_ is filled with mercury when the tap is in
use, so that if, for example, the end _C_ be attached to a flask, and
_D_ to an apparatus for exhausting the flask, it will be possible to
close the flask by turning off the tap _E_, and if no air be allowed
access through _D_, the vacuum produced in the flask at _C_ cannot be
affected by air leaking through the tap at _A_ or _B_.

A passage _F_ must be drilled from the bottom of the plug _E_ to meet
_G_, in order that when the plug is in position no residue of air shall
be confined within _B_, whence it might gradually leak into any
apparatus connected to it.

It is obvious, however, that this tap does not protect a flask sealed
to _C_ from the entrance of air through _D_, which, in fact, is the
direction in which air is most likely to effect an entrance. When using
one of these taps as part of an apparatus for supplying pure oxygen, I
have guarded against this by attaching a trap (Fig. 33) to the end _D_,
_C_ being joined to the delivery tube from the gas-holder. The structure
and mode of action of the trap are as follows:--

A narrow tube _G_ is joined to _D_ of Fig. 34, and terminates in the
wide tube _I_, which is connected above to _H_, and below to the
air-trap _J_. _J_ is connected at _K_, by a piece of flexible tube, to a
reservoir of mercury, from which mercury enters the air-trap, and
passing thence to _I_, can be employed for filling the V-trap _HLG_. The
air-trap _J_ is in the first instance filled with mercury, and then
serves to intercept any stray bubbles of air that the mercury may carry
with it. The particular form of the trap shown at _HLG_ was adopted
because with it the arm _LG_ is more readily emptied of mercury than
with any other form of trap made of small tube that I have tried. It has
been used in my apparatus in the following manner:--_H_ was connected
with a vessel to be filled with pure oxygen, the tap _E_ closed, and the
rise of mercury above _L_ prevented by a clamp on the flexible tube; the
vessel to be filled and the trap were then exhausted by a Sprengel pump,
and oxygen allowed to flow into the exhausted space by opening _E_, the
operation of exhausting the tubes and admitting oxygen being repeated as
often as necessary.

To prevent access of air to _E_ on disconnecting the vessel at _H_, the
mercury was allowed to flow into the trap till it reached to _MM_. _E_
was then closed, and _H_ exposed without danger of air reaching _E_, the
leng