Oil 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. 



XLVIII. Experiments and Observations on Light which has 

 permeated coloured Media^ and on the Chemical Action of 

 the Solar Spectrum. By Robert Hunt.* 



IV/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 t •" 



However, the production of a coloured picture of the 

 spectrum by Sir John Herschel, and some effects 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 my first hopes, 

 I have gathered much that is curious and certainly instruct- 

 ive. 



Photographic Papers. 



1. By saturating paper with different chlorides and mu- 



* Gommunicated by the Author. 



t " The History and Practice of Photogenic Drawing, &c., by L. J. M. 

 Daguerre. Translated by J. S. Memes, LL.D." 



