MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 



suffice. But how is it possible to restrict the chemical 

 force of the spectrum to the region of the more refrangi- 

 ble rays, in face of the fact that compounds of silver 

 such as the iodide, which have heretofore been 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 

 colors of flowers, which vary indefinitely in their tints, 

 originate under the influence of rays of many different 

 refrangibilities, and are bleached or destroyed by spec- 

 trum colors complementary to their own, and, therefore, 

 varying indefinitely in their refrangibility. 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 undulations promoting their oxidation, some their 

 deoxidation. These actions are not limited to decompo- 

 sition ; they extend to combinations. 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 effect. The dia- 

 gram so generally used to illustrate the calorific, lumi- 

 nous, and chemical parts of the spectrum serves only to 

 mislead. 



While thus we find that chemical action may take 

 place throughout the entire length of the spectrum, the 

 remarks that have been made in a previous Memoir 

 (XXVI.) respecting the differences of calorific distribu- 



