4X4 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 



facts respecting the bleaching or decolorization of chlo- 

 rophyl by light. He used an ethereal solution of that 

 substance : 



"The first action of light is perceived in the mean red 

 rays, and it attains a maximum incomparably greater at 

 that point than elsewhere. The next part affected is in 

 the indigo, and accompanying it there is an action from 

 + 10.5 to +36.0 of the same scale (Herschel's) beginning 

 abruptly in Fraunhofer's blue. So striking is this whole 

 result that some of my earlier spectra contained a per- 

 fectly neutral space, from 5.0 to +20.5, in which the 

 chlorophyl was in no way changed, while the solar 

 picture in the red was sharp and of a dazzling white. 

 The maximum in the indigo was also bleached, pro- 

 ducing a linear spectrum, as follows : 



in which the orange, yellow, and green rays are neutral. 

 These, it will be remembered, are active in forming 

 chlorophyl. Upon longer exposure, the subordinate 

 action along the yellow, etc., occurs, but not until the 

 other portions are perfectly bleached. 



"In Sir J. Herschel's experiments there remained a 

 salmon color after the discharge of the green. This is 

 not seen when chlorophyl is used, and is due to a color- 

 ing matter in the leaf, soluble in water, but insoluble in 

 ether." 



I have quoted these results in detail because they il- 

 lustrate in a striking manner the law that vegetable col- 

 ors are destroyed by rays complementary to those that 

 have produced them, and furnish proof that rays of every 

 refrangibility may be chemically active. 



At this point I abstain from adding other instances 

 showing that chemical changes are brought about in 

 every part of the spectrum. The list of cases here pre- 

 sented might be indefinitely extended, if these did not 



