CONDITIONS OF CHEMICAL CHANGE 67 



measure its original property of temporary irresponsiveness to 

 stimulation by light. The explanation of this phenomenon, 

 which had received the name of " photochemical induction," was 

 supplied by the assumption that the complex is slowly broken 

 down in the dark into the substances from which it was built 

 up — chlorine, water and hydrogen. The hypothesis in question 

 was originally advanced by Mellor and afterwards modified by 

 Bevan so as to account for the fact discovered by Draper and 

 confirmed by Bevan that the inert period is of much shorter 

 duration when the chlorine is exposed to light before being 

 mixed with the hydrogen. The modification of the hypothesis 

 necessitated by the confirmation of Draper's observation was 

 obvious and simple. The complex must be formed in two 

 stages : in the first, water molecules become united with chlorine 

 molecules ; in the second, the complexes thus produced become 

 attached to molecules of hydrogen : the subsequent fate of the 

 final product depending, as stated above, on whether it be 

 permitted to break up in the light or in the dark. When 

 examined in the light of the qualitative facts on which it was 

 based, the hypothesis was not unconvincing. Bevan, moreover, 

 maintained that, by means of experiments on the formation by 

 expansion of clouds in moist chlorine and electrolytic gas, he 

 had obtained evidence of the existence of peculiar compounds 

 — presumably the postulated complex — which could act like 

 gaseous ions as condensation nuclei for steam. 



The view that hypotheses based on the assumption of the 

 formation of intermediate complex compounds could adequately 

 account for the cardinal or subsidiary phenomena observed in 

 the study of the photochemical action of chlorine on hydrogen 

 was contested by Burgess and the writer at the Cambridge 

 meeting of the British Association in 1904 and also in the 

 Proceedings of the Chemical Society, for two distinct reasons. 

 The first objection was grounded on certain observations 

 relating to the preliminary inert period recorded by Draper and 

 confirmed by the authors. Under certain conditions when 

 electrolytic gas was exposed to light, no hydrogen chloride was 

 formed during a considerable period of time — sometimes in our 

 experiments exceeding two hours— and then the rate of formation 

 of the chloride rose in less than ten minutes to its maximum 

 value. This result was contrary to the requirements of 

 the intermediate complex hypotheses, according to any of 



