MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTKUM. 



OF THE CHEMICAL ACTION OF RADIATIONS ON SUBSTANCES. 



Having offered the foregoing evidence in support of 

 the first proposition considered in this Memoir, which 

 was to the effect 



" That so far from chemical influences being restricted 

 to the more refrangible rays, every part of the spectrum, 

 visible and invisible, can give rise to chemical changes, 

 or modify the molecular arrangement of bodies," I now 

 pass to the second, which is 



" That the ray effective in producing chemical or mo- 

 lecular changes in any special substance is determined 

 by the absorptive property of that substance." 



This involves the conception of selective absorption, 

 as I have formerly shown (Philosophical Magazine, Sept., 

 1841), Memoir XVII. A ray which produces a maxi- 

 mum effect on one substance may have no effect on an- 

 other. Thus the rays which change chlorophyl are not 

 those which change silver iodide. 



In the examination of this subject I shall select two 

 well-known instances, presenting the fewest elements and 

 the simplest conditions. They are, 1st, the decomposition 

 of silver iodide, the basis of so many photographic prepa- 

 rations ; 2d, the production of hydrochloric acid by the 

 union of its two constituents, chlorine and hydrogen a 

 mixture of these gases being exceedingly sensitive to 

 liht. 





1st. Of the Decomposition of Silver Iodide. 



There are two forms in which the silver iodide has 

 been used for photographic purposes : 1st, when prepared 

 by the action of the vapor of iodine on metallic silver, as 

 in the daguerreotype tablet; 2d, when nitrate of silver 

 is decomposed by iodide of potassium, or other metallic 

 iodides. These preparations differ strikingly in their ac- 



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