Chemical Force in the Spectrum, 429 



bered. are active in forming chlorophyl. Upon longer exposure 

 the subordinate action along the yellow &c. occurs, but not 

 until the other portions are perfectly bleached. 



" In Sir J. HerschePs experiments there remained a salmon- 

 colour after the discharge of the green. This is not seen when 

 chlorophyl is used, and is due to a colouring-matter in the leaf, 

 soluble in water, but not soluble in ether/' 



I have quoted these results in detail, because they illus- 

 trate in a striking manner the law that vegetable colours are de- 

 stroyed by rays complementary to those that have produced them, 

 and furnish proof that rays of every refrangibility may be che- 

 mically active. 



At this point I abstain from adding other instances showing 

 that chenncal changes are brought about in every part of the 

 spectrum. The list of cases here presented might be indefinitely 

 extended if these did not suffice. But how is it possible to 

 restrict the chemical force of the spectrum to the region of the 

 more refrangible rays, in face of the fact that compounds of 

 silver, such as the iodide, which have been heretofore mainly 

 relied upon to support that view, and in fact originated it, are 

 now proved to be affected by every ray, from the invisible ultra- 

 red to the invisible ultra-violet ? how, when it is proved that the 

 decomposition of carbonic acid, by far the most general and most 

 important of the chemical actions of light, is brought about, not 

 by the more refrangible, but by the yellow rays ? The delicate 

 colours of flowers, v/hich vary indefinitely in their tints, originate 

 under the influence of rays of many different refrangibilities, and 

 are bleached or destroyed by spectrum-colours complementary to 

 their own, and therefore varying indefinitely in their refrangibi- 

 lity. Towards the indigo ray the stems of plants incline ; from 

 the red their roots turn away. There is not a wave of light that 

 does not leave its impress on bitumens and resins — some un- 

 dulations promoting their oxidation, some their deoxidation. 

 These actions are not limited to decomposition ; they extend to 

 combination. Every ray in the spectrum brings on the union of 

 chlorine and hydrogen. 



The conclusion to which these facts point is, then, that it is 

 erroneous to restrict the chemical force of the spectrum to the 

 more refrangible, or, indeed, to any special region. There is not 

 a ray, visible or invisible, that cannot produce a special chemical 

 efi*ect. The diagram so generally used to illustate the calorific, 

 luminous, and chemical parts of the spectrum serves only to 

 mislead. 



Whilst thus we find that chemical action may take place 

 throughout the entire length of the spectrum, the remarks that 

 have been made in the previous memoir (Phil. Mag. August 



