On Light and the chemical action of the Spectrum, 267 
as the first brown layer of the salt A described. Yet the de- 
position of this basic iron-salt must have happened under sin- 
gular circumstances, for it is a well-known fact that a solution 
of iron- vitriol, oxidized by the atmosphere, is precipitated as 
a five-fold basic salt. It is likewise difficult to explain how 
the yellow layer containing alkali suddenly succeeded the dark- 
brown ; it may indeed be supposed, that at the commencement 
of this decomposition of the iron pyrites the alum-slate re- 
sisted for some time all action, until it was attacked, and its 
alkali dissolved by the sulphuric acid, which commenced the 
formation of a new salt. But if this mode of explanation has 
much appearance of probability, the sudden cessation of the 
one product of decomposition, and the commencement of the 
second, is a strange fact. That the gypsum, as the more 
easily soluble substance, is found on the inferior part of the 
ceiling of the cavern is, on the other hand, easily conceived. 
The lime in it undoubtedly acted no unimportant part at the 
deposition of the iron salts described, aiding in their precipi- 
tation by saturating the acid. 
XL VI II. Experiments and Observations on Light which has 
permeated coloured Media^ and on the Chemical Action of 
the Solar Spectrum, By Robert Hunt.^ 
l\/r GAY-LUSSAC, when speaking of the beautiful dis- 
*^^-*-* covery of M. Daguerre, said, The palette of the 
painter is not very rich in colour, black and white compose 
the whole. The image in its natural and varied colours may 
remain long, perhaps for ever, a thing hidden from human 
sagacity f 
However, the production of a coloured picture of the 
spectrum by Sir John Herschel, and some efects produced 
by Mr. Talbot, together with some delicate tinting which I 
observed, when, during the summer of 1839, I was engaged 
in copying some flowers of Nature’s richest painting, led me 
to think coloured photographs within the range of probabi- 
lities, and induced me to pursue a train of experiments from 
which, although little has resulted to heighten niy first hopes, 
I have gathered much that is curious and certainly instruct- 
ive. 
Photographic Papers, 
1. By saturating paper with different chlorides and mu- 
* Communicated by the Author. 
t “ The History and Practice of Photogenic Drawing, &c., by L. J. M. 
Daguerre. Translated by J. S. Memes, LL.D.’* 
