MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 499 



of their want of sensitiveness. 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 por- 

 tions 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 men- 

 struum. They may therefore be developed by its use. 

 Practically, care has to be taken to moderate the solvent 

 action and to check it at the proper time. The former 

 is accomplished by dilution with some other appropriate 

 liquid, the latter by the affusion of a stream of water. 



The substance I have used is West India bitumen dis- 

 solved in benzine, and developed by a mixture of ben- 

 zine and alcohol. 



The bitumen solution being poured on a glass plate 

 in a dark room, and drained off 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 termination beyond H. Every ray in the spectrum 

 acts. The proof is continuous except where the Fraun- 

 hofer 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. 



3d. Case of Carbonic Acid. 



The decomposition of carbonic acid by plants under 

 the influence of sunshine is undoubtedly the most im- 

 portant of all actino-chemical facts. The existence of the 

 vegetable world, and indeed it may be said the existence 

 of all living beings, depends upon it. 



I first effected this decomposition in the solar spec- 



