MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 495 



stricted to the more refrangible rays, every part of the 

 spectrum, visible arid invisible, can give rise to chemical 

 changes, of modify the molecular arrangement of bodies. 



2d. That the ray effective in producing chemical or 

 molecular changes in any special substance is determined 

 by the absorptive property of that substance. 



I may here remark that both these propositions were 

 maintained by me many years ago : an example of the 

 first will be found in the Philosophical Magazine (Dec., 

 1842), and of the second in a paper in the same journal, 

 " On some Analogies between the Phenomena of the 

 Chemical Rays and those of Radiant Heat " (Sept., 1841), 

 Memoir XVII. 



The opinion commonly held respecting the distribu- 

 tion of chemical force in the spectrum is mainly founded 

 on the behavior of some of the compounds of silver. 

 These darken when exposed to the more refrangible 

 rays, and, unless correct methods of examination be re- 

 sorted to, seem to be unaffected by the less refrangible. 

 Hence it has been supposed that in the higher parts of 

 the spectrum a special principle prevails, to which the 

 designation of " actinic rays " is often applied an inap- 

 propriate iteration. In these pages I use the derivatives 

 of CIKTIQ not in this restricted sense, but as expressive of 

 radiations of every kind. This is their proper significa- 

 tion. 



Every part of the spectrum, no matter what its re- 

 frangibility may be, can produce chemical changes, and 

 therefore there is no special localization of force in any 

 limited region. Out of a large body of evidence that 

 might be adduced, I select a few prominent instances. 



1st. Case of the Compounds of Silver. 



Silver is the basis of the most important photographic 

 sensitive substances. Its iodide, bromide, and chloride, 



