should be employed.

[Illustration: FIG. 30.]

When sulphuric acid is to be employed in such a joint, or when for any
other reason the use of an india-rubber tube is undesirable, the joint
may consist of a hollow stopper _B_ (Fig. 31), made of glass tube, and
ground to fit the neck of a thistle funnel _A_. _A_ and _B_ are joined
respectively to the pieces of apparatus to be connected, and connection
is made by placing _B_ in position in the neck of _A_; the joint is made
air-tight by introducing mercury with strong sulphuric acid above it
into the cup _A_. The joint may be rendered air-tight by introducing
sulphuric acid only into the cup. But this plan must not be adopted if
the interior of the apparatus is to be exhausted, as sulphuric acid is
easily forced between the ground glass surfaces by external pressure.
Mercury, however, will not pass between well-ground glass surfaces, and
is therefore to be employed for connecting apparatus which is to be
exhausted, and, if necessary, protected by a layer of strong sulphuric
acid to completely exclude air.

[Illustration: FIG. 31.]

Tubes placed horizontally may be joined by a glycerine or mercury joint
such as is shown in Fig. 32. The two tubes _A_ and _B_ are joined as
before by an india-rubber connection _C_, or one may be ground to fit
the other, and the joint is then enclosed within a larger jacketing-tube
_D_, with a mouth at _F_, which is filled with glycerine or mercury. _D_
is easily made by drawing out both ends of a piece of tube, leaving them
large enough to pass over the connection at _C_, however, and piercing
one side at _F_.

[Illustration: FIG. 32.]

=Vacuum Taps.=--It is not necessary to enter into a description of the
construction of ordinary glass taps, which can be purchased at very
reasonable prices. It may be remarked here, however, as a great many of
them are very imperfectly ground by the makers, that they may easily be
made air-tight by hand-grinding with camphorated turpentine and fine
emery, finishing with rotten-stone. A well-ground tap, which is well
lubricated, should be practically air-tight under greatly reduced
pressure for a short period; but when it is necessary to have a tap
which absolutely forbids the entrance of air into apparatus, one of the
following may be employed:--

[Illustration: FIG. 33.]

[Illustration: FIG. 34.]

(1.) _Mr. Cetti's Vacuum Tap_ (Fig. 34): This tap is cupped at _A_ and
sealed at _B_, and the cup _A