plain the air in the branch BCDE will now be pressed upon by a weight
equal to twice 28 inches of mercury, or twice the weight of the
atmosphere; and experience shows, that, in this case, the included air,
instead of filling the tube from B to E, only occupies from C to E, or
exactly one half of the space it filled before. If to this first column
of mercury we add two other portions of 28 inches each, in the branch
AB, the air in the branch BCDE will be pressed upon by four times the
weight of the atmosphere, or four times the weight of 28 inches of
mercury, and it will then only fill the space from D to E, or exactly
one quarter of the space it occupied at the commencement of the
experiment. From these experiments, which may be infinitely varied, has
been deduced as a general law of nature, which seems applicable to all
permanently elastic fluids, that they diminish in volume in proportion
to the weights with which they are pressed upon; or, in other words,
"_the volume of all elastic fluids is in the inverse ratio of the weight
by which they are compressed_."
The experiments which have been made for measuring the heights of
mountains by means of the barometer, confirm the truth of these
deductions; and, even supposing them in some degree inaccurate, these
differences are so extremely small, that they may be reckoned as
nullities in chemical experiments. When this law of the compression of
elastic fluids is once well understood, it becomes easily applicable to
the corrections necessary in pneumato chemical experiments upon the
volume of gas, in relation to its pressure. These corrections are of two
kinds, the one relative to the variations of the barometer, and the
other for the column of water or mercury contained in the jars. I shall
endeavour to explain these by examples, beginning with the most simple
case.
Suppose that 100 cubical inches of oxygen gas are obtained at 10
(54.5 ) of the thermometer, and at 28 inches 6 lines of the barometer,
it is required to know what volume the 100 cubical inches of gas would
occupy, under the pressure of 28 inches[58], and what is the exact
weight of the 100 inches of oxygen gas? Let the unknown volume, or the
number of inches this gas would occupy at 28 inches of the barometer, be
expressed by x; and, since the volumes are in the inverse ratio of
their superincumbent weights, we have the following statement: 100
cubical inches is to x inversely as 28.5 inches of pressure is to 28.0
inches; or directly 28 : 28.5 :: 100 : x = 101.786--cubical inches, at
225