PRODUCTION OF SOUND BY RADIANT ENERGY. 189 



Such a marked reenforcement of the soudcI resulted that he was in- 

 duced to try lampblack alone. 



About a teaspoonf ul of lampblack was placed in a test-tube and 

 exposed to an intermittent beam of sunlight. The sound produced 

 was much louder than any heard before. 



Upon smoking a piece of plate-glass, and holding it in the inter- 

 mittent beam with the lampblack surface toward the sun, the sound 

 produced was loud enough to be heard, with attention, in any part of 

 the room. With the lampblack surface turned from the sun, the sound 

 was much feebler. 



Mr. Tainter repeated these experiments for me immediately upon 

 my return to Washington, so that I might verify his results. 



Upon smoking the interior of the conical cavity shown in Fig. 1, 

 and then exposing it to the intermittent beam, with the glass lid in 

 position as shown, the effect was perfectly startling. The sound was 

 so loud as to be actually painful to an ear placed closely against the 

 end of the hearing-tube. 



The sounds, however, were sensibly louder when we placed some 

 smoked wire-gauze in the receiver, as illustrated in the drawing (Fig. 1). 



When the beam was thrown into a resonator, the interior of which 

 had been smoked over a lamp, most curious alternations of sound and 

 silence were observed. The interrupting disk was set rotating at a 

 high rate of speed, and was then allowed to come gradually to rest. 

 An extremely feeble musical tone was at first heard, which gradually 

 fell in pitch as the rate of interruption grew less. The loudness of the 

 sound produced varied in the most interesting manner. Minor reen- 

 forcements were constantly occurring, which became more and more 

 marked as the true pitch of the resonator was neared. When at last 

 the frequency of interruption corresponded to the frequency of the 

 fundamental of the resonator, the sound produced was so loud that it 

 might have been heard by an audience of hundreds of people. 



The effects produced by lampblack seemed to me to be very ex- 

 traordinary, especially as I had a distinct recollection of experiments 

 made in the summer of 1880 with smoked diaphragms, in which no such 

 reenforcement was noticed. 



Upon examining the records of our past photophonic experiments 

 we found in vol. vii, p. 57, the following note : 



Experiment V. Mica diaphragm covered with lampblack on side exposed to 

 light. 



Result : distinct sound about same as without lampblack. A. G. B., July 

 18, 1880. 



Verified the above, but think it somewhat louder than when used without 

 lampblack. S. T., July 18, 1880. 



Upon repeating this old experiment we arrived at the same result 

 as that noted. Little if any augmentation of sound resulted from 



