CHEMICAL ACTION OF THE SOLAR RADIATIONS. 157 
in many of the instances, that electricity is an active exciting agent appears 
proved; but in the phosphorescence, produced by the solar rays, we have 
effects which can scarcely be referred so easily to electrical effect. 
If sulphuret of calcium (Canton’s phosphorus) or the sulphuret of barium 
(the Bolognian stone) are exposed to sunshine they become luminous in the 
dark. If a paper covered with either of these substances is rendered luminous 
by exposure to sunshine, and is put under the influence of the solar spectrum, 
two very dissimilar actions occur; over that portion of the spectrum where 
the chemical rays exert their maximum power the phosphorescence is greatly 
increased, but that portion on which the least refrangible rays fall, is com- 
pletely darkened. If the phosphorescent body is rendered luminous by the 
action of the actinic radiations, this phosphorescence is immediately destroyed 
by the momentary action of the calorific rays of the red spaces of the spec- 
trum. This latter effect is not a mere formation of heat, since by the agency 
of artificial heat we can increase the amount of phosphorescence which is 
excited by the rays at the chemical end of the spectrum. Seebeck appears 
to have been the first to notice this peculiar property of the red rays. 
In 1839 Edmund Becquerel first directed attention to the electricity deve- 
loped during the chemical action excited by solar agency. Plates of platina, 
being placed in acidulated water, were connected with a delicate galvano- 
meter ; and the needle, after the first disturbance having come to rest at zero, 
the spectral radiations, commencing with the red, were thrown upon one of 
the plates. Neither the red, orange, yellow or green rays produced any 
action ; the blue and indigo induced a slight disturbance ; but the violet rays 
and the dark rays beyond the violet gave very decided indications of action 
by the deflections of the galvanometer. These experiments were repeated by 
me with many modifications*. I never obtained any deflections of the gal- 
vanometric needles by any rays below the green; we must therefore conclude 
that electro-chemical action is due to the most refrangible rays. At the York 
meeting of the British Association I produced some experiments, showing 
that certain electro-chemical decompositions which took place in the dark, 
giving rise to delicate metallic precipitates, were entirely prevented by expo- 
sure to sunshine. It is my intention to prosecute this line of inquiry with 
all care at the earliest opportunity. 
In Poggendorff’s ‘Annalen’ for 1842, M. Ludwig Moser announced the 
discovery of some very remarkable phenomena which he attributed to light. 
These are, ‘‘If a surface has been touched in any particular parts by any 
body, it acquires the property of precipitating all vapours which adhere to it, 
or which combine chemically with it, on these spots, differently to what it 
does on the other untouched parts.” Dr. Draper described some similar 
phenomena in 1840. In three papers, which have been translated and 
published in the ‘ Scientific Memoirs,’ Moser has stated all the results which 
he obtained; the deductions from these were, that light was susceptible of 
becoming latent, and that it was continually being radiated as ‘* invisible 
light”’ from all bodies, different bodies giving off rays of different refrangibi- 
lity. After a very searching examination of all the phenomena, I arrived at 
conclusions widely differing from those of Moser, and I was induced to refer 
them all to the influence of calorific radiationst. M. Fizeaut states his 
belief that the effects observed are the result of organic matter being trans- 
ferred from one surface to another, and Professor Grove has expressed him- 
self favourable to the same view. I believe, however, that invisible heat 
* Researches on Light, by Robert Hunt, p. 213, 
Ὑ On Thermography. Phil. Mag. Dec. 1842. ὁ Comptes Rendus, Noy. 1842, 
