74 SCIENCE PROGRESS 



considerations, we put forward the hypothesis 1 that the light 

 which falls on the moist mixture of chlorine and hydrogen is 

 absorbed, in the first instance, by the coloured component 

 (the chlorine) and after it has been absorbed is degraded into 

 heat ; during the process of degradation, the energy passes 

 through various forms. Now it is conceivable that the distri- 

 bution of the various kinds of vibration of which the degrading 

 energy is composed will depend in certain cases largely on 

 the presence in the system of even small quantities of foreign 

 bodies. A difference in the rate of chemical change might 

 clearly be expected as a result of a marked difference in the 

 character of the energy through which the light passes as it is 

 degraded into heat. We shall see below how far this view has 

 been confirmed by subsequent discoveries. 



A statement had been made many years before this explana- 

 tion of the facts was put forward and had remained uncontested, 

 concerning the influence of the proportions of chlorine and 

 hydrogen on the rate of interaction of the gases, which, if true, 

 would have necessitated a profound modification, possibly a 

 complete abandonment, of our hypothesis. It had been an- 

 nounced, both by Draper and by Bunsen and Roscoe, that the 

 most sensitive mixture was one composed of exactly equivalent 

 proportions of the two gases, a slight excess of either having the 

 effect of reducing very appreciably the responsiveness of the 

 mixture to light. Bunsen and Roscoe assert that an excess of 

 three parts of hydrogen in a thousand reduces the rate of inter- 

 action from ioo to 37'8 and that one part of chlorine in a 

 hundred reduces it from ioo to 60. This effect required re- 

 investigation, especially as we suspected that there was a source 

 of error in Bunsen and Roscoe's experiment. Our experiments 

 were at first unsuccessful, owing no doubt to the circumstance 

 that the chlorine and hydrogen used to dilute the mixture con- 

 tained impurities. An appreciable retardation in the rate of 

 formation of hydrogen chloride was brought about by the 

 addition of a small quantity of either constituent to the electro- 

 lytic gas but its magnitude was variable. It was only after a 

 method had been devised for the preparation of chlorine and 

 hydrogen quite uncontaminated with destructible inhibitive 

 impurity and containing very little oxygen that the experiments 

 furnished consistent results. These results demonstrated con- 



1 fourn. Chem, Soc. 1906, 89, 1433. 



