Sound by Radiant Energy. 513 



closed by a flat plate of glass. A brass tube leading into the 

 cavity served for connexion with the hearing-tube. When 

 this conical cavity was stuffed with worsted or other fibrous 

 materials, the sounds produced were much louder than when 

 a test-tube was employed. This form of receiver is shown in 

 figure 1. 



Mr. Tainter next collected silks and worsteds of different 

 colours, and speedily found that the darkest shades produced 

 the best effects. Black worsted especially gave an extremely 

 loud sound. 



As white cotton-wool had proved itself equal, if not superior, 

 to any other white fibrous material before tried, he was anxious 

 to obtain coloured specimens for comparison. Not having 

 any at hand, however, he tried the effect of darkening some 

 cotton-wool with lampblack. Such a marked reinforcement 

 of the sound resulted that he was induced to try lampblack 

 alone. 



About a teaspoonful 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 

 intermittent beam with the lampblack surface towards 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 

 figure 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, figure 1. 



When the beam was thrown into a resonator, the interior 

 of which had been smoked over a lamp, most curious alter- 

 nations 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 

 reinforcements were constantly occurring, which became more 



