the external air an opportunity of mixing with the air in the
inside more or less. The candle _c_, at the other end of the wire is
very convenient for holding under a jar standing in water, in order to
burn as long as the inclosed air can supply it; for the moment that it
is extinguished, it may be drawn through the water before any smoke can
have mixed with the air.

In order to draw air out of a vessel which has its mouth immersed in
water, and thereby to raise the water to whatever height may be
necessary, it is very convenient to make use of a glass _syphon_, fig.
13, putting one of the legs up into the vessel, and drawing the air out
at the other end by the mouth. If the air be of a noxious quality, it
may be necessary to have a syringe fastened to the syphon, the manner of
which needs no explanation. I have not thought it safe to depend upon a
valve at the top of the vessel, which Dr. Hales sometimes made use of.

If, however, a very small hole be made at the top of a glass vessel, it
may be filled to any height by holding it under water, while the air is
issuing out at the hole, which may then be closed with wax or cement.

If the generated air will neither be absorbed by water, nor diminish
common air, it may be convenient to put part of the materials into a
cup, supported by a stand, and the other part into a small glass
vessel, placed on the edge of it, as at _f_, fig. 1. Then having, by
means of a syphon, drawn the air to at convenient height, the small
glass vessel may be easily pushed into the cup, by a wire introduced
through the water; or it may be contrived, in a variety of ways, only to
discharge the contents of the small vessel into the larger. The distance
between the boundary of air and water, before and after the operation,
will shew the quantity of the generated air. The effect of processes
that _diminish_ air may also be tried by the same apparatus.

When I want to admit a particular kind of air to any thing that will not
bear wetting, and yet cannot be conveniently put into a phial, and
especially if it be in the form of a powder, and must be placed upon a
stand (as in those experiments in which the focus of a burning mirror is
to be thrown upon it) I first exhaust a receiver, in which it is
previously placed; and having a glass tube, bended for the purpose, as
in fig. 14, I screw it to the stem of a transfer of the air pump on
which the receiver had been exhausted, and introducing it through