ON THE PRODUCTION OF SOUND BY LIGHT. 815 



continuously by heating up to the point of fusion, and that the resist- 

 ance suddenly increased in passing from the solid to the liquid con- 

 dition. It was early discovered that exposure to sunlight hastens the 

 change of selenium from one allotropic form to another ; and this ob- 

 servation 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-conduct- 

 ing foi'm. It occurred to "Willoughby Smith that, on account of the 

 high resistance of crystalline selenium, it might be usefully employed 

 at the shore-end of a submarine cable, in his system of testing and 

 signaling 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 fourteen hundred megohms a 

 resistance equivalent to that which would be offered by a telegraph- 

 wire Ion sr enough to reach from the earth to the sun ! But the resist- 

 ance was found to be extremely 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 se- 

 lenium 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, the 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 suffi- 

 cient to cause a marked deflection of the needle of the galvanometer 

 connected with the selenium, and the lighting of a piece of magnesium 

 wire caused the selenium to measui'e 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, coincident with the maxi- 

 mum of the heat-rays ; but, according to Adams, the maximum effect 

 is produced in the greenish-yellow or most luminous part of the spec- 

 trum. Lord Rosse exposed selenium to the action of non-luminous 

 radiations from hot bodies, but could produce no effect ; whereas a 

 thermopile under similar circumstances gave abundant indications of a 

 current. He also cut off the heat-rays from luminous bodies by the 

 interposition of liquid solutions, such as alum, between the selenium 

 and the source of light, without affecting the power of the light to 

 reduce the resistance of the selenium ; whereas the interposition of 

 these same substances almost completely neutralized the effect upon 

 the thermopile. Adams found that selenium was sensitive to the cold 



