Chemical Force in the Spectrum, 4^27 



4th. Case of the Colours of Flowers. 



The production and destruction of vegetable colours by the 

 agency of light has, of course, long been a matter of common 

 observation. Little, however, has been done in the special ex- 

 amination of the facts, and that little, for the most part, by 

 Herschel. 



We have only to examine his memoir in the Philosophical 

 Transactions (Part II. 1842) to be satisfied that nearly every 

 radiation can produce effects. Thus the yellow stain imparted 

 by the Corchorus japonica to paper is whitened by the green, 

 blue^ indigo, and violet rays. The rose- red of the Ten-weeks 

 Stock is, in like manner, changed by the yellow, orange, and 

 red. The rich blue tint of the Viola odorata, turned green by 

 sodium carbonate, is bleached by the same group of rays. 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 colours being affected by some, others by 

 other rays. It is, however, desirable that the general principle 

 at which Herschel arrived, viz. that the luminous rays are chiefly 

 effective, should be more closely examined. Some important 

 physiological explanations turn on that principle. These so- 

 called luminous rays are such as can impress the retina, which, 

 like organic colours, 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 maximum being in the yellow. 



It is, however, to a former experimenter, Grothuss, that we 

 owe the discovery of the law under which these decompositions 

 of the colours of flowers take place. This law in repeated in- 

 stances 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 colour, are those which 

 by their union produce a tint complementary to the colour de- 

 stroyed.^^ Even the partial establishment of this law, already 

 accomplished, is sufficient to prove that chemical effects are not 

 limited to the more refrangible places of the spectrum, but can 

 be occasioned by any ray. 



5 th. Case of the union of Chlorine and Hydrogen. 

 In the Philosophical Magazine (December 1843) may be 

 found the description of an actinometer invented by me, depend- 

 ing for its indications on the combination of chlorine and hy- 

 drogen, those gases having been evolved in equal volumes from 



