72 SCIENCE PROGRESS 



exceedingly minute amount of the vapour. I estimate that a 

 sensitive mixture of chlorine and hydrogen which contains 

 one molecule of nitrogen chloride to 1,000,000 molecules of 

 chlorine and hydrogen is at least ioo times less sensitive 

 to light than a similar mixture which contains none ot the 

 vapour. The inhibitory effect of oxygen discovered by Bunsen 

 and Roscoe is surprisingly large but that of nitrogen chloride 

 is very many times greater. As photochemical changes are so 

 sensitive to the influence of common impurities, it is not 

 surprising that so little progress has been made in the eluci- 

 dation of the laws which control chemical transformations 

 induced by the agency of light. 



It will now be convenient to relinquish for the present the 

 inquiry into the phenomenon of photochemical inhibition, in 

 order that we may pass on to the discussion of a question to 

 which an answer must be found before we can formulate any 

 views concerning the mechanism of the influence of light 

 in promoting certain chemical changes. When white light 

 traverses a mixture of chlorine and hydrogen, we know that 

 some of the rays are extinguished, since the emergent beam 

 is coloured. Is the whole of this abstracted light absorbed 

 by the one coloured constituent, chlorine, without the inter- 

 vention of the hydrogen ; or is a certain proportion of the 

 light extinguished as a result of the chemical change which is 

 proceeding, the amount being proportional to the change ? 

 More than one authority has asserted that the latter is the 

 correct view. Bunsen and Roscoe interpret some of their 

 experiments with the aid of the assumption that the absorbed 

 rays can be divided into two distinct parts, those which are 

 absorbed by the constituents of the mixture in virtue of the 

 optical properties of these and those which affect the chemical 

 changes. If this view were correct, a mixture of air and 

 chlorine in equal volumes would absorb less light than a 

 mixture in equivalent proportions of chlorine and hydrogen. 

 Bunsen and Roscoe claim to have shown experimentally that 

 such is the case ; but in order to interpret the results of their 

 experiments, they were compelled to assume that a formula 

 which is only strictly applicable to monochromatic light could 

 for all practical purposes be used to interpret the results ol 

 experiments performed with composite light. This objection 

 was fully realised by Bunsen and Roscoe at the time. Burgess 



