Djxon and Poole — Photosynthesis and the Electronic Theory. 65 



are approximately equal in photosynthetie effectiveness, while the small part 

 of this latter range between bi-ijxfi and 512;((U (transmitted by a green screen) 

 was apparently ineffective. Hence the presence of a second crest on the 

 curve of photosynthesis produced by wave-lengths of less than 500/i/<, 

 corresponding to the observed sensitizing action to violet light, is definitely 

 established ; and we may conclude that the wave-lengths which are effective 

 in displacing the electrons of chlorophyll, as indicated by its sensitizing action 

 on the photographic plate, are the same as those which are effective in 

 photosynthesis. The view that light energy becomes available for photosyn- 

 thesis through the intermediary of the electrons, arrived at from d i^^ri 

 reasoning, is in this way in accordance with experimental results. 



The presence of the electropositive element magnesium in the chlorophyll 

 molecule is significant in this connexion. 



Granting, then, that the first action of the light is to disturb some of the 

 electrons in the chlorophyll molecule, the question arises as to whether the 

 electrons receive sufficient energy to cause their expulsion from the molecule 

 altogether, or whether the only result is a displacement of the electron 

 within the molecular system itself. In the first ease we might suppose 

 chlorophyll to be capable of causing some chemical change in adjacent 

 molecules, without itself taking part in the reaction. In the second, we 

 must consider that its own chemical character is altered by the light, so that 

 it takes a direct part in the resulting reactions. On the first hypothesis we 

 would expect chlorophyll to differ from almost all other substances (except 

 the alkali metals) in showing the photo-electric effect under the action of 

 visible light. Even the light which passes through a red screen should 

 produce an appreciable effect. 



Pkeliminaey Expeeiments. 



In order to test this point, some preliminary rough experiments were 

 made. Two methods were adopted. In the first, an attempt was made to 

 observe a photo-electric discharge by means of a gold-leaf electroscope, to which 

 was connected an insulated brass plate about 15 cms. square, which could be 

 coated with chlorophyll. Close to this plate and parallel to it was supported 

 a piece of copper wire gauze, which was earthed. An arc lamp was arranged 

 so that a beam of light from it could be so directed as to pass through the 

 gauze and strike the plate. The lamp was at a sufficient distance to prevent 

 the ions generated in the arc from reaching the plate and gauze. When the 

 brass plate was charged positively and the light turned on, no motion of the 

 gold leaf, as observed through a small telescope, was apparent. On the 

 contrary, when the plate and electroscope were negatively charged, the gold 



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