ting against one another, but not really
touching (Fig. 64). The instant that I take out the sealing-wax you see
they join together and become one (Fig. 65). Two soap-bubbles,
therefore, enable us to detect electricity, even when present in minute
quantity, just as two water fountains did.

[Illustration: Fig. 64.]

[Illustration: Fig. 65.]

We can use a pair of bubbles to prove the truth of one of the
well-known actions of electricity. Inside an electrical conductor it is
impossible to feel any influence of electricity outside, however much
there may be, or however near you go to the surface. Let us, therefore,
take the two bubbles shown in Fig. 56, and bring an electrified stick of
sealing-wax near. The outer bubble is a conductor; there is, therefore,
no electrical action inside, and this you can see because, though the
sealing-wax is so near the bubble that it pulls it all to one side, and
though the inner one is so close to the outer one that you cannot see
between them, yet the two bubbles remain separate. Had there been the
slightest electrical influence inside, even to a depth of a
hundred-thousandth of an inch, the two bubbles would have instantly come
together.

[Illustration: Fig. 66.]

There is one more experiment which I must show, and this will be the
last; it is a combination of the last two, and it beautifully shows the
difference between an inside and an outside bubble. I have now a plain
bubble resting against the side of the pair that I have just been using.
The instant that I take out the sealing-wax the two outer bubbles join,
while the inner one unharmed and the heavy ring slide down to the bottom
of the now single outer bubble (Fig. 66).

And now that our time has drawn to a close I must ask you whether that
admiration and wonder which we all feel when we play with soap-bubbles
has been destroyed by these lectures; or whether now that you know more
about them it is not increased. I hope you will all agree with me that
the actions upon which such common and every-day phenomena as drops and
bubbles depend, actions which have occupied the attention of the
greatest philosophers from the time of Newton to the present day, are
not so trivial as to be unworthy of the attention of ordinary people
like ourselves.

PRACTICAL HINTS.

I hope that the following practical hints may be found useful by those
who wish themselves to successfully perform the experiments already
described.

_Dr