216 Intelligoice and Miscellaneous Articles. 



several chemical reactions taking place simultaneously, and are not 

 due to contrary action, exercised on the part of the solar rays, on 

 the iodide of silver alone. 



Another fact which I will call attention to is, that the least refran- 

 gible part of the spectrum, instead of possessing a negative action 

 upon the iodide of silver, exercises a continuous influence upon most 

 of the salts of silver alone, such as the iodide, the bromide, and the 

 chloride ; and also that the experiments upon which this proposition 

 is founded have been verified by the commissioners of the Academy, 

 charged with the examination of one of my memoirs. 



In the foregoing, the only point discussed was the 'influence of 

 light upon iodide of silver, or the simple salts of that metal. When 

 plates of silver are exposed successively to the vapours of iodine, 

 bromine or chlorine, the mixtures obtained may give rise to various 

 kinds of chemical reaction, of which the result only is appreciable. 

 For this reason these mixtures must not be employed without great 

 caution, and the Daguerreotype plates must be used as little as pos- 

 sible in experimenting as to the nature of the active rays. 



In order to show how far the mixture of sensitive materials is 

 capable of influencing the eff^ects of the spectrum, I will direct atten- 

 tion to the following observation of Sir J. Herschel : — 



If paper be prepared, first with a strong solution of lead and after- 

 wards with bromate of potash and nitrate of silver, a surface will be 

 produced which will speedily become black on exposure to the light ; 

 on being presented to the spectrum, the black tint will be produced 

 in the most refrangible rays, as far as green. But if the paper has 

 been blackened by previous exposure to the light, on being covered 

 with a dilute solution of iodide of potassium and exposed to the blue 

 part of the spectrum, the paper will become white. This result 

 proves that the iodide of potassium is decomposed, and that the silver 

 which stained the paper being iodized, and coming in contact with 

 an alkaline iodide, ceases to be aff"ected by the light ; the paper will 

 therefore remain of a j^ellowish-white in that portion of the spectrum 

 on which the reaction takes place. 



If the layer of iodide of potassium emjDloyed is produced from a 

 dilute solution of that salt, the paper not only becomes white in the 

 violet part of the spectrum, but also becomes darker in the red rays, 

 and even beyond, a neutral line being in the middle. It would ap- 

 pear therefore, from an examination of the image thus obtained, that 

 two contrary results were produced ; viz. the destruction of the co- 

 lour in the violet part and its augmentation in the red. These 

 efi^ects may be easily explained as two distinct chemical reactions : 

 first, the action of the light upon the iodide of silver, the colouring of 

 which had commenced ; second, the action of the light to effect the 

 decomposition of the iodide of potassium, and the iodizing of the 

 silver arising from the sub-iodide formed by the first reaction. As 

 the red part of the spectrum contains those rays which continue the 

 chemical action commenced upon the salts of silver, and as the 

 first reaction is only commenced, the latter has most influence in 

 that part of the spectrum. The second reaction is, on the contrary, 



