two, the most convenient plan is to have a small T-shaped glass tube
which can be joined by one arm of the T to the blow-pipe by means of a
short piece of india-rubber tube, and be connected by its vertical limb
with a sufficient length of india-rubber pipe, one-eighth of an inch in
diameter inside, to reach to the floor, after which it may be connected
by any kind of pipe with the gas supply. The gas can be stopped either
by pinching the india-rubber tube with the left hand, if that is at
liberty, or by treading on it if both hands are occupied. Meanwhile air
can be blown in by the other arm of the T, and the end closed by the
tongue when gas alone is required. This end of the tube should be
slightly spread out when hot by rapidly pushing into it the _cold_ tang
of a file, and twisting it at the same time, so that it may be lightly
held by the teeth without fear of slipping.
If a light T-piece or so great a length of small india-rubber tube
cannot be obtained, then the mouth must be removed from the pipe and the
india-rubber tube slipped in when air is to be changed for gas. This
makes the manipulation more difficult, but all the experiments, except
the one with three bubbles, can be so carried out.
The pipe must in every case be made to enter the highest point of a
bubble in order to start an internal one. If it is pushed horizontally
through the side, the inner bubble is sure to break. If the inner bubble
is being blown with gas, it will soon tend to rise. The pipe must then
be turned over in such a manner that the inner bubble does not creep
along it, and so meet the outer one where penetrated by the pipe. A few
trials will show what is meant. The inner bubble may then be allowed to
rest against the top of the outer one while being enlarged. When it is
desired after withdrawing the pipe to blow more air or gas into either
the inner or the outer bubble, it is not safe after inserting the pipe
again to begin to blow at once; the film which is now stretched across
the mouth of the pipe will probably become a third bubble, and this,
under the circumstances, is almost certain to cause a failure. An
instantaneous withdrawal of the air destroys this film by drawing it
into the pipe. Air or gas may then be blown without danger.
If the same experiment is performed upon a light ring with cotton and
paper attached, the left hand will be occupied in holding this ring, and
then the gas must be controlled by the foot, or by a friend. The light
ring should be quite two inches in diameter. If, when the inner bubble
has begun to carry away the ring, &c., the paper is caught hold of, it
is possible, by a judicious pull, to cause the two bubbles to leave the
ring and so escape into the air one inside the other. For this purpose
the smallest ring that will carry the paper should be used. With larger
rings the same effect may be produced by inclining the ring, and so
allowing the outer bubble to peel off, or by placing the mouth of the
pipe against the ring and blowing a third bubble in real contact with
the ring and the outer bubble. This will assist the peeling process.
To blow three bubbles, one inside the other two, is more difficult. The
following plan I have found to be fairly certain. First blow above the
ring a bubble the size of a large orange. Then take a small ring about
an inch in diameter, with a straight wire coming down from one side to
act as a handle, and after wetting it with the solution, pass it
carefully up through the fixed ring so that the small ring is held well
inside the bubble. Now pass the pipe, freshly dipped in the solution,
into the outer or No. 1 bubble until it is quite close to the small
ring, and begin to blow the No. 2 bubble. This must be started with the
pipe almost in contact with the inner ring, as the film on this ring
would destroy a bubble that had attained any size. Withdraw the pipe,
dip it into the liquid, and insert it into the inner bubble, taking care
to keep these two bubbles from meeting anywhere. Now blow a large
gas-bubble, which may rest against the top of No. 2 while it is growing.
No. 2 may now rest against the top of No. 1 without danger. Remove pipe
from No. 3 by gently lowering it, and let some gas into No. 2 to make it
lighter, and at the same time diminish the pressure between Nos. 2 and
3. Presently the small ring can be peeled off No. 2 and removed
altogether. But if there is a difficulty in accomplishing this, withdraw
the pipe from No. 2 and blow air into No. 1 to enlarge it, which will
make the process easier. Then remove the pipe from No. 1. The three
bubbles are now resting one inside the other. By blowing a fourth
bubble, as described above, against the fixed ring, No. 1 bubble will
peel off, and the three will float away. No. 1 can, while peeling, be
transferred to a light wire ring from which paper, &c. are suspended.
