of Selenium and Sulphur to Light* 179 



the electromotive force of the battery employ ed, being gene- 

 rally diminished as the battery-power was increased ; (2) that 

 the resistance of a bar AB was generally not the same for 

 current in the direction AB as for a current in the direction 

 BA ; -(3) that the passage of a battery-current was always 

 followed, when the battery had been disconnected, by a 

 secondary or polarization-current in the opposite direction, it 

 being clearly proved that this secondary current was not due 

 to any thermoelectric action, either in the selenium itself or 

 in any other part of the circuit. 



The authors do not, however, appear to have considered 

 that the observed behaviour of selenium was to be explained 

 by actual electrolysis, but rather that the molecular structure 

 or crystalline condition of the substance was altered or modi- 

 fied by the action of a current of electricity in such a manner 

 as to produce effects analogous to those which would have 

 occurred if the selenium were an electrolyte and actually de- 

 composed by the current. As to the possible influence of 

 light, the following are their words *: — " Light, as we know, 

 in the case of some bodies, tends to promote crystallization, 

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

 tends to promote crystallization in the exterior layers, 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 dimi- 

 nution 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." 



Attention has lately been again directed to the subject of 

 selenium, and its behaviour under the influence of light, by 

 the publication, by Mr. C. E. Fritts of New York, of a new 

 and extremely ingenious method of constructing selenium 

 cellsf. He melts a thin film of selenium upon "a plate of 

 metal w r ith which it will form a sort of chemical combina- 

 tion During the process of melting and crystallizing, 



the selenium is compressed between the metal plate upon 

 which it is melted and another plate of steel or other substance 



with which it will not combine The non-adherent plate 



being removed after the cell has become cool, [he] then covers 

 that surface with a transparent conductor of electricity, which 

 may be a thin film of gold-leaf. .... The whole surface of 



* Proc. Roy. Soc. 1876, p. 117. 



+ Proc. American Assoc. 1884. Reproduced in the ( Electrical Review/ 

 March 7, 1885, p. 208. 



N 2 



