place to which the
water has descended; add another measure of air, and again mark the
place of the water, and so on, till all the water be displaced. It is of
great consequence that, during the course of this operation, the bottle
and jar be kept at the same temperature with the water in the cistern;
and, for this reason, we must avoid keeping the hands upon either as
much as possible; or, if we suspect they have been heated, we must cool
them by means of the water in the cistern. The height of the barometer
and thermometer during this experiment is of no consequence.

When the marks have been thus ascertained upon the jar for every ten
cubical inches, we engrave a scale upon one of its sides, by means of a
diamond pencil. Glass tubes are graduated in the same manner for using
in the mercurial apparatus, only they must be divided into cubical
inches, and tenths of a cubical inch. The bottle used for gaging these
must hold 8 oz. 6 gros 25 grs. of mercury, which exactly
corresponds to a cubical inch of that metal.

The method of determining the volume of air or gas, by means of a
graduated jar, has the advantage of not requiring any correction for the
difference of height between the surface of the water within the jar,
and in the cistern; but it requires corrections with respect to the
height of the barometer and thermometer. But, when we ascertain the
volume of air by weighing the water which the jar is capable of
containing, up to the marks EF, it is necessary to make a farther
correction, for the difference between the surface of the water in the
cistern, and the height to which it rises within the jar. This will be
explained in the fifth section of this chapter.

SECT. IV.

_Of the method of Separating the different Gasses from each other._

As experiments often produce two, three, or more species of gas, it is
necessary to be able to separate these from each other, that we may
ascertain the quantity and species of each. Suppose that under the jar
A, Pl. IV. Fig. 3. is contained a quantity of different gasses mixed
together, and standing over mercury, we begin by marking with slips of
paper, as before directed, the height at which the mercury stands within
the glass; then introduce about a cubical inch of water into the jar,
which will swim over the surface of the mercury: If the mixture of gas
contains any muriatic or sulphurous acid gas, a rapid and considerable
absorption will instantly take place, from t