McCi.ELi.AND AND FiTZGicRALi) — Photo-ekctric Discharge. 3 



The chlorophyll was extracted by acetone, using in different experiments ivy 

 leaves, leaves of nettles, and of the evergreen euonymus. We may quote the 

 results for nettle leaves, which were similar to the others. 



The leaves were pounded in a mortar, put into a glass vessel, and left for 

 some hours in contact with acetone, and the liquid then filtered off. This 

 liquid when exposed to ultra-violet light showed practically no activity. 

 "Wlien diluted with a large quantity of water the activity was greatly 

 increased, the maximum effect being obtained when the acetone exti-act 

 formed less than one per cent of the whole. The actual numbers for one 

 experiment are given below : — 



Volume of acetone extract added 





to 100 CCS. of distilled water. 



Actirity 



2 drops, . . .' . 



3-5 



6 drops, .... 



6-0 



20 drops = 0"5 c.c.. 



. . 9-0 



1 „ . . 



6-5 



2 „ . . 



5-0 



4 „ . . 



2-5 



8 „ . . 



1-5 



Afresh mixture was taken at every observation, so as to eliminate fatigue 

 effects. The activities are as before given on the scale on which a copper 

 plate would be represented by 100. The effect was therefore a maximum 

 when about '5 c.c. of this particular acetone solution was added to 100 c.c. of 

 water, and the addition of more of the solution diminished the activity. 



The explanation appears to be as follows : — The active substance, 

 probably chlorophyll, is in solution in the acetone, and in this solution it 

 is inactive. When a few drops of the acetone solution are added to the 

 water, the substance is thrown out of solution, and shows its photo-electric 

 effect. When more and more acetone solution is added, we reach a point when 

 the effect of the additional acetone is to dissolve more completely the active 

 substance, and therefore the activity decreases. This can be verified by the 

 addition of small quantities of pure acetone at different stages. For example, 

 in the case of one acetone solution, '2 c.c. added to 100 c.c. of water gave 

 on an arbitrary scale an activity of 150. An additional 5 c.c. reduced this 

 to 72 ; but if, instead of adding the additional 5 c.c. of acetone solution, we 

 added 5 c.c. of pure acetone, the effect was reduced to 55. These and similar 

 observations made it clear that the photo-electric effect in these experiments 

 depends essentially on the quantity of the active substance out of solution, as 

 it is inactive when dissolved in acetone. 



[1*] 



