CONDITIONS OF CHEMICAL CHANGE 73 



and the writer 1 have submitted this important question to 

 re-examination, using an experimental method free from the 

 objection indicated above. Light from Harcourt's standard 

 pentane lamp, after it had traversed a column of a mixture of 

 equal volumes of chlorine and oxygen enclosed at atmospheric 

 pressure in a cylinder with transparent ends, was permitted 

 to fall on the insolation vessel of a Bunsen and Roscoe actino- 

 meter containing gas uncontaminated with any destructible 

 inhibitor ; the intensity of the light was then determined by 

 measuring the rate of interaction of the chlorine and hydrogen. 

 A mixture of chlorine and hydrogen in equal volumes and at 

 the same pressure was then substituted for the mixture of 

 chlorine and oxygen in the cylinder through which the light 

 passed before falling on the actinometer. The intensity of 

 the light proved to be the same in both cases. A mixture of 

 chlorine with an equal volume of hydrogen is therefore not 

 less transparent than a similar mixture of chlorine and oxygen. 

 The chemical change does not cause light to be absorbed ; it 

 is the light absorbed by the chlorine which stimulates the 

 molecules of the two gases to interact. Our conclusion that 

 an absorption of light does not occur as a direct result of a 

 chemical change has been confirmed recently by several workers 

 engaged on investigations relating to other photochemical 

 changes 2 and is, I believe, now generally accepted as true. 



We were now in the possession of two fundamental facts 

 on which to base a working hypothesis. . Firstly, the energy 

 which brings about the change is derived solely from the 

 light absorbed by the chlorine in virtue of the selective 

 absorption exercised by the latter; secondly, certain impurities 

 have an enormous effect in retarding the interaction of the 

 chlorine and hydrogen. At the time when these two facts 

 were established, the investigation of R. W. Wood on the 

 resonance spectra of the elements was being carried on and 

 was attracting considerable attention. Wood's work had de- 

 monstrated the great complexity of the vibrations set up in the 

 atoms and molecules of the elements by the stimulating effect 

 of light and it was known that these vibrations could be pro- 

 foundly modified by traces of impurities. Influenced by these 



1 Journal Chem. Soc. 1906, 89, 1399. 



2 Winther, Zeitsch. wiss. Photochem. 1908, 8, 242 ; Weigert, Zeitsch. 

 fllektrochem. 1908, 596, 



