Dixon and Ball — Photosynthesis and the Electronic Theory. 43t 



air and was at room temperature, showed a sti-onger deposit of silver after 

 thirty seconds' exposure than did the experimental plate after an hour's 

 exposure. Furthermore, it should be noticed that the effectiveness of the 

 sensitizer is considerably reduced at this temperature, for it is always 

 observed that the contrast between the sensitized and untreated portions 

 is much greater on the film exposed at room temperature than on that 

 submerged under liquid air during exposure. 



These experiments show that the photographic film may be used as a 

 means of testing the electronic disturbances caused by light in substances 

 distributed through it. It will be noticed that the effects on the film will be 

 summative, and, therefore, given time, even when the number of electrons 

 displaced per second is small, sensible effects may be produced. 



Sensitization by Chlorophyll. 



It is well known that chlorophyll, like many dyes, acts as a sensitizer to 

 the photographic film, and the fact that its presence renders the photographic 

 film sensitive to those wave frequencies which it absorbs is strong evidence 

 that, in its case also, exposure to light leads to the displacement of electrons. 

 This conclusion would be rendered almost certain if sensitization by 

 chlorophyll could be shown to be effective at the temperature of liquid air. 



Nearly all experimenters on photographic sensitization by chlorophyll 

 comment on its uncertainty. 'Ihe uncertainty of its action is particularly 

 noticeable when it is used in conjunction with gelatine photographic films. 

 As gelatine plates are so manageable, and give such excellent results with 

 other pigments, it seemed worth while to carry out some experiments to try 

 and ascertain the conditions necessary for their sensitization. 



Experiments on the Sensitization of Gelatine Films. 



Untreated photographic gelatine films were immersed in alcoholic extract 

 of fresh leaves, alcoholic extract of dry leaf-powder, and alcoholic extract of 

 leaves killed by exposure to steam. None of these preparations of 

 crystalloidal chlorophyll gave certain results. 



Similarly untreated plates immersed in or smeared with colloidal 

 chlorophyll were equally uncertain. 



The addition of different quantities of ammonia to the sensitizing bath — 

 although some good plates were obtained by this method — did not render 

 sensitization certain. Neither did the acidnlation of the bath produce better 

 results. 



An effort to change the sign of the colloidal chlorophyll by the addition 

 of calcium sulphate did not secure sensitization. 



