he 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
in the water apparatus. It is likewise necessary to register the
difference between the surface of the mercury in the cistern and that in
the jar, and the height of the barometer and thermometer, at the end of
each experiment.

When all the gas or gasses absorbable by water and potash are absorbed,
water is admitted into the jar to displace the mercury; and, as is
described in the preceding section, the mercury in the cistern is to be
covered by one or two inches of water. After this, the jar is to be
transported by means of the flat dish BC, Pl. V. Fig. 9. into the water
apparatus; and the quantity of gas remaining is to be ascertained by
changing it into a graduated jar. After this, small trials of it are to
be made by experiments in little jars, to ascertain nearly the nature of
the gas in question. For instance, into a small jar full of the gas,
Fig. 8. Pl. V. a lighted taper is introduced; if the taper is not
immediately extinguished, we conclude the gas to contain oxygen gas;
and, in proportion to the brightness of the flame, we may judge if it
contain less or more oxygen gas than atmospheric air contains. If, on
the contrary, the taper be instantly extinguished, we have strong reason
to presume that the residuum is chiefly composed of azotic gas. If, upon
the approach of the taper, the gas takes fire and burns quietly at the
surface with a white flame, we conclude it to be pure hydrogen gas; if
this flame is blue, we judge it consists of carbonated hydrogen gas;
and, if it takes fire with a sudden deflagration, that it is a mixture