McClici.land and Fiizgkrald — Photo-electric Discharge. 7 



Another solution of hydroquinone which we tested had an activity of 80, 

 and this was increased when treated with lead peroxide to 115, or 15 percent, 

 more than a freshly cleaned copper plate. 



The two substances isomeric with hydroquinone, resorcine, and pyro- 

 catechine, gave by like treatment similar results, and showed high 

 activities. 



A very high photo-electric activity was obtained with pyrogallic acid. 

 It is very soluble in water, and a solution which showed an activity of 60 

 had this activity increased, when shaken up with potassium permanganate, 

 to as high a value as 400, or four times that of copper. 



Tannin and gallic acid solutions showed little increased activity when 

 treated with lead peroxide, but responded to potassium permanganate. 



As an example of a substance of a different type we tried /3-naphthol, 

 which has two benzene rings and one hydroxyl group. Dissolved in weak 

 alkali it showed an activity of 80, and this was increased to 170 by treatment 

 with very dilute permanganate. 



Solutions in water of maltose and dextrose were tried. They gave a small 

 activity, and it could not be increased by treatment with lead peroxide or 

 permanganate. Methyl alcohol, ethyl alcohol, acetone, ether, aldehyde, and 

 liquid esters were tested and found to be practically inactive. 



The greatest effects and the greatest increases by the use of oxidizing 

 agents were therefore found with closed-ring compounds, the open chain 

 compounds giving small effects. 



Summary. 



1. A table is given showing the photo-electric effects of a number of leaves 

 and flowers. The maximum effect obtained from any type of leaf was about 

 10 per cent, of that from copper. 



2. Acetone was used to extract chlorophyll from leaves. The acetone 

 solution was inactive, but became active when largely diluted with water, the 

 solid being then thrown out of solution. The photo-electric effect decays very 

 rapidly under the action of the light. 



3. It is shown that an active substance is obtained from leaves by 

 immersing them in distilled water. When the water is near boiling point, a 

 short time is sufficient. Large effects are obtained in this way, the water 

 solution in some cases being half as active as copper. When comparing the 

 effects obtained in this way with those obtained by the use of acetone, we 

 must remember that a few drops of the acetone solutions added to a large 



