No. 7.] BELL — THE PHOTOPHONE. 399 



" crystalline " selenium, or, as Regnault called it, " metallic " sele- 

 nium. It was selenium of this kind that Hittorff found to be a 

 conductor of electricity at ordinary temperatures. He also found 

 that its resistance to the passage of an electrical current diminished 

 continuously by heating up to the point of fusion, and thnt the 

 resistance suddenly increased in passing from the solid to the 

 liquid condition. It was early discovered that exposure to sun- 

 light hastens the change of selenium from one allotropic form t) 

 another; and this observation is significant in the light of recent 

 discoveries. 



Although selenium has been known for the last sixty years it 

 has not yet been utilized to any extent in the arts, and it is still 

 considered simply as a chemical curiosity. It is usually supplied 

 in the form of cylindrical bars. These bars are sometimes found 

 to be in the metallic condition ; but more usually they are in the 

 vitreous or non-conducting form. It occurred to Willoughby 

 Smith that, on account of high resistance of crystalline selenium, 

 it might be usefully employed at the shore-end of a submarine 

 cable, in his system of testing and signalling during the process of 

 submersion. Upon experiment, the selenium was found to have 

 all the resistance required — some of the bars employed measuring 

 as much as 1400 megohms — a resistence equivalent to that which 

 would be offered by a telegraph wire long enough to reach from 

 the earth to the sun ! But the resistance was found to be ex- 

 tremely variable. Experiments were made to ascertain the cause 

 of this variability. Mr. May, Mr. Willoughby Smith's assistant, 

 discovered that the resistance was less when the selenium was 

 exposed to light than when it was in the dark. 



In order to be certain that temperature had nothing to do with 

 the effect, selenium was placed in a vessel of water so that the 

 light had to pass through from one to two inches of water in order 

 to reach the selenium. The approach of a lighted candle was 

 found to be sufficient to cause a marked deflection of the needle 

 of the sjalvanometer connected with the selenium, and the lio;htine: 

 of a piece of magnesium wire caused the selenium to measure less 

 than half the resistance it did the moment before. 



These results were naturally at first received by scientific 

 men with some incredulity, but they were verified by Sale, Draper, 

 Moss and others. When selenium is exposed to the action of the 

 solar spectrum, the maximum effect is produced, according to Sale, 

 just outside the red end of the spectrum, in a point nearly co- 



