1901.] Effect of Light and Electric Radiation on Matter. 169 



or an electrometer. In the carrying out of this method there are, how- 

 ever, many practical difficulties. First of all, in making a voltaic com- 

 bination, some kind of electrolyte is necessary, but unfortunately all 

 electrolytes are opaque to electric radiation. This difficulty could, 

 however, be obviated to some extent by taking an electrolyte which is 

 almost a non-conductor (e.g., amylic alcohol) so as to be partially 

 transparent to electric waves. But the second difficulty is far more 

 serious. Owing to the diffuse action of the comparatively long electric 

 waves it is impossible to shield one plate while exposing the other. If 

 both the plates are equally acted on, there would then be no electro- 

 motive variation. From these considerations any attempt to detect 

 the effect of electric radiation by electromotive variation appeared to 

 be hopeless. It was only after the conclusion of another line of 

 investigation on the electromotive variation produced by mechanical 

 stimulus that I learnt that the effects of the same stimulus on two 

 pieces of the same metal, forming a voltaic element, are different if 

 the molecular conditions of the two are not originally the same. 

 Under such a condition a P.D. exists between the two, and stimula- 

 tion of both causes a variation of the existing electromotive force. 



From the similarities of the effects of radiation and mechanical 

 strain (see the following paper) I was convinced that with radiation, 

 too, I would get unequal effects on the two plates having a slight 

 initial electromotive difference. The effect of radiation would then be 

 to produce a variation of the original electromotive force. And if the 

 effects are at all parallel to those observed in the conductivity varia- 

 tion method (as diminution or increase of resistance) we may likewise 

 expect to find a diminution or increase of electromotive force. 



In carrying out experiments to verify the above suppositions, I soon 

 found my anticipations to be fully justified. I at first made a cell by 

 taking two varieties of silver. A piece of cotton wool moistened with 

 amylic alcohol was placed in a glass tube. Ag and Ag' were placed on 

 opposite sides of the moistened cotton ; this formed a voltaic element. 

 Two electrodes compressed the powder, till a current was observed to 

 flow. The amylic alcohol acted as the electrolyte. Very careful 

 adjustment of pressure is necessary, as in the case of receivers for 

 exhibition of conductivity variation. In order that the effect observed 

 might be purely due to electromotive variation and not to the variation 

 of conductivity, the cell was connected with a capillary electrometer. 

 On allowing electric radiation to act on the cell, there was produced a 

 variation of electromotive force ; continued radiation even produced a 

 reversal. The electric radiator — a rod used in the previous experiments 

 — ^was also used in this case : owing to the opacity of the electrolyte 

 the intensity of radiation has to be strong. The radiator was placed 

 at a distance of 6 inches from the cell. 



