60 FRAGMENTS 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- 
