AMERICAN JOURNAL OF SCIENCE. 
[THIRD SERIES.] 
Art. X V.— Upon a modification of Wheatsione’s Microphone and 
its applicability to Radiophonic Researches; by ALEXANDER 
GRAHAM BELL. 
[A paper read before the Philosophical ee of Washington, D. C., June 11, 
1881. 
In August, 1880, I directed attention to the fact that thin 
disks or diaphragms of various materials become sonorous when 
exposed to the action of an intermittent beam of sunlight, and 
I stated my belief that the sounds were due to molecular dis- 
turbances produced in the substance composing the diaphragm.* 
Shortly afterwards Lord Raleigh undertook a mathematical 
investigation of the subject, and came to the conclusion that the 
audible effects were caused by the bending of the plates under 
unequal heating.t This explanation has recently been called 
in question by Mr. Preece, who has expressed the opinion that 
although vibrations may be produced in the disks by the action 
of the intermittent beam, such vibrations are not the cause of 
the sonorous effects observed. According to him, the aerial 
disturbances that produce the sound arise spontaneously i in the 
air itself by sudden expansion due to heat communicated from 
the dia pratt e increase of heat giving rise to a fresh 
pulse of air. Mr. Preece was led to Fools the theoretical 
ss of Lord Raleigh on account of the failure of experi- 
ments undertaken to test the theory. 
n Association for the Advancement of erage: ae 27, 1880. 
ieee a xxiii, p. 274. t Roy. Soc., March 10, 1 
Am. Jour. Sct. e Series, Vor, XXII, No, 128. perenne 1881. 
