be drained from both bubbles
simultaneously. Care must be taken after this that the inner bubble is
not allowed to come against either wire ring, nor must the pipe be
passed through the side where the two bubbles are very close together.
To peel off the lower ring it should be pulled down a very little way
and then inclined to one side. The peeling will then start more readily,
but as soon as it has begun the ring should be raised so as not to make
the peeling too rapid, otherwise the final jerk, when it leaves the
lower ring, will be too much for the bubbles to withstand.

Bubbles coloured with fluorescine, or uranine, do not show their
brilliant fluorescence unless sunlight or electric light is concentrated
upon them with a lens or mirror. The quantity of dye required is so
small that it may be difficult to take little enough. As much as can be
picked up on the last eighth of an inch of a pointed pen-knife will be,
roughly speaking, enough for a wine-glassful of the soap solution. If
the quantity is increased beyond something like the proportion stated,
the fluorescence becomes less and very soon disappears. The best
quantity can be found in a few minutes by trial.

To blow bubbles containing either coal-gas or air, or a mixture of the
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