i86 THE POPULAR SCIEKCE MONTHLY. 



has been by the Bussey Institute, in connection with the Arnold 

 Arboretum, at Brookline, Massachusetts. Under the able and judi- 

 cious management of Professor Sargent, it has already fine plantations 

 of forest-trees, has diffused much valuable information in regard to the 

 growth and importance of trees, and has secured the planting of a 

 large number in various parts of the country. On the foundation of 

 such institutions will naturally be built up in due time schools of 

 instruction in forestry like those of Europe, which will have a recog- 

 nized and permanent place among us. The European schools of for- 

 estry will form the subject of another article. 







PEODUCTION OF SOUND BY EADIANT ENEEGY.* 



By ALEXANDER GRAHAM BELL. 



IN a paper read before the American Association for the Advance- 

 ment of Science, last August, I described certain experiments made 

 by Mr. Sumner Tainter and myself which had resulted in the construc- 

 tion of a " Photophoney^ or apparatus for the production of sound by 

 light ; f and it will be my object to-day to describe the progress we 

 have made in the investigation of photophonic phenomena since the 

 date of this communication. 



In my Boston paper the discovery was announced that thin disks 

 of very many different substances emitted sounds when exposed to the 

 action of a rapidly-interrupted beam of sunlight. The great variety 

 of material used in these experiments led me to believe that sonorous- 

 ness under such circumstances would be found to be a general property 

 of all matter. 



At that time we had failed to obtain audible effects from masses of 

 the various substances which became sonorous in the condition of thin 

 diaphragms, but this failure was explained upon the suj^position that- 

 the molecular disturbance produced by the light was chiefly a surface 

 action, and that under the circumstances of the experiments the vibra- 

 tion had to be transmitted through the mass of the substance in order 

 to affect the ear. It was therefore supposed that, if we could lead to 

 the ear air that was directly in contact with the illuminated surface, 

 louder sounds might be obtained, and solid masses be found to be as 

 sonorous as thin diaphragms. The first experiments made to verify 



*A paper read before the National Academy of Arts and Sciences, April 21, 1881. 

 (From author's advance-sheets.) 



f "Proceedings of American Association for the Advancement of Science," August 27, 

 1880 ; see, also, "American Journal of Science," vol. xx, p. 305 ; " Journal of the American 

 Electrical Society," vol. iii, j). 3; "Journal of the Society of Telegraph Engineers and 

 Electricians," vol. ix, p. 40 1 ; " Annales de Chimie et de Physique," vol. xxi. 



