Intelligence and Miscellaneous A7-ticles. 217 



at its height In the violet. Thus these appearances of inverse action 

 in the colouring do not arise from two distinct effects, positive and 

 negative, produced by the rays upon the same sensitive surface, but 

 are owing to two distinct chemical reactions, which predominate 

 respectively in the red and violet parts. 



If the blackened paper be covered with a fresh layer of iodide of 

 potassium, it will begin to turn white at tlie least refrangible part, 

 and the neutral line will again approach the red ; if a sufficient 

 quantity of iodide be used, the paper will turn white from the violet 

 to the red ; but if a very strong solution of iodide were employed, 

 the paper would whiten, even in the dark ; so violent is the action 

 of the iodide of potassium upon metallic silver. 



These results clearly prove that several chemical actions may take 

 place simultaneously in the mixtures of sensitive substances, the re- 

 sults only of which are obsen'able. Analogous effects must neces- 

 sarily be produced on employing iodized plates of silver, and after- 

 wards exposing them to the vapour of bromine or to chlorine ; and 

 perhaps even when using plates iodized according to M. Daguerre's 

 plan. In fact, under these circumstances, the iodide, chloride, or 

 bromide of silver, are in direct contact with the metallic silver ; and 

 as, by the decomposition of these salts, through the action of the 

 light, subsalts are formed, the result is that iodine, chlorine and bro- 

 mine are exposed directly to the above-named salts, and even to the 

 metallic silver itself, at the moment when the solar action makes its 

 influence felt. These reactions, which are sufficiently complex, be- 

 come more so by the iodides, chlorides, and bromides of silver being 

 submitted to the action of rays which always act with the same 

 energy in the violet part of the spectrum ; whilst in the red portion 

 the rays react with greater energy, owing to certain chemical actions 

 naving commenced. 



It is therefore essential to distinguish between the chemical re- 

 actions effected under the influence of light upon sensitive substances 

 alone, and upon combinations of them. This has not been done by 

 .Messrs. Foucault and Fizeau : they have considered a Daguerreotype 

 plate as offering a separate sensitive surface, whilst it is only by a 

 mixture of substances that different effects can be produced in the 

 various parts of the spectrum, as is proved by Herschel's experi- 

 ment, and without the existence of rays acting in an inverse direc- 

 tion. Thus it has been proved by experiment, that the solar rays, 

 although of various degrees of refrangibility, only act in one way 

 upon iodide of silver ; whilst a mixture of this substance with other 

 matters may occasion several chemical reactions acting conjointly 

 and hiding the princij)al effect. 



If the light acts only in one way upon iodide of silver (the con- 

 tinuous rays included), the case may be different on other sensitive 

 substances, and the rays may act sometimes positively and sometimes 

 negatively. It is known, in fact, that each sensitive substance is 

 difft-rcntly affected by the solar rays ; this might be cxi)lained by 

 stating that each sensitive substance receives the rays in a manner 

 peculiar to itself. I will cite as an example an observation of Wol- 



Phil. Mas'. S. 3. Vol. .'^0. No. 200. March 1817. Q 



