2 Proceedings of the Roi/al Irish Academy. 



of the vessel used to contain them so that no discharge of electrons took place 

 from the vessel itself. 



The photo-electric effect is in aU cases expressed in terms of the discharge 

 from freshly cleaned copper under similar conditions. A copper plate was 

 used as a standard, and care taken that the leaves or other surface under 

 examination were of the same size and in same position as the standard. 



To obtain approximate uniformity in the light the terminals were cleaned 

 very frequently, and all volatilized matter removed from the quartz. 



The electrometer used gave a deflection of 1300 scale diAdsions per volt, 

 so that a small photo-electric effect could be detected, but as the source of 

 light was strong, and the exposed surface large, we usually had quite a large 

 capacity joined to the electrometer. 



Photo-Electric Activities of various leaves and flowers. 



Leaves from a great many trees and shrubs, and flowers of various colours, 

 were tested. The results may be summarized by stating that green leaves 

 crave an effect varying from 3 per cent, to 10 per cent, of that from copper, 

 while flowers of various colours gave smaller effects, usually less than 1 per 

 cent, of that from copper. 



The following table contains a few of the leaves and flowers tested with 

 the corresponding activities on a scale on which copper is represented 

 by 100 :— 



Copper plate freshly cleaned, 



100 



Leaves of Sycamore tree, 



10 



Leaves of shrub Euonymus, . . . . 



10 



Leaves of Horse Chestnut, 



8 



Leaves of Arum lUy, 



8 



Leaves of Plane tree 



7 



Ivy leaves, ....... 



5 



Fresh grass, 



3 



Chrysanthemum leaves, .... 



10 



White Chrysanthemum flowers, . 



IS 



Pink Chiysanthemum flowers, . 



1-0 



Yellow Chrysanthemum flowers. 



0-8 



Ked Chrysanthemum flowers, 



0-5 



The different beha\aour of gieen leaves and flowers suggested that chloro- 

 phyll maj' be the chief active agent. Hence we made experiments with 

 chlorophyll extracts prepared in the laboratory from various types of leaves. 



