light 

 and subsequently destroyed so as to form hydrogen chloride 

 and water, the production and decomposition taking place in 

 accordance with the law of mass. According to this view, 

 hydrogen chloride could not be generated in the system by the 

 direct interaction of molecules of chlorine and hydrogen but 

 only by the breaking-down of the unstable system in which they 

 were associated with water. It was claimed that by this explana- 

 tion it was possible to account satisfactorily for the accelerative 

 influence of moisture on the change and also for the initial inert 

 period which is generally observed when a mixture of freshly 

 prepared chlorine and hydrogen is exposed to light. Moreover, 

 the hypothesis supplied an intelligible account of another well- 

 known peculiarity of a mixture of chlorine and hydrogen, 

 namely, that when it has been exposed to light until the rate of 

 formation of hydrogen chloride is constant {i.e. until the con- 

 centration of the complex attains its maximum value) and is 

 then left in the dark during several hours, it regains in some 



1 A description and diagram of an actinometer with which most of the ex- 

 periments on chlorine and hydrogen described in this article can be carried out 

 are given at the end of the article. 



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