ot equally valuable.

If the stopper is to fit a tube contracted like _B_, it must be
constructed from a piece of tube that will pass through the contraction
at _B_. The tail _GF_ will not do such good service as it does in the
case of a tube which has been opened out to receive its stopper, but it
will help to guide the stopper, and should be retained.

When the stopper has been ground into its place, melt off the tail at
_F_. The flame must be applied very cautiously, as glass which has been
ground is particularly apt to crack on heating. To avoid all risk of
this, the tail may simply be cut off, and its edges filed smooth with a
file moistened freely with camphorated turpentine.

The stoppers of bottles are not made exactly in the manner described
above, though, on occasion, a new stopper may be made for a bottle by
following those directions. Ill-fitting stoppers, which are very common,
can be very easily re-ground with emery and camphorated turpentine.

CHAPTER IV.

_MAKING THISTLE FUNNELS, U-TUBES, ETC.--COMBINING THE PARTS OF
COMPLICATED APPARATUS--MERCURY, AND OTHER AIR-TIGHT JOINTS--VACUUM
TAPS--SAFETY TAPS--AIR-TRAPS._

In Chapter III. the simpler operations used in making the separate parts
of which apparatus is composed have been described. In this Chapter
finished apparatus will be described, and the combination of the
separate parts into the more or less complicated arrangements used in
experiments will be so far explained as to enable the student to set up
such apparatus as he is likely to require. I have thought it would be
useful that I should add a short account of various contrivances that
have come much into use of late years for experimenting under reduced
pressure, such as safety taps, air-traps, vacuum joints, etc.

[Illustration: FIG. 24.]

=Electrodes.=--On page 38 (Fig. 13) is shown a simple form of electrode
sealed into a glass tube, which for many purposes answers very well. But
frequently, in order that there may be less risk of leakage between the
glass and the metal, the latter is covered for a considerable part of
its length with solid glass, which at one extremity is united to the
apparatus. In Fig. 24 _W_ is the metal core of the electrode, and _G_
the glass covering around it. The wire is fused into the glass, and the
glass is then united to the apparatus; a little white enamel should be
applied at one end and combined with the glass by fusion.

=U-Tubes.=--A U-tube