Mi MOIR XL] THE FOHCE INCLUDED IN PLANTS. 

 R Y G B I 



185 



Fig- &. 



O, Y, G, B, I, V red, orange, yellow, green, blue, indigo, 

 violet of the spectrum. 



Many repetitions of this experiment satisfied me that 

 it is the yellow and adjacent regions of the spectrum 

 which occasion the greening of plants; the heat rays 

 and the chemical rays have nothing whatever to do 

 with it. 



Next I attempted the decomposition of carbonic acid 

 in the spectrum, and succeeded. I read before the Amer- 

 ican Philosophical Society in Philadelphia, at its centen- 

 nial celebration in 1843, an account of this experiment 

 as published in Memoir X. 



In addition to the special interest of these experiments 

 on plant life, they had a very important bearing on the 

 general principles of actino - chemistry. They proved 

 that it is altogether incorrect to suppose that chemical 

 changes are brought about by the more refrangible rays 

 only. They showed that every ray has its proper chem- 

 ical function ; for instance, the violet in the decomposi- 

 tion of compounds of silver, the yellow in the case of car- 

 bonic acid. And hence I proposed to abandon the con- 

 ception of a tripartite division of the spectrum into heat, 

 light, and chemical radiations, and to designate radia- 

 tions by their wave-lengths, or, better still, by their mini- 



