﻿758 Dr. E. B. Ludlam on the Action of 



electronegative element upon which we can experiment, 

 showed any similar photoelectric effect. 



Lenard has found that in the case of oxygen a shorter 

 wave-length is necessary to produce ionization than is 

 required to convert the oxygen into ozone. Shenstone 

 curried out an experiment * to discover if any ozone-like 

 modification of chlorine was produced by means of the silent 

 electric discharge, but his result was purely negative. 



Chlorine, being green, absorbs light in the visible portion 

 of the spectrum, and there is no doubt that light of this 

 wave-length is effective in producing chemical change. It 

 was, therefore, of interest to ascertain whether any ionization 

 of the gas was measurable when it was exposed to light of 

 various wave-lengths, including such light as is effective in 

 ionizing oxygen. In this connexion it is worthy of note that 

 the spectrum of chlorine in the ultra-violet region does not 

 consist of lines but of a band, and in this region the emission 

 spectrum is not the same as the absorption spectrum ; from 

 this it may be anticipated that the mechanism of the absorp- 

 tion in the ultra-violet band is not the same as in the portion 

 consisting of lines, and this band absorption might fully 

 prevent the ejection of electrons. 



But, of all the problems connected with chlorine, the most 

 fascinating is the mechanism of its union with hydrogen under 

 the influence of light. Under suitable conditions the union 

 takes place with explosion, but, that this may occur, the 

 gases must be mixed in equal volumes, they must be free 

 from certain impurities which impede the reaction, the 

 illumination must be powerful, and the gases must not 

 be absolutely dry. 



The actual amount of energy supplied by the light is 

 negligible compared with that liberated by the reaction, 

 therefore the light plays the part of the trigger to the 

 explosion, and the new equilibrium which it sets up is not 

 displaced by the energy supplied by the light. 



It was mainly with a view to obtain some knowledge of 

 the mechanism of this catalytic trigger action that the work 

 described in this paper was undertaken, for one point of first 

 importance to settle is the question as to whether light is 

 capable of ionizing; chlorine. 



Before dealing with the actual experiments, it may be 

 serviceable to state briefly some of the most recent results 

 obtained by Lenard and his collaborators, as these have not 

 Vet appeared in a publication of wide circulation f. 



* Shenstone, J. C. S. Trans, vol. Ixxi. p. 487. 



t See Lenard & Kamsauer, Trans. Heidelberg Academy. Aug. 2, 

 1910 to Aug. 4. 1911, five parts. 



