906 EXPERIMENT STATION RECORD. 



produce very different effects from the former. Draper demonstrated 

 long ago that the orange rays are the most active in decomposing car- 

 bonic acid in the leaves. This conclusion was fully confirmed by the 

 researches of Sachs, Cailletet, and of the author made nearly 30 years 

 ago. The reasons for this special action of the rays of this part of the 

 spectrum were not investigated until the Russian physiologist, Timiria- 

 zeff, took up the subject. He found that the rays which are most active 

 in decomposing carbon dioxid are the orange and yellow, which are 

 absorbed by chlorophyll when the latter is examined with the spectro- 

 scope. The same fact has been beautifully demonstrated by Engelmann. 

 He received a ray of light upon a prism so placed under the objective 

 of a microscope that on looking through the instrument the different 

 rays of the spectrum could be seen. He then put a drop of water on a 

 slide and added a filament of green alga and some putrefactive bacteria, 

 which were aerobic. It was observed that the bacteria congregated in 

 great numbers on that part of the alga lighted by the yellow and 

 orange rays. In the green region only a few were observed and these 

 finally collected in the blue portion. In other words, the bacteria col- 

 lected in the different rays in numbers approximately proportionate to 

 their activity in assisting the decomposition of the carbon dioxid by 

 chlorophyll. 



Evidently the rays which pass freely through the chlorophyll exert 

 no action. So it happens, as shown above, that the extreme red or the 

 green rays are without effect on the decomposition of carbon dioxid. 

 On the other hand, the orange and blue rays are retained and absorbed 

 by the chlorophyll, and thus made available for the work of decompos- 

 ing carbon dioxid. The fact that orange rays are much more effective 

 than the blue is easily explained. The decomposition of the carbon 

 dioxid, with the evolution of oxygen, requires an expenditure of energy 

 equal to that involved in the burning of carbon in oxygen. In order, 

 therefore, that the rays may be effective for reducing carbon dioxid 

 they must be not only readily absorbed but sufficiently warm. The 

 orange rays, which are situated at the side of the spectrum where the 

 heat rays are concentrated, are far more energetic than the blue because, 

 in addition to being readily absorbed, they are warmer than the latter. 



The decomposition of carbon dioxid can only be brought about by the 

 aid of outside energy. This energy is supplied by the sun's rays. 

 Since decomposition of carbon dioxid in the chlorophyll cells is the 

 source of the organic constituents of plants, and since these substances 

 are essential to the life of animals, we see that all living beings on the 

 earth's surface owe their activity primarily to the sun. 



PRODUCTION OF CARBON COMPOUNDS BY DECOMPOSITION OF CARBON 



DIOXID. 



It does not suffice to know that the leaves, saturated with water, 

 absorb the carbon dioxid of the air and under the influence of the 

 sun's rays evolve oxygen. It is necessary to define this process and 



