tly. This should
be washed before it is used again for filling the gazogene.

_Bubbles balanced against one another._

These experiments are most conveniently made on a small scale. Pieces of
thin brass tube, three-eighths or half an inch in diameter, are
suitable. It is best to have pieces of apparatus, specially prepared
with taps, for easily and quickly stopping the air from leaving either
bubble, and for putting the two bubbles into communication when
required. It should not be difficult to contrive to perform the
experiments, using india-rubber connecting tubes, pinched with spring
clips to take the place of taps. There is one little detail which just
makes the difference between success and failure. This is to supply a
mouth-piece for blowing the bubble, made of glass tube, which has been
drawn out so fine that these little bubbles cannot be blown out suddenly
by accident. It is very difficult, otherwise, to adjust the quantity of
air in such small bubbles with any accuracy. In balancing a spherical
against a cylindrical bubble, the short piece of tube, into which the
air is supplied, must be made so that it can be easily moved to or from
a fixed piece of the same size closed at the other end. Then the two
ends of the short tube must have a film spread over them with a piece of
paper, or india-rubber, but there must be _no_ film stretched across the
end of the fixed tube. The two tubes must at first be near together,
until the spherical bubble has been formed. They may then be separated
gradually more and more, and air blown in so as to keep the sides of the
cylinder straight, until the cylinder is sufficiently long to be nearly
unstable. It will then far more evidently show, by its change of form,
than it would if it were short, when the pressure due to the spherical
bubble exactly balances that due to a cylindrical one. If the shadow of
the bubbles, or an image formed by a lens on a screen, is then measured,
it will be found that the sphere has a diameter which is very accurately
double that of the cylinder.

_Thaumatrope for showing the Formation and Oscillations of Drops._

The experiment showing the formation of water-drops can be very
perfectly imitated, and the movements actually made visible, without any
necessity for using liquids at all, by simply converting Fig. 35 (at end
of book) into the old-fashioned instrument called a thaumatrope. What
will then be seen is a true representation, because the