on the Action of Light on Selenium. 299 



(2) That the sensitiveness is different at different parts of the 

 same piece. 



(3) That in general the direction of the current is from the 

 less towards the more illuminated portion of the selenium, but that, 

 owing to accidental differences in molecular arrangement, this di- 

 rection is sometimes reversed. 



The currents produced in the selenium by the action of light 

 do not resemble the thermoelectric currents due to heating of 

 the junctions between the platinum electrode and the selenium ; 

 for in many cases the current produced was most intense when 

 the light was focused on points of the selenium not coinciding 

 with the junctions ; also the current was produced suddenly on 

 exposure, and on shutting off the light the needle at once fell to 

 zero ; the gradual action due to gradual cooling was entirely 

 wanting. 



When the light fell upon a junction, the current passed from 

 the selenium to the platinum through the junction, which is not 

 in accordance with the place assigned to selenium in the thermo- 

 electric series of metals. 



Experiments were next undertaken in order to examine what 

 effect would be produced on the strength of a current which 

 was passing through a piece of selenium in the dark when a beam 

 of light was allowed to fall upon it. 



The results obtained from these experiments were as follows > — 



With pieces of selenium of low resistance and with a weak cur- 

 rent passing through them — 



(1) When light falls on the end of the selenium at which the 

 current from the positive pole of the battery is entering the metal, 

 it opposes the passage of the current. 



(2) When light falls on the end of the selenium at which the 

 current is leaving the metal, it assists the passage of the current. 



With pieces of selenium of a high resistance we found that in 

 all cases the action of light tended to facilitate the passage of the 

 battery-current, whichever was its direction. 



We also found that in those pieces which appeared so little sen- 

 sitive to light that no independent current was developed in them 

 by exposure, yet, when a current due to an external electromotive 

 force was passing through them, the exposure to light facilitated 

 the passage of the current. 



The results of the experiments described in this paper furnish a 

 possible explanation of the character of the action which takes place 

 when light falls upon a piece of selenium which is in a more or less 

 perfect crystalline condition. 



When a stick of vitreous selenium has been heated to its point 

 of softening, if it were possible to cool the whole equally and very 

 slowly, then the whole of the molecules throughout its mass would 

 be able to take up their natural crystalline positions, and the whole 

 would then be in a perfectly crystalline state, and would conduct 

 electricity and heat equally well throughout its mass. But from 

 the nature of the process it is evident that the outer layers will 



