Dixon and Poolk — Photosynthesis and the Electronic Theory. 73 



electrometer quadrants. This decrease does not appreciably affect the 

 difference caused by the light. In this case the increase caused was about 

 0'3 scale division per minute, representing a current of 7 x 10"° E.s.u. 



The following table shows the results obtained with different sub- 

 stances : — 



Substance on Test Plate. 



Leaf-powder Irept in the dark in a stoppered bottle for 9 months, 

 „ freshly prepared, ..... 



Zinc plate coated with chlorophyll from acetone solution, 

 Same a second test at 1-5 mm. air pressure, 

 Ziuc plate coated with colloidal chlorophyll from acetone and 

 water solution, ...... 



Same with red screen interposed in path of light beam, 



Zino plate freshly cleaned with carborundum cloth (at 2 mm.). 



Same at atmospheric pressure, .... 



Same with pressure again reduced to 2 mm., 



In these tests the air-pressure, unless otherwise stated, was 2 mm. The 

 last three figures show how very much more active the zinc plate was than 

 the chlorophyll, and indicate the presence of a certain amount of ultra-violet 

 light, in spite of all the glass in the path of the beam. They also show the 

 very great increase in the current caused by reducing the pressure, and also 

 the photo-electric fatigue of the zinc. 



The red screen used in one test was a Wratten gelatine colour filter, 

 No. 29. This test, which was not a very satisfactory one, as the electro- 

 meter movements were slightly irregular, shows that the light which passes 

 through the screen produces little, if any, effect. 



. Tests with a Source rich in Ultra- Violet Light. 



As M'Clelland and FitzGerald (7) obtained a considerable photo-electric 

 effect with leaf-extracts, using the light from a spark, it seems probable that 

 the small effect shown above is due to the ultra-violet light which passes 

 through the glass. It appeared to be worth while to repeat some of the tests 

 with a source rich in ultra-violet light. 



Accordingly the half-watt lamp was replaced by an open carbon arc, and 

 a quartz condenser substituted for the glass one, A quartz window was 

 fitted to the testing vessel, and a speculum metal mirror used instead of the 

 silvered glass one. The water beaker was removed, and heating was reduced 

 by using a much less concentrated beam than before. The rest of the 

 apparatus was unaltered. 



