RADIANT HEAT AND ITS RELATIONS 87 



medium which accepts their vibrations, and transmits them 

 through space. But suppose the waves generated by one 

 system of molecules to impinge upon another system, how 

 will the waves be affected ? Will they be stopped, or will 

 they be permitted to pass ? Will they transfer their mo- 

 tion to the molecules on which they impinge, or will they 

 glide round the molecules, through the intermolecular 

 spaces, and thus escape? 



The answer to this question depends upon a condition 

 which may be beautifully exemplified by an experiment 

 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. Stopping 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 neighbor, 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 vibration. You cannot hear it sound. Detached 

 from its case, the amount of motion which it can commu- 

 nicate to the air is too small to be sensible at any dis- 

 tance. When the dismounted fork is brought close to the 

 mounted one, but not into actual contact 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 



