184 THE FORCE INCLUDED IN PLANTS. [MEMOIR XL 



though the less refrangible radiations, those of heat, 

 were inoperative, the more refrangible, the chemical or 

 deoxidizing, would decompose carbonic acid readily. 



But some collateral experiments had thrown a diffi- 

 culty in the way of this conclusion. I had caused seeds 

 to germinate in three little closets (Fig. 21, page 172), into 

 which, by means of panes of colored glass, or through 

 troughs filled with colored liquids red, yellow, and vio- 

 let light respectively could be admitted I remarked 

 with very great surprise that the seeds in the red-light 

 closet and those in the violet one were just as much etio- 

 lated as they would have been had they grown in dark- 

 ness; those in the yellow closet promptly assumed a 

 green color, and developed themselves as well as if grow- 

 ing under natural circumstances. 



But the light that comes through stained glass and 

 colored solutions is far from being homogeneous ; it con- 

 tains rays of many refrangibilities. I therefore deter- 

 mined to attempt the greening of plants and the decom- 

 position of carbonic acid by their leaves phenomena 

 which, as has been said, are equivalent in the solar 

 spectrum itself. 



I arranged things so as to have a horizontal solar spec- 

 trum of several inches in length kept motionless by a he- 

 liostat. I had previously caused to germinate in a wooden 

 box filled with earth, and of corresponding length, a crop 

 of seeds. They were etiolated, or blanched, for the ger- 

 mination had taken place in the dark. These young 

 plants I placed so as to receive the spectrum. Very soon 

 those that were in the yellow space turned green, but 

 those in the extreme red and extreme violet underwent 

 no change, though the exposure might be kept up the 

 whole day. 



In Fig. 23, a a is the box containing the germinating 

 seeds, and placed so as to receive the colored spaces R, 



