410 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 



tram, as may be seen in Memoir X. The results obtain- 

 ed by me at that time from the direct spectrum experi- 

 ment, that the decomposition of* carbonic acid is effected 

 by the less, not by the more refrangible rays, have been 

 confirmed by all recent experimenters, who differ only 

 as regards the exact position of the maximum. In the 

 discussions that have arisen this decomposition has often 

 been incorrectly referred to the green parts of plants. 

 Plants which have been caused to germinate and grow 

 to a certain stage in darkness are etiolated. Yet these, 

 when brought into the sunlight, decompose carbonic 

 acid, and then turn green. The chlorophyl thus pro- 

 duced is the effect of the decomposition, not its cause. 

 Facts derived from the visible absorptive action of chlo- 

 rophyl do not necessarily apply to the decomposition of 

 carbonic acid. The curve of the production of chlorophyl, 

 the curve of the destruction of chlorophyl, the curve of 

 the visible absorption of chlorophyl, and the curve of the 

 decomposition of carbonic acid, are not all necessarily co- 

 incident. To confound them together, as is too frequent- 

 ly done, is to be led to incorrect conclusions. 



Two different methods may be resorted to for deter- 

 mining the rays which accomplish the decomposition of 

 carbonic acid: 1st, the place of maximum evolution of 

 oxygen gas in the spectrum may be determined ; 2d, 

 the place in which young etiolated plants turn green. 



I resorted to both these methods, and obtained from 

 them the same results. The rays which decompose car- 

 bonic acid are the same which turn etiolated plants 

 green. They may be designated as the yellow with the 

 orange on one side and a portion of green on the other. 

 Though the form of experimentation does not admit of a 

 close reference to the fixed lines, I think we are almost 

 justified in supposing that the point of maximum action 

 is in the yellow. It must be borne in mind that the 



