th of the arms of the trap being sufficient to provide against the
effects of any changes of temperature and pressure that could occur.

A delivery tube may be connected to _H_ and filled with mercury, by
closing _E_ and raising the mercury reservoir. All air being in that
way expelled from the delivery tube, and the supply of mercury cut off
by clamping the tube from the reservoir, oxygen can be delivered from
the tube by opening _E_, when it will send forward the mercury, and pass
into a tube placed to receive it without any risk of air being derived
from the delivery tube.

[Illustration: FIG. 35.]

(2.) _Gimmingham's Vacuum Tap_,[15] shown in Fig. 35, consists of three
parts. A tube _A_ is ground to fit the neck of _B_. _B_ is closed at its
lower end, and has a hole _d_ drilled through it; when _B_ is fitted to
_C_, _d_ can be made to coincide with the slit _e_. When _A_, _B_, _C_
are fitted together, if _d_ meet _e_, there is communication between any
vessels attached to _A_ and any other vessel attached to _C_, entrance
of external air being prevented by mercury being placed in the cups of
_C_ and _B_. The tap may be opened and closed at pleasure by rotating
_B_.

[15] From _Proceedings of Royal Society_, vol. XXV. p. 396.

If _A_ has to be removed, _C_ may be converted into a mercury joint _pro
tem._ by letting a little mercury from the upper cup fall into the tube
and cover _d_, the tap being closed. This mercury must be removed by a
fine pipette in order to use the tap again. It should be noted, however,
that though external air cannot enter by way of the ground glass joints,
there is no absolute protection against the passage of air between _A_
and _C_, or vessels joined to _A_ and _C_, even when the tap is closed.
The passage of air from _A_ to _C_ depends upon the grinding and
lubrication of the joint at _C_.

=Lubricating Taps.=--For general purposes resin cerate answers very
well. In special cases burnt india-rubber, or a mixture of burnt
india-rubber and vaseline will answer well, or vaseline may be used
alone. Sulphuric acid and glycerine are too fluid. When a lubricant is
wanted that will withstand the action of ether, the tap may be
lubricated by sprinkling phosphorus pentoxide upon it, and exposing it
to air till the oxide becomes gummy. The joint must then be protected
from the further action of the air if possible. For example, if a safety
tap be used the cup may be filled with mercury.

=