CHEMICAL ACTION OF THE SOLAR RADIATIONS. 141 
point so brilliant that the eyes were scarcely able to endure it ; yet the muriate 
of silver remained exposed more than two hours to this brilliant point of light 
without undergoing any sensible alteration. On-the other hand, when exposed 
to the other pencil, which was much less bright and less hot, it was blackened 
in less than six minutes*.” This is the earliest intimation we have of any 
indication that the luminous and chemical powers may be due to dissimilar 
agencies. On this, the Commissioners remark :—‘ If we wish to consider 
solar light as composed of three distinct substances, one of which occasions 
light, another heat, and the third chemical combinations; it will follow that 
each of these substances is separable by the prism into an infinity of different 
modifications, like light itself; since we find by experiment, that each of 
the three properties, chemical, colorific and calorific, is spread, though un- 
equally, over a certain extent of the spectrum. Hence we must suppose, on 
that hypothesis, that there exists three spectrums one above another; namely 
a calorific, a colorific and a chemical spectrum. We must likewise admit that 
each of the substances which compose the three spectrums, and even each 
molecule of unequal refrangibility which constitutes these substances, is en- 
dowed, like the molecules of visible light, with the property of being polar- 
ized by reflection, and of escaping from reflection in the same positions as 
the luminous molecules, ἅς. Some other objections to M. Berard’s views 
are then urged. The experiment, already named, by Dr. Young on the che- 
mical action of the dark rings, and analogous ones, to be yet noticed, by 
M. Edmund Becquerel and Professor Miller, go to show, that whether the 
chemical agency is due to the same principle which produces light or not, it 
certainly obeys nearly all the same general laws. 
It was stated by Arago and others, that M. Charles, an experimentalist of 
some celebrity, had a process by which he was enabled to produce portraits 
by the aid of light. He died however without disclosing his secret ; and even 
the Abbé Moigno, always anxious to claim for France the honour of any 
discovery, admits that Charles left “‘ no authentic document to attest his dis- 
covery ;” and he consequently gives to Wedgwood the merit of being the 
originator of photography. M. Niepce, of Chalons on the Saéne, communi- 
cated to our Royal Society, in 1827, an account of his experiments, upon 
which it would appear he had been engaged since 1814. This memoir was 
not printed by the Royal Society, owing to the circumstance that M. Niepce 
refused to publish the secret of the process by which he produced the pictures 
he then exhibited, some of which are now in the possession of Mr. Robert 
Brown of the British Museum. 
-- The discovery of Niepce appears to have been, that the luminous rays have the 
property of solidifying several resinous substances, thus rendering the parts 
which had been exposed less soluble than these which have been preserved in 
shadow. He appears to have also observed, that resins thus changed by the 
influence of sunshine returned to their original state when kept for a short time 
in the dark. We have not however any exact statement of the researches of 
Niepce, which appear to have been extensive, but devoted principally to the 
production of what he calied heliographic pictures. In 1829 Niepce associated 
himself with Daguerre, to whom we owe the iodized silver plate and the disco- 
very of that peculiar condition induced by the solar rays, which regulates the 
deposit of mercurial vapour ; the distinguishing feature of the well-known Da- 
guerreotype process. In addition to this, Niepce discovered that silver plates 
could be rendered sensitive to solar agency by being washed “‘ with a decoction 
of the herb Shepherd’s purse (Thlaspi Bursa-pastoris),fumes of phosphorus, and 
* Annales de Chimie, vol. Ixxxv. p. 309. 
