1876.] 



the Action of Light on Selenium. 



117 



through a piece o£ 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 current 



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 batterj- 

 current, whichever was its direction. 



We also found that in those pieces which appeared so little sensitive 

 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 ciu-rent. 



The results of the experiments described in this paper furnish a pos- 

 sible explanation of the character of the action ^^"hich 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 ^\ ould be able to 

 take up their natural crystalline positions, and the whole would then bs 

 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 coo] the most rapidly, and we 

 shall have, in passing from the outside to the centre, a series of strata 

 in a more and more perfect crystalline condition. 



Light, as we know in the case of some bodies, tends to promote crvs- 

 tallization, and, when it falls on the surface of such a stick of selenium, 

 probably tends to promote crystallization in the exterior la vers, and 

 therefore to produce a flow of energy from within outwards, which 

 under certain circumstances appears, in the case of selenium, to produce 

 an electric current. 



The crystallization produced in selenium by light may also account for 

 the diminution in the resistance of the selenium when a current from a 

 battery is passing through it, for in changing to the crystalline state 

 selenium becomes a better conductor of electricity. 



