PHILOSOPHICAL SOCIETY OF WASHINGTON. 151 



was attached by its central point only to the strip uuder trial, and 

 was then submitted to the action of the beam, as shown in Fig. 6. 



It was presumed that if the vibration of the diaphragm (B) had 

 been due to a pushing force acting on the strip (A,) the addition 

 of the strip (Dj would not interfere with the effect. But if, on 

 the other hand, it had been due to the longitudinal expansion and 

 contraction of the strip, (A,) the sound would cease, or, at least, be 

 reduced. The beam of light falling upon strip (D) was now inter- 

 rupted as before by the rapid rotation of a perforated disk, which 

 was allowed to come gradually to rest. 



No sound was heard excepting at a certain speed of rotation, 

 when a feeble musical tone became audible. 



This result is confirmatory of the first. 



The audibility of the effect at a particular rate of interruption 

 suggests the explanation that the strip (D) had a normal rate of 

 vibration of its own. 



When the frequency of the interruption of the light corres- 

 ponded to this, the strip was probably thrown into vibration after 

 the manner of a tuning fork, in which case a to-and»fro vibration 

 would be propagated down its stem or central support to the strip 



(A.)_ 



This indirectly proves the value of the experiment. 



The list of solid substances that have been submitted to experi- 

 ment in my laboratory is too long to be quoted here, and I shall 

 merely say that we have not yet found one solid body that has 

 failed to become sonorous under proper conditions of experiment.* 



Experiments with Liquids. 



The sounds produced by liquids are much more difficult to ob- 

 serve than those produced by solids. The high absortive power 

 possessed by most liquids would lead one to expect intense vibra. 

 tions from the action of intermittent light, but the number of son- 

 orous liquids that have so far been found is extremely limited, and 

 the sounds produced are so feeble as to be heard only by the 

 greatest attention and under the best circumstances of experiment. 



* Carbon and thin microscopic glass are mentioned in my Boston paper as non- 

 responsive, and powdered chlorate of potash in the communication to the French 

 Academy, (Comtes Rendus, vol. xcl, p. 595.) All these substances have since 

 yielded sounds xinder more careful conditions of experiment. 



