round the jar. If we have been operating in mercury, we begin by
displacing the mercury from the jar, by introducing water in its stead.
This is readily done by filling a bottle quite full of water; having
stopped it with your finger, turn it up, and introduce its mouth below
the edge of the jar; then, turning down its body again, the mercury, by
its gravity, falls into the bottle, and the water rises in the jar, and
takes the place occupied by the mercury. When this is accomplished, pour
so much water into the cistern ABCD as will stand about an inch over the
surface of the mercury; then pass the dish BC, Pl. V. Fig. 9. under the
jar, and carry it to the water cistern, Fig. 1. and 2. We here exchange
the gas into another jar, which has been previously graduated in the
manner to be afterwards described; and we thus judge of the quantity or
volume of the gas by means of the degrees which it occupies in the
graduated jar.

There is another method of determining the volume of gas, which may
either be substituted in place of the one above described, or may be
usefully employed as a correction or proof of that method. After the air
or gas is exchanged from the first jar, marked with slips of paper, into
the graduated jar, turn up the mouth of the marked jar, and fill it with
water exactly to the marks EF, Pl. IV. Fig. 3. and by weighing the water
we determine the volume of the air or gas it contained, allowing one
cubical foot, or 1728 cubical inches, of water for each 70 pounds,
French weight.

The manner of graduating jars for this purpose is very easy, and we
ought to be provided with several of different sizes, and even several
of each size, in case of accidents. Take a tall, narrow, and strong
glass jar, and, having filled it with water in the cistern, Pl. V. Fig.
1. place it upon the shelf ABCD; we ought always to use the same place
for this operation, that the level of the shelf may be always exactly
similar, by which almost the only error to which this process is liable
will be avoided. Then take a narrow mouthed phial which holds exactly 6
oz. 3 gros 61 grs. of water, which corresponds to 10 cubical
inches. If you have not one exactly of this dimension, choose one a
little larger, and diminish its capacity to the size requisite, by
dropping in a little melted wax and rosin. This bottle serves the
purpose of a standard for gaging the jars. Make the air contained in
this bottle pass into the jar, and mark exactly the