518 Prof. A. G-. Bell on the Production of 



2. To tost whether this had been the case, an additional strip 

 (D) was attached by its central point only to the strip under 

 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 (D) 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 interrupted 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 interrup- 

 tion suggests the explanation that the strip D had a normal 

 rate of vibration of its own. "When the frequency of the in- 

 terruption of the light corresponded 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 ex- 

 periment 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 ivith Liquids. 



The sounds produced by liquids are much more difficult to 

 observe than those produced by solids. The high absorptive 

 power possessed by most liquids would lead one to expect in- 

 tense vibrations from the action of intermittent light; but the 

 number of sonorous liquids that have so far been found is ex- 

 tremely limited, and the sounds produced are so feeble as to 

 be heard only by the greatest attention and under the best 

 circumstances of experiment. In the experiments made in 

 my laboratory, a very long test-tube was filled with the 

 liquid under examination, and a flexible-rubber tube was 



* Carbon and thin microscope glass are mentioned in my Boston 

 paper as non-responsive, and powdered chlorate of potash in the com- 

 munication to the French Academy ( Comptes Mendus, vol. xcl. 

 p. 595). All these substances have since yielded sounds under more 

 careful conditions of experiment. 



