82 
MR. P. V. SEVAN ON THE COMBINATION OF HYDROGEN 
immediately follows, and the final volume is one which is less than the original 
volume. 
When the light is cut off, after or before the steady velocity of contraction has 
been reached, the rate of contraction at first increases, reaches a maximum, and then 
decreases until the index comes to rest. 
The phenomena to be investigated thus fall under three heads :—1. The initial 
expansion. 2. The final contraction (when light has been cut off). 3. The period of 
induction proper. 
1, The Initial Expansion * 
This expansion can, as we have seen, be made exceedingly large. But with light of 
suitable intensity it can he kept within manageable limits. 
PpiINGSIIEIM has worked at this phenomenon, but his conclusions have not been 
confirmed in this investigation. The following experiments were therefore made very 
carefully and in considerable detail. 
* The results of this section were pul:)lished in the ‘Proceedings of the Cambridge Philosophical 
Society,’ vol. 11, Part IV., 1902. J. W. Mellok published almost simultaneously the fact that hydrogen 
chloride is formed during the momentary illumination producing the initial expansion. He also afterwards 
arrived at the conchrsion of § 5 with regard to the expansion of damp chlorine alone under the influence of 
light. Mellor’s investigations are in the ‘Journal of the Chemical Society,’ 1901, 1902. 
