elope of the balloon is extended, and its size enlarged. Now withdraw
it from the fire and note what happens.

As the air inside gets cold again, the elastic envelope of the balloon
gradually shrinks, until it has been reduced to its former size. What
has been taking place during this experiment with regard to the elastic
envelope and the atoms thereof? May we not say that there has been a
vibration or oscillation, among the particles which go to make up the
elastic envelope, that forms the surface of the balloon? Certainly there
has been some form of motion, and that motion took first the form of an
expansion, and then contraction of the individual particles; and we have
only to conceive of this process being repeated quickly and
continuously, to form a mental picture of what takes place in any
aetherial elastic envelope or shell that surrounds the sun.

The illustration is not, however, perfect, because we have made the
source of heat to be outside instead of inside the elastic envelope, as
is the case with the sun and its aetherial atmosphere or envelope. We
will therefore slightly modify the experiment, and take two balloons,
_A_, _B_, one smaller than the other, and put the smaller one _A_ into
the interior of the larger one, inflating the smaller one, so that it
can be situated in the middle of the larger one, the latter having twice
the diameter of the smaller one, as in the diagram (Fig. 6). To the neck
of the smaller balloon _A_ we will attach an india-rubber tube which
ends in a closed bulb _C_. We have now the two balloons inflated. Let us
press the bulb _C_ and notice what happens. The effect will be exactly
the same as it was when we brought the balloon in contact with the heat
of the fire in the first experiment--that is, the elastic envelope will
be again expanded. As soon as we take the pressure from the bulb _C_ the
envelope, being elastic, seeks to recover its original position, with
the result that it springs back to its original size. If we pressed the
bulb _C_ 20 times per minute, we should get 20 vibrations of the
particles of the envelopes per minute, and if we pressed it 1000 times
per minute, we should get 1000 vibrations among the particles of the
elastic envelope, so that the number of vibrations would correspond to
the number of times we pressed the bulb. Now how did this vibration
reach the elastic envelope of the balloon _B_ from the balloon _A_?

[Illustration: Fig: 6.]

The reply is, b