been
removed. This process is repeated until the divided tube is filled. A
table of results is prepared, showing the number of divisions occupied
by each known volume of mercury introduced.

In subsequently using the tube the volumes of the gases measured in it
must be ascertained from the table of values thus prepared.

In observing the level of the mercury, unless a cathetometer is
available, a slip of mirror should be held behind the mercury close to
the tube, in such a position that the pupil which is visible on the
looking-glass is divided into two parts by the surface of the mercury.

A correction must be introduced for the error caused by the meniscus of
the mercury. As the closed end of the tube was downwards when each
measured volume of mercury was introduced, and as the surface of mercury
is convex, the volume of mercury in the tube when it is filled to any
division _l_ (Fig. 41) is represented by _A_ of 1. But in subsequently
measuring a gas over mercury in the same tube, when the mercury stands
at the same division _l_, the volume of the gas will be as represented
by _B_ of 2, which is evidently somewhat greater than _A_. This will be
seen still more clearly in 3, where _a_ represents the boundary of the
mercury, and _b_ the boundary of the air, when the tube is filled to the
mark _l_ with mercury or a gas over mercury respectively.

[Illustration: FIG. 41.]

It is plain that when the level of the mercury in measuring a gas is
read at _l_, the volume of the gas is greater than the volume of the
mercury recorded, by twice the difference between the volume _A_ of
mercury measured, and that which would fill the tube to the level _l_,
if its surface were plane.

The usual mode of finding the true volume of a gas collected over
mercury is as follows:--

Place the graduated tube mouth upwards, introduce some mercury, and,
after removing all bubbles, note the division at which it stands. Then
add a few drops of solution of mercuric chloride; the surface of the
mercury will become level, read and record its new position. Then, in
any measurement, having observed that the mercury stands at _n_
divisions of the tube, add twice the difference between the two
positions of the mercury to _n_, and ascertain the volume which
corresponds to this reading from the table of capacities.

=To Calibrate the Tube of a Thermometer.=--Detach a thread of mercury
from half an inch to one inch in length from the body of the