Sound by Radiant Energy. 511 



upon the supposition that the molecular disturbance produced 

 by the light was chiefly a surface action, and that under the 

 circumstances of the experiments the vibration had to be trans- 

 mitted 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 this hypothesis pointed towards success. A beam of 

 sunlight was focused into one end of an open tube, the ear 

 being placed at the other end. Upon interrupting the beam, 

 a clear, musical tone was heard, the pitch of which depended 

 upon the frequency of the interruption of the light, and the 

 loudness upon the material composing the tube. 



At this stage our experiments were interrupted, as circum- 

 stances called me to Europe. 



While in Paris a new form of the experiment occurred to 

 my mind, which would not only enable us to investigate the 

 sounds produced by masses, but would also permit us to test 

 the more general proposition, that sonorousness, wider the in- 

 fluence of intermittent light, is a property common to all matter. 



The substance to be tested was to be placed in the interior 

 of a transparent vessel, made of some material which (like 

 glass) is transparent to light, but practically opaque to sound. 

 Under such circumstances the light could get in, but the 

 sound produced by the vibration of the substance could not 

 get out. The audible effects could be studied by placing the 

 ear in communication with the interior of the vessel by means 

 of a hearing-tube. 



Some preliminary experiments were made in Paris to 

 test this idea ; and the results were so promising that they 

 were communicated to the French Academy on the 11th of 

 October, 1880, in a note read for me by M. Antoine Breguet *. 

 Shortly afterwards I wrote to Mr. Tainter, suggesting that he 

 should carry on the investigation in America, as circumstances 

 prevented me from doing so myself in Europe. As these ex- 

 periments seem to have formed the common starting-point for 

 a series of independent researches of the most important 

 character, carried on simultaneously, in America by Mr. 

 Tainter, and in Europe by M. Mercadier f, Prof. Tyndall f , 



* Comities Benclus, vol. xcl. p. 595. 



t " Notes on Radiophonv," Comptes Rcndus, Dec. 6 and 13, 1880, and 

 Feb. 21 and 28, 1881 ; Phil. Mag. Jan. 1881, p. 78. See also Journal de 

 Physique, vol. x. p. 53. 



\ "Action of an Intermittent Beam of Radiant Heat upon Gaseous 

 Matter," Proc. Royal Society, Jan. 13, 1881, vol. xxxi. p. 307. 



202 



