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 the strong tendency these two
gasses have, especially the former, to combine with, or be absorbed by
water. If the water only produces a slight absorption of gas hardly
equal to its own bulk, we conclude, that the mixture neither contains
muriatic acid, sulphuric acid, or ammoniacal gas, but that it contains
carbonic acid gas, of which water only absorbs about its own bulk. To
ascertain this conjecture, introduce some solution of caustic alkali,
and the carbonic acid gas will be gradually absorbed in the course of a
few hours; it combines with the caustic alkali or potash, and the
remaining gas is left almost perfectly free from any sensible residuum
of carbonic acid gas.
After each experiment of this kind, we must carefully mark the height at
which the mercury stands within the jar, by slips of paper pasted on,
and varnished over when dry, that they may not be washed off when placed
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