communicates with the burette. The burette is connected by a rubber tube
and a Y-piece, either with another burette or with a piece of ordinary
combustion-tube of about the same size. The wide-mouthed bottle contains
either a short test-tube or an ordinary phial with its neck cut off. In
working the apparatus the weighed substance is put in the bottle and the
re-agent which is to act on it, in the test-tube; the cork is then
inserted. The liquid in the two burettes is next brought to the same
level, either by pouring it in at A or running it out at B. The level of
the liquid in the apparatus for correcting variation in volume is then
read and noted. Next, after seeing that the level of the liquid in the
burette has not changed, turn the bottle over on its side so that the
re-agent in the test-tube shall be upset into the bottle. Then, as the
volume of the gas increases, lower the liquid in the burette by running
it out at B, and at the same time keep the level in A half an inch or so
lower than that in the burette. When the action has finished bring the
liquid in the two vessels to the same level and read off the burette.
This part of the work must always be done in the same manner.

[Illustration: FIG. 31.]

_The volume corrector for gas analysis_ is a graduated glass tube of 120
c.c. capacity inverted over a narrow glass cylinder of mercury. It
contains 0.2 or 0.3 c.c. of water and a volume of air, which, if dry and
under standard conditions, would measure 100 c.c. The actual volume
varies from day to day, and is read off at any time by bringing the
mercury inside and outside to the same level. This is done by raising or
lowering the tube, as may be required. Any volume of gas obtained in an
assay can be corrected to standard temperature and pressure by
multiplying by 100 and dividing by the number of c.c. in the corrector
at the time the assay is made.