Chemical Force in the Spectrum. 425 



erroneous to suppose that chemical force is restricted to the 

 more refrangible portions of the spectrum. 



2nd. Case of Bitumens and Resins. 



These substances are of special interest in the history of pho- 

 tography, since^ in the hands of Niepce, they were probably the 

 first on which impressions in the camera were obtained and 

 fixed. Their use has been abandoned in consequence, as it 

 seems to me, of an incorrect opinion of their want of sensitive- 

 ness. Properly used they are scarcely inferior to chloride of 

 silver. 



The theory of their use is very simple. Alcohol, ether, and 

 various volatile oils respectively dissolve certain portions of these 

 substances. If such a solution be spread in a thin film upon 

 glass, as in the collodion operation, and parts of the surface be 

 then exposed to light, the portions so exposed become insoluble 

 in the same menstruum ; they may therefore be developed by 

 its use. Practically, care has to be taken to moderate the sol- 

 vent action_, and to check it at the proper time. The former is 

 accomplished by dilution with some other appropriate liquid ; 

 the latter, by the aflPusion of a stream of water. 



The substance I have used is West-Indian bitumen dissolved 

 in benzine, and developed by a mixture of benzine and alcohol. 



The bitumen solution being poured on a glass plate in a dark 

 room, and drained ofi" as in the operation of collodion, leaves a 

 film sufficiently thin to be iridescent. This is exposed to the 

 spectrum for five minutes, and then developed. 



The beginning of the impression is below the line A, its ter- 

 mination beyond H. Every ray in the spectrum acts ; the 

 proof is continuous, except where the Praunhofer lines fall. A 

 better illustration that the chemical action of the spectrum is 

 not restricted to the higher rays, but is possessed by all, could 

 hardly be adduced. 



3rd. Case of Carbonic Acid. 



The decomposition of carbonic acid by plants under the in- 

 fluence of sunshine is undoubtedly the most important of all 

 actino-chemical facts. The existence of the vegetable world, 

 and, indeed, it may be said, the existence of all living things, 

 depends upon it. 



I first efi'ected this decomposition in the solar spectrum, as 

 may be found in a memoir in the Philosophical Magazine 

 (Sept. 1843). The results ascertained by me at that time from 

 the direct-spectrum experiment, that the decomposition of car- 

 bonic acid IS efi'ected by the less, not by the more refrangible 

 rays, have been confirmed by all recent experimenters, who 



