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XXXIII. 



PHOTOSYNTHESIS AND THE ELECTEONIC THEORY (II). 



By HENEY H. DIXON, Sc.D., F.E.S., 



University Professor of Botany in Trinity College, Dublin ; 



AND 



NIGEL G. BALL, MA., 

 Assistant to the University Professor of Botany, Trinity College, Dublin. 



[Read December 20, 1921. Published Feekuaky 2, 1922.] 



It has been pointed out that the energy required in photosynthesis to form 

 carbohydrates from carbon dioxide and water is derived from the light 

 energy rendered available by the displacement of electrons of the chlorophyll 

 molecule. This view is necessitated by the fact that the wave frequencies 

 absorbed by chlorophyll are those which are effective in photosynthesis (1). 



Previous experiments have shown that the number of electrons ejected 

 from the molecules of chlorophyll when illuminated by visible light could not 

 carry anything like sufficient energy to make the formation of the observed 

 quantities of carbohydrates possible external to the chlorophyll. Hence it 

 would seem to follow that the necessary electrons are not ejected from, but 

 are merely displaced in the chlorophyll molecule (1). This displacement may 

 be supposed to render some atomic group in the chlorophyll molecule reactive 

 leading to the fixation of carbon dioxide, to the separation of a carbohydrate 

 from its complex molecule, and to reconstitution of the original from the 

 residue and water, with the evolution of oxygen. 



This paper contains an account of some experiments carried out with a 

 view to testing further the photo-electric properties of chlorophyll by means 

 of a photographic method. 



According to the generally received view, the latent image of the photo- 

 graphic film is formed by the displacement of electrons on exposure to light. 

 The development of the plate reveals the ionization so caused, and converts 

 the latent image into the photographic image. A strong confirmation of this 

 view is to be found in the observation that the latent image may be formed 

 at a temperature so low that no chemical action is possible (2). 



An extension of this theory explains the action of sensitizers. Light of 

 wave frequencies, which are unable to form a latent image in the untreated 



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