er be
fastened to a heavy foot, in which case a side tube must be joined to
it, as in Fig. 47, or it may be open at both ends and be held in a
clamp. It is well to put a cone of card-board on the open end (Fig. 48),
if the sound is to be heard by many at a time. If the experimenter alone
wishes to hear as well as possible when faint sounds are produced, he
should carry a piece of smooth india-rubber tube about half an inch in
diameter from the open end to his ear. This, however, would nearly
deafen him with such loud noises as the tick of a watch.

_Bubbles and Ether._

Experiments with ether must be performed with great care, because, like
the bisulphide of carbon, it is dangerously inflammable. The bottle of
ether must never be brought near a light. If a large quantity is
spilled, the heavy vapour is apt to run along the floor and ignite at a
fire, even on the other side of a room. Any vessel may be filled with
the vapour of ether by merely pouring the liquid upon a piece of
blotting-paper reaching up to the level of the edge. Very little is
required, say half a wine-glassful, for a basin that would hold a gallon
or more. In a draughty place the vapour will be lost in a short time.
Bubbles can be set to float upon the vapour without any difficulty. They
may be removed in five or ten seconds by means of one of the small light
rings with a handle, provided that the ring is wetted with the soap
solution and has _no_ film stretched across it. If taken to a light at a
safe distance the bubble will immediately burst into a blaze. If a
neighbouring light is not close down to the table, but well up above the
jar on a stand, it may be near with but little risk. To show the burning
vapour, the same wide tube that was used to blow out the candle will
answer well. The pear shape of the bubble, owing to its increased weight
after being held in the vapour for ten or fifteen seconds, is evident
enough on its removal, but the falling stream of heavy vapour, which
comes out again afterwards, can only be shown if its shadow is cast upon
a screen by means of a bright light.

_Experiment with Internal Bubbles._

For these experiments, next to a good solution, the pipe is of the
greatest importance. A "churchwarden" is no use. A glass pipe 5/16 inch
in diameter at the mouth is best. If this is merely a tube bent near the
end through a right angle, moisture condensed in the tube will in time
run down and destroy the bubble occasion