MODIFIED BY COLOURED GLASS MEDIA, ETC. 
261 
then combining them again with the upper surface ? It is impossible to admit that 
the red radiation is endowed at the same time with the property of separating and 
the property of reuniting the same elements. We must then attribute it to a parti- 
cular force — electricity perhaps, which might accompany each radiation, and which, 
under the influence of the one, would act positively, and negatively under the other, 
without changing the chemical compound. In one case this influence would give 
the affinity for mercury, and in the other destroy it. 
At all events, we must look for another explanation of the phenomenon than the 
one which has hitherto been received, viz. the decomposition of the iodide of silver 
by the action of light. It is true that light decomposes iodide of silver, forming a 
subiodide, but this seems to require a longer time than that during which the sur- 
face is endowed with the property of attracting the vapours of mercury. In fact, the 
last property is communicated nearly instantaneously, which is not the case for the 
decomposition of the iodide by the action of light. 
