RADIANT HEAT AND ITS RELATIONS. 81 



impinge, or will they glide round the molecules, through 

 the intermolecular spaces, and thus escape ? 



The answer to this question depends upon a condi- 

 tion which may be beautifully exemplified by an experi- 

 ment on sound. These two tuning-forks are tuned 

 absolutely alike. They vibrate with the same rapidity, 

 and, mounted thus upon their resonant cases, you hear 

 them loudly sounding the same musical note. Slopping 

 one of the forks, I throw the other into strong vibration, 

 and bring that other near the silent fork, but not into 

 contact with it. Allowing them to continue in this 

 position for four or five seconds, and then stopping the 

 vibrating fork, the sound does not cease. The second 

 fork has taken up the vibrations of its neighbour, and 

 is now sounding in its turn. Dismounting one of the 

 forks, and permitting the other to remain upon its 

 stand, I throw the dismounted fork into strong vibra- 

 tion. You cannot hear it sound. Detached from its 

 case, the amount of motion which it can communicate 

 to the air is too small to be sensible at any distance. 

 When the dismounted fork is brought close to the 

 mounted one, but not into actualcontact with it, out of 

 the silence rises a mellow sound. Whence comes it ? 

 From the vibrations which have been transferred from 

 the dismounted fork to the mounted one. 



That the motion should thus transfer itself through 

 the air it is necessary that the two forks should be in 

 perfect unison. If a morsel of wax not larger than a 

 pea be placed on one of the forks, it is rendered thereby 

 powerless to affect, or to be affected by, the other. It 

 is easy to understand this experiment. The pulses of 

 the one fork can affect the other, because they are per- 

 fectly timed. A single pulse causes the prong of the 

 silent fork to vibrate through an infinitesimal space. 

 But just as it has completed this small vibration. 



