ON THE PRODUCTION OF SOUND BY LIGHT. 813 



majority of cases, but strong sulphuric acid slowly acts on it, the action 

 becoming rapid if heat be applied. Strong nitric acid acts on it with 

 some energy, causing its entire destruction, and in a similar manner it 

 is destroyed by the prolonged action of chlorine, bromine, or iodine ; 

 although these reagents, when their action is kept under control, pro- 

 duce a vulcanizing or strengthening effect. Abridged from Journal 

 of the Society of Arts. 



ON THE PEODUCTIOX OF SOUND BY LIGHT* 



By ALEXANDER GEAHAM BELL. 



IN" bringing before you some discoveries made by Mr. Sumner 

 Tainter and myself, which, having resulted in the construction of 

 apparatus for the production and reproduction of sound by means of 

 light, it is necessary to explain the state of knowledge which formed 

 the starting-point of our experiments. I shall first describe the re- 

 markable substance selenium, and the manipulations devised by va- 

 rious experimenters ; but the final result of our researches has extended 

 the class of substances sensitive to light-vibrations, until we can pro- 

 pound the fact of such sensitiveness being a general property of all 

 matter. We have found this property in gold, silver, platinum, iron, 

 steel, brass, copper, zinc, lead, antimony, German silver, Jenkin's 

 metal, Babbitt's metal, ivory, celluloid; gutta-percha, hard rubber, soft 

 vulcanized rubber, paper, parchment, wood, mica, and silvered glass ; 

 and the only substances from which we have not obtained results are 

 carbon and thin microscopic glass. We find that when a vibratory 

 beam of light falls upon these substances they emit sounds, the pitch 

 of which depends upon the frequency of the vibratory change in the 

 light. We find, further, that, when we control the form or character 

 of the light-vibration on selenium, and probably on the other sub- 

 stances, we control the quality of the sound and obtain all varieties of 

 articulate speech. We can thus, without a conducting wire, as in 

 electric telephony, speak from station to station, wherever we can pro- 

 ject a beam of light. We have not had opportunity of testing the 

 limit to which this photophonic influence can be extended, but we have 

 spoken to and from points two hundred and thirteen metres apart ; 

 and there seems no reason to doubt that the results will be obtained at 

 whatever distance a beam of light can be flashed from one observatory 

 to another. The necessary privacy of our experiments hitherto has 

 alone prevented any attempts at determining the extreme distance at 

 which this new method of vocal communication will be available. I 

 shall now speak of selenium. 



* Lecture delivered before the American Association for the Advancement of Science, 

 in the Institute of Technology, Boston, August 27, 1880. 



