156 REPORT—1850. 
goes on much more rapidly in light than in darkness, and that peculiar 
chemical changes take place in virtue of some solar force. 
These phzenomena stand at present as isolated facts, and serve only to 
show how extensive a field of inquiry is indicated, into which the experimen- 
talist has scarcely ventured. I am not prepared at present to support the 
view which I once entertained, in common with Dr. Draper and others, that 
the results obtained afford evidence of the absorption of any solar radiation. 
We know 80 little of the constitution of molecules, and of the peculiar powers 
grouped under the name of molecular forces, and variously referred to as ca- 
pillarity, endosmose, catalysis, allotropic and epipolic forces, that it is neces- 
sary to pause in our consideration of this intricate question. 
The subject of this Report has been investigated with considerable caution 
by M. Edmund Becquerel*. His conclusions in general do not widely differ 
from those of the other investigators already named; but from these and a 
subsequent series of investigations on the dark lines of the chemical spectrum, 
M. Becquerel expresses his conviction that all chemical change is the result 
of light—luminous power.—‘‘Je crois qu’on peut conclure de l’ensemble des 
faits que j’ai réunis dans ce travail, que les phénoménes lumineux chimiques 
et phosphorogéniques proviennent d’un seul et méme agent, dont I’action est 
modifiée suivant la nature de la matiére sensible exposée ἃ son influence et 
la genre de modification dont cette substance est susceptible.” The fact 
noticed by E. Becquerel, and also by Professor Miller and Dr. Draper, that 
the chemical spectrum has the same inactive or dark lines as the luminous 
spectrum, has been thought by some to be conclusive as to the identity of 
the chemical and luminous rays; but although it certainly proves that the 
agency in both cases obeys a similar law of motion, or is subject to the same 
wave interference, it does not appear necessarily to follow that the two phez- 
nomena, so broadly distinguished in their effects, are the result of a precisely 
similar cause. 
It has been already stated that M. Edmund Becquerel distinguishes the 
most chemically active rays as exciting rays, and the least refrangible rays of 
the spectrum as continuing rays, since he finds that the chemical change 
commenced by one set of rays is capable of being continued by the other set. 
Shortly after this announcement, M. Gaudin found that the red, orange and 
yellow rays not only continue the action on iodized plates, but that they de- 
velope without mercury an image having the same appearance as that pro- 
duced by mercurial vapour. 
This class of phenomena has been also investigated by M.Claudet, so well 
known for the success with which he has prosecuted Daguerreotype portrait- 
ure}. This experimentalist states, as the result of his inquiries, that upon 
silver plates, prepared simply with iodine, all the rays ‘‘ have the property of 
decomposing the iodide of silver in a longer or shorter time, as they have that 
of producing the affinity for mercury on the bromo-iodide of silver; with the 
difference, that on the former compound the separate actions of the several 
rays continue each other, and that on the second compound these separate 
actions destroy each other. We can understand, that in the first case, all 
the rays are capable of operating the same decomposition; and that in the 
second, the affinity for mercury, when imparted by one ray, is destroyed by 
another.” 
The phenomena of phosphorescence have attracted much attention, and 
ΓΝ Effets produit sur les Corps par les cog bse Solaires. Ann. Ch. et Ph. yol. ix. N.S. 
p. 2 
+ Researches on the Theory of the principal Phenomena of Photography in the Daguer- 
reotype Process. Phil. Mag. November 1849. 
