1881.] on Schuitnn and ils ajyplu'aiions to the PhotojJionp, dc. 525 



quite opaque to light; its melting-point also is considerably higher, 

 being '217' C. 



Vitreous selenium, if melted and kept for a certain length of time 

 at a temperature between its own fnsing-j)()iiit and tliat of crystalline 

 selenium, will erystalliso; and I think I am right in saying, from 

 casual observation, tliorgh I have made no experiments to verify the 

 point, that the length of time necessary for crystallisation depends 

 upon the degree of temperature, being proportionately shorter as the 

 temperature approaches 217° C. 



Vitreous selenium is an exceedingly bad conductor of electricity ; 

 it is, indeed, an almost perfect insulator. Crystalline selenium is a 

 moderately good conductor, and it possesses this very remarkable 

 property, which has been utilised in the photophone and other inven- 

 tions, that it ccmducts better in the light than in the dark, the change 

 in its resistance to the passage of a current of electricity through it 

 varying, according to Professor W. G. Adams, as the square root of 

 the illuminating power. 



Let a galvanometer be connected to the two poles of a battery by 

 means of two copper wires. The passage of a current of electricity 

 will at once be denoted by the deflection of the magnetic needle ; or, 

 if a little mirror is attached to the needle, and a beam of light bo 

 reflected from it upon a scale, the movement of the spot of light will 

 indicate the movement of the needle. Let now one of the wires bo 

 cut, and the two ends be joined together by a piece of crystalline 

 selenium. The spot of light will again move, but its deflection will 

 be very much less than it was before, showing that the resistance of 

 the selenium is very much greater than that of the wire. Moreover, 

 if the piece of selenium be alternately exposed to and screened from 

 a beam of light, the deflection will be greater when it is in the light 

 than when it is in the dark, showing a corresponding variation in its 

 resistance. This remarkable property of selenium was first aimounced 

 and exhibited by Mr. Willoughby Smith in 1873. But tlie eflfects 

 produced by the simple arrangement which I have just described 

 are small, and very delicate instruments are required for their 

 observation. 



Since that date several devices have been proposed for exaggerating 

 the effect, but they all depend upon tlie fact that the amount of 

 the variation increases with the extent of the selenium surface acted 

 upon. It has lately been the fashion to call these arrangements 

 " cells," which, in most cases at all events, seems to be a very inap- 

 propriate name. It has been suggested that they should be termed 

 " rheostats," a name which well expresses the purposes for wliich 

 they are generally used, and is less likely to lead to confusion than 

 the other. In deference to custom, however, I shall to-night call 

 them by the usual name. 



The simplest selenium cell which could be devised, would be 

 made by placing two short pieces of copper wire parallel to each 

 other, and very near together, and connecting them by a narrow strip 



Vol. IX. (No. 73.) 2 o 