This description sounds complicated, but after a little practice the
process can be carried out almost with certainty in far less time than
it takes to describe it; in fact, so quickly can it be done, and so
simple does it appear, that no one would suppose that so many details
had to be attended to.
_Bubbles and Electricity._
These experiments are on the whole the most difficult to perform
successfully. The following details should be sufficient to prevent
failure. Two rings are formed at the end of a pair of wires about six
inches long in the straight part. About one inch at the opposite end
from the ring is turned down at a right angle. These turned-down ends
rest in two holes drilled vertically in a non-conductor such as ebonite,
about two or three inches apart. Then if all is right the two rings are
horizontal and at the same level, and they may be moved towards or away
from one another. Separate them a few inches, and blow a bubble above or
below each, making them nearly the same size. Then bring the two rings
nearer together until the bubbles just, and only just, rest against one
another. Though they may be hammered together without joining, they will
not remain long resting in this position, as the convex surfaces can
readily squeeze out the air. The ebonite should not be perfectly warm
and dry, for it is then sure to be electrified, and this will give
trouble. It must not be wet, because then it will conduct, and the
sealing-wax will produce no result. If it has been used as the support
for the rings for some of the previous experiments, it will have been
sufficiently splashed by the bursting of bubbles to be in the best
condition. It must, however, be well wiped occasionally.
A stick of sealing-wax should be held in readiness under the arm, in a
fold or two of _dry_ flannel or fur. If the wax is very strongly
electrified, it is apt to be far too powerful, and to cause the bubbles,
when it is presented to them, to destroy each other. A feeble
electrification is sufficient; then the instant it is exposed the
bubbles coalesce. The wax may be brought so near one bubble in which
another one is resting, that it pulls them to one side, but the inner
one is screened from electrical action by the outer one. It is important
not to bring the wax very near, as in that case the bubble will be
pulled so far as to touch it, and so be broken. The wetting of the wax
will make further electrification very uncertain. In showing the
difference between an inner and an outer bubble, the same remarks with
regard to undue pressure, electrification, or loss of time apply. I have
generally found that it is advisable in this experiment not to drain the
drops from both the bubbles, as their weight seems to steady them; the
external bubble may be drained, and if it is not too large, the process
of electrically joining the outer bubbles, without injury to the inner
one, may be repeated many times. I once caused eight or nine single
bubbles to unite with the outer one of a pair in succession before it
became too unwieldy for more accessions to be possible.
* * * * *
It would be going outside my subject to say anything about the
management of lanterns. I may, however, state that while the experiments
with the small bubbles are best projected with a lens upon the screen,
the larger bubbles described in the last lecture can only be projected
by their shadows. For this purpose the condensing lens is removed, and
the bare light alone made use of. An electric arc is far preferable to a
lime-light, both because the shadows are sharper, and because the
colours are so much more brilliant. No oil lamp would answer, even if
the light were sufficient in quantity, because the flame would be far
too large to cast a sharp shadow.
In these hints, which have in themselves required a rather formidable
chapter, I have given all the details, so far as I am able, which a
considerable experience has shown to be necessary for the successful
performance or the experiments in public. The hints will I hope
materially assist those who are not in the habit of carrying out
experiments, but who may wish to perform them for their own
satisfaction. Though people who are not experimentalists may consider
that the hints are overburdened with detail, it is probable that in
repeating the experiments they will find here and there, in spite of all
my care to provide against unforeseen difficulties, that more detail
would have been desirable.
Though it is unusual to conclude such a book as this with the fullest
directions for carrying out the experiments described, I believe that
the innovation in the present instance is good, more especially because
many of the experiments require none of the elaborate apparatus which so
often is necessary.
THE END.
_Richard Clay & Sons, Limited, London & Bungay._
[Illustration: Fig. 35
THAUMATROPE for showing the formation and oscillation of drops.]