Existence of the Luminiferous JEtlier. 489 



molecular condition of the substance selenium." Lord Rosse* 

 instituted some experiments to determine whether the effect 

 is to be attributed to heat or to light. He concludes that the 

 effect is due to the action of light and not to that of heat, and 

 that the variation of the resistance is proportional rather to 

 the square root of the intensity of illumination than to the in- 

 tensity simply ; but no definite law of this kind could be 

 deduced. Dr. Werner Siemens confirmed Lord Rosse's results, 

 and, in addition, proved that the electrical properties are modi- 

 fied by the temperature at which the selenium has been pro- 

 duced!. His experiments lead him to the conclusion that the 

 effect of the light upon the selenium is to produce a change 

 in the molecular condition of the substance near its surface, by 

 which it is more or less converted from an electrolytic con- 

 ductor into a metallic conductor, involving a liberation of spe- 

 cific heat upon that surface which is exposed to the light-rays. 

 The exhaustive experiments of Prof. W. Gr. Adams and Mr. 

 R. E. Day led the former to propound two possible hypo- 

 theses by which the action of light on selenium may be ex- 

 plained: — (1) that the light falling on the selenium causes an 

 electromotive force in it, which opposes a battery-current pass- 

 ing through it, the effect being similar to the effect due to 

 polarization in an electrolyte; and (2) that the effect of light 

 is to cause a change on the surface analogous to the change 

 which it produces on the surface of a phosphorescent body, 

 and that in consequence of this change the electric current 

 is enabled to pass more readily over the surface of the sele- 

 nium %. These experimenters proved, among other things, that 

 the effect was directly proportional to the square root of the 

 intensity of the illumination, and also that an electric current 

 was produced in a piece of crystalline selenium when suddenly 

 exposed to light. One of the conclusions to which they were 

 led is significant. They consider that the first current sent 

 through the selenium causes a more or less permanent " set " 

 of the molecules, in consequence of which the passage of the 

 succeeding currents is more resisted in that direction than it 

 is in the opposite one. They also proved that this effect was 

 considerably modified if the bar was exposed to light, thus 

 conclusively showing that the waves of light impinging on the 

 material molecules of the selenium affected those molecules in 

 in such a manner as to cause them to be less affected by the 

 passage of an electric current through them. The subject 



* Phil. Mag. vol. xlvii. No. 311. 



t Monatsbericht der Akademie der Wissenschaften zu Berlin, 1875, p. 80, 

 and January 1876. 



\ Proe. Roy. Soc. vol. xxiii. p. 535. 



