60 PR A OMENTS OF SCIENCE. 



they glide round the molecules, through the intermolecu- 

 lar 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 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 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 

 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 under- 

 stand this experiment. The pulses of the one fork can 

 affect the other, because they are perfectly 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, another pulse is ready to strike it. Thus, 

 the impulses add themselves together. In the five seconds 

 during which the forks were held near each other, the 

 vibrating fork sent 1,280 waves against its neighbor and 

 those 1,280 shocks, all delivered at the proper moment, all, 

 as I have said, perfectly timed, have given such strength 

 to the vibrations of the mounted fork as to render them 

 audible to all. 



Another curious illustration of the influence of syn- 

 chronism on musical vibrations, is this: Three small gas- 



