on the Action of Light on Selenium. 297 



ends of the wire electrodes of the selenium were made to dip into 

 two little mercury-cups fixed on a plate of ebonite, and then were 

 connected to the binding-screws of the Wheatstone-bridge arrange- 

 ment. Thus by reversing the position of the electrodes the di- 

 rection of the current through the selenium was reversed. The 

 positive direction of the current was always determined at the 

 commencement of each series of experiments by means of a 

 delicately suspended magnetic needle. 



A few preliminary experiments were made to determine whether 

 the change of resistance with change of direction of the current 

 had any connexion with the position of the selenium or the di- 

 rection of the current with respect to the magnetic meridian. 

 No such connexion was found to exist. 



From the results obtained from a great many experiments 

 made to determine the diminution of resistance with increased 

 battery-power, and the change of resistance with a change of the 

 direction of the current, the following conclusions were drawn : — 



(1) That on the whole there is a general diminution of resist- 

 ance in the selenium as the battery-power is increased. 



(2) The first current through the selenium, if a strong one, 

 causes a permanent set of the molecules, in consequence of which 

 the passage of the current through the selenium during the re- 

 mainder of the experiments is more resisted in that direction than 

 it is when passing in the opposite direction. 



(3) The passage of the current in any direction produces a set 

 of the molecules which facilitates the subsequent passage of a 

 current in the opposite, but obstructs one in the same direction. 

 Hence when two currents are sent through successively, after 

 a very small interval, in the same direction, the resistance ob- 

 served in the second case, even with the higher battery-power, 

 is often equal to or greater than it was before. 



The results of these experiments seeming to indicate that the 

 conductivity of selenium is electrolytic, a number of experiments 

 were undertaken in order to discover whether after the passage of 

 an electric current through a piece of selenium any distinct evidence 

 of polarization could be detected. It was then found that, after 

 passing the current from a voltaic battery for some time through 

 the selenium, and after having disengaged the electrodes from 

 the battery and connected them with a galvanometer, a current, 

 in some cases of considerable intensity, in the opposite direction 

 to that of the original battery-current, passed through the galva- 

 nometer. This proved that the passage of the battery-current sets 

 up polarization in the selenium. 



AH the results hitherto described were obtained with the selenium 

 kept in the dark. 



We then tried to discover whether on exposing the selenium to 

 light during the passage of the polarization-current any change in 

 the intensity of that current would be produced. We found 

 that in several cases there was a distinct change : in most instances 

 the action of the light assisted the passage of the current ; but in 



