66 Scientific Proceedings, Royal Dublin Society. 



leaf steadily collapsed under the influence of the light. This motion is due 

 to the negative charge being carried off by the expulsion of the electrons 

 from the brass plate by the light, and its rate depends on the photo-electric 

 activity of the brass plate. The plate was now coated with a skin rich in 

 chlorophyll, deposited there byevaporating on it an acetone extract of pulverized 

 nettle leaves. It was then found, on replacing the plate in its former 

 position, that there was no evidence of any photo-electric discharge, no 

 motion of the leaf being observed whether the plate was charged positively 

 or negatively, so that even when acted on by a source rich in ultra-violet 

 light, as the arc is, the chlorophyll gave no evidence of the expulsion of 

 electrons when tested in this manner. The same was, of course, true when 

 the ultra-violet light was cut off by a picric acid screen. 



In the second method employed for these preliminary experiments, a 

 change in conductivity was looked for in a medium containing chlorophyll 

 when exposed to light. It seemed possible that, if electrons are expelled by 

 light, their action as current-carriers might be revealed in an alteration of 

 resistance. 



In the first instance the sap crushed from leaves was used for this 

 experiment. The sap was introduced into a very narrow, deep, electrolytic 

 cell with glass walls. Into this cell dipped two long platinum electrodes 

 coated with platinum black, connected to a Kohlrausch conductivity 

 apparatus. Measurements of the conductivity when the cell was obscured, 

 and illuminated, indicated no difference of resistance. But the conductivity 

 of the sap is so high compared with the current due to possible photo- 

 electrons that a positive result could scai-cely be hoped for in this case, unless 

 the chlorophyll dispersed througli the medium possessed a very high photo- 

 electric activity. 



A modification of the experiment, using a suspension of colloidal 

 chlorophyll, obtained by diluting an acetone leaf-extract with water, in place 

 of the sap, proved equally negative. 



Even when the sap was replaced by a medium having an extremely high 

 resistance a negative result was obtained. Thus no change with illumination 

 could be detected in the current flowing through a small block of paraffin 

 wax in which chlorophyll was dissolved, under a pressure-gradient of nearly 

 1000 volts per centimetre, the minute leakage current being measured by 

 means of a sensitive electrometer. 



"We could not, however, expect to detect a very small photo-electric effect 

 by the electroscope metliod described above. The ordinary gold-leaf electro- 

 scope is not very sensitive for small changes of pressure, as several volts are 

 required to cause a motion of the leaf equivalent to one scale division. Its 



