4 Proceedings of the Royal Irish Academy. 



As mentioned above, similar results were obtained with acetone extracts 

 from various types of leaves, and we also made similar measurements with 

 chlorophyll purchased as pure and afterwards dissolved in acetone. Of 

 course the percentage in the water of an acetone solution required to 

 give the maximum effect depends on the concentration of the particular 

 solution. 



Fatigue JSffects of Acetone Extracts. 



The active substance extracted from leaves by means of acetone showed 

 very rapid fatigue effects ; and in work of the type described above, the time 

 of exposure must be carefully attended to, if concordant results are to be 

 obtained. The fatigue effect with these extracts is certainly far more rapid 

 than in the case of metals. Not only is the fatigue more rapid, but there 

 appears to be a permanent destruction of photo-electric power produced by 

 the light to a considerable depth in the liquid. For example, in one 

 experiment drops of acetone extract were added to water sufficient to give 

 approximately the maximum activity. This was measured as quickly as 

 possible and found to be 140 on our arbitrary scale. The light was kept on, 

 and the activity fell off very rapidly. The liquid, which was 1'5 cms. deep, 

 was then stirred up^o as to renew the surface, and the terminals and quartz 

 plate were carefully cleaned, but it was impossible to restore the activity to 

 more than 90. These numbers indicate a destruction of photo-electric power 

 to a considerable depth in the liquid, and not merely a change, temporary or 

 permanent, in the surface layer. In the case of the active substances we are 

 dealing with, the photo-electric discharge is apparently accompanied by a 

 permanent change in the substance itself. 



To test directly whether the ultra-violet rays we were using could 

 penetrate to a considerable depth, we interposed in the path of the rays a 

 vessel with a quartz bottom. When a layer of water 1 cm. deep was in the 

 vessel, the rays after passing through it gave a maiked photo-electric 

 discharge from a copper plate. The transparency was, however, greatly 

 diminished hy the addition of a very few drops of the acetone extract to 

 the water. 



From their known properties it is to be expected that the chlorophyll 

 molecules would break up readily under the action of light. 



Experiments with water Extracts from Leaves and from Flowers 

 of various kinds. 



We now decided to treat leaves directly with distilled water, and to test 

 the water for a photo-electric effect. The first experiment tried was with 



