LIGHT AND ELECTRIFICATION. 181 



from a discharging mirror had become at all inactive, or 

 whether it was still equally competent to exert a dis- 

 charging action on any second surface which it met. The 

 latter seems to be the truth ; whatever fatigue may be 

 experienced by a surface none seems experienced by a 

 light; or if the light was affected, there was too great a 

 balance of power left to make its deterioration of quality 

 conspicuous. It is, however, an important question whether 

 the short waves which are especially operative are really 

 consumed in the act of effecting the electric discharge, or 

 whether they are uninjured ; and hence the experiment 

 was carefully and repeatedly performed. No question of 

 energy arises, because the energy of the discharge may 

 be simply and entirely electrical. 



NON-METALLIC SUBSTANCES. 



Elster and Geitel found that luminous paint was 

 peculiarly active as a discharging agent, and they tried 

 other phosphorescent bodies too, also several minerals. 

 In my experience some minerals discharge positive .more 

 readily than negative, but any surface to be effective should 

 be dry. A lump of dry soft red sandstone fell in potential 

 a given amount in 150 seconds when charged positively, 

 whereas when charged negatively it fell much more slowly 

 or not at all. On the other hand many non-metallic bodies 

 imitate metals in their more rapid loss of negative elec- 

 tricity. Thus a piece of gas-carbon lost negative in 18 

 seconds, positive in 150 ;- lamp-black lost negative in 10 

 seconds, positive in 120. 



PLANTS. 



Several growing plants, and cuttings of others, were 

 tried in my laboratory last June, with the result that several 

 of them discharged positive more readily than negative ; 

 but on the whole the rates of discharge for the two kinds of 

 electrification from the leaves of plants under the action of 

 light are more nearly equal than are the rates of discharge 

 from most other substances. Some facts suggest that 

 the time of year, i.e., the state of the plant, has an 



