506 ~—- Prof. Roscoe on the Influence of Light upon Chlorine. 
cally, separately through long tubes which could be exposed to 
the direct solar rays; and on observing the photo-chemical pro- 
perties of the mixed chlorine and hydrogen (by gas-light), first, 
when the separated gases had been strongly insolated, and 
secondly, when they had been carefully protected from light, we 
found that in the one case the combination did not take place 
more rapidly than it did in the second; conclusively proving 
that the previous insolation of the separated gases, and hence 
any allotropie modification of either gas, cannot account for this 
slow action on the first exposure to light. Dr. Draper’s objec- 
tion, that we did not expose the gases for a sufficient time to 
solar action, is not valid, inasmuch as the direct solar light to 
which they were separately exposed was several thousand times 
as intense as the small gas-flame which in four minutes effected 
the maximum action. 
The phenomenon in question depends upon a totally different 
cause. It belongs to a most important, but hitherto disregarded 
phase of general chemical action. In all chemical combinations 
and decompositions a certain time must elapse before the full 
action is set up. In no case does chemical action commence 
instantly on bringing the bodies under the requisite conditions ; 
and in many instances the length of time which elapses before 
the full action occurs is very considerable, so that the progres- 
sive phases of the combination can be distinctly traced. 
The phenomenon which we have considered is a striking ex- 
ample of this general condition of chemical action, of this increase 
of the chemical activity with the time under which the forces 
act. This general mode of action has been named by us Che- 
mical Induction. We have shown in our paper on Photo-chemical 
Induction, that precisely similar relations are observable in the 
photographic phenomena, and also in pure chemical actions ; our 
investigations form, in fact, starting-poimts for the consideration 
of time as affecting chemical action, a subject which I am at pre- 
sent engaged in following out. 
It. is almost superfluous for me to mention, regarding Dr. 
Draper’s experiment on the solar decomposition of chloride of 
silver, that before one can believe that the black substance inso- 
luble in nitric acid is either a modification of the metal or “a 
something which is not silver,” we must know that it is not a 
subchloride, particularly as Wéhler states that suboxide of silver 
receives a metallic lustre on burnishing. 
I am yours truly, 
Henry E. Roscor. 
