412 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 



by those less refrangible than the yellow. The green 

 (chlorophyl) of the Elder leaf is changed by the extreme 

 red. 



It is needless to extend this list of examples. The 

 foregoing establish the principle that every part of the 

 spectrum displays activity, some vegetable colors being 

 affected by one, others by other rays. It is, however, 

 desirable that the general principle at which Herschel ar- 

 rived viz., that the luminous rays are chiefly effective 

 should be more closely examined. Some important phys- 

 iological explanations turn on that principle. These so- 

 called luminous rays are such as can impress the retina, 

 which, like organic colors, is a carbon compound. There 

 are strong reasons for inferring that carbon is affected 

 mainly by rays the wave-lengths of which are between 

 those of the extreme red and extreme violet, the maxi- 

 mum being in the yellow. 



It is, however, to a former experimenter, Grotthus, that 

 we owe the discovery of the law under which these de- 

 compositions of the colors of flowers take place. This 

 law in repeated instances was verified by Herschel, and 

 more recently by myself. It may be thus expressed : 

 " The rays which are effective in the destruction of any 

 given vegetable color are those which by their union 

 produce a tint complementary to the color destroyed." 

 Even the partial establishment of this law, already ac- 

 complished, is sufficient to prove that chemical effects 

 are not limited to the more refrangible portions of the 

 spectrum, but can be occasioned by any ray. 



6th. Case of the Union of Chlorine and Hydrogen. 



In Memoir XVIII. may be found the description of 

 an actinometer invented by me, depending for its indi- 

 cations on the combination of chlorine and hydrogen, 

 these gases having been evolved in equal volumes from 



