426 Dr. J. W. Draper on the Distribution of 



differ only as regards the exact position of the maximum. In 

 the discussions that have arisen^ this decomposition has often 

 incorrectly been 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 produced is the effect of the decomposition, 

 not its cause. Facts derived from the visible absorptive action 

 of chlorophyl 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 coincident. To con- 

 found them together, as is too frequently done, is to be led to 

 incorrect conclusions. 



Two different methods may be resorted to for determining 

 the rays which accomplish the decomposition of carbonic acid. 



1st. The place of maximum evolution of oxygen gas in the 

 spectrum may be determined. 2nd. 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 carbonic acid are the 

 same which turn etiolated plants green. They may be desig- 

 nated as the yellow,, with the orange on one side and a portion 

 of the green on the other. Though the form of experimenta- 

 tion does not admit a close reference to the fixed lines, I think 

 we are justified in supposing that the point of maximum action 

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

 increasing concentration of the rays occasioned by the peculiarity 

 of prismatic dispersion towards the red end, will give a decep- 

 tive preponderance in that direction. AVithout entering further 

 into this discussion, it is sufficient for my present purpose to 

 understand that the decomposition in question is accomplished 

 by rays between the fixed lines B and F. 



The two absorptive media, potassium bichromate and cupro- 

 ammonium sulphate, so often and so usefully employed in 

 actino-chemical researches, corroborate this conclusion. Plants 

 cannot decompose carbonic acid, nor can they turn green, in rays 

 that have passed through a solution of the latter salt. They 

 accomplish both these results in rays that have passed through 

 the former. 



The decomposition of carbonic acid, and the production of 

 chlorophyl, by the less refrangible rays of the spectrum, afford 

 thus a striking illustration that chemical changes may be brought 

 about by other than the so-called chemical rays. 



