of carbonic acid;
if the lime-water is not rendered turbid, we may be certain that no
sensible quantity of that acid remains in the air.

The rest of the air which has served for combustion, and which chiefly
consists of azotic gas, though still mixed with a considerable portion
of oxygen gas, which has escaped unchanged from the combustion, is
carried through a third tube 28, 29, of deliquescent salts, to deprive
it of any moisture it may have acquired in the bottles of alkaline
solution and lime-water, and from thence by the tube 29, 30, into a
gazometer, where its quantity is ascertained. Small essays are then
taken from it, which are exposed to a solution of sulphuret of potash,
to ascertain the proportions of oxygen and azotic gas it contains.

In the combustion of oils the wick becomes charred at last, and
obstructs the rise of the oil; besides, if we raise the wick above a
certain height, more oil rises through its capillary tubes than the
stream of air is capable of consuming, and smoke is produced. Hence it
is necessary to be able to lengthen or shorten the wick without opening
the apparatus; this is accomplished by means of the rod 31, 32, 33, 34,
which passes through a leather-box, and is connected with the support of
the wick; and that the motion of this rod, and consequently of the wick,
may be regulated with the utmost smoothness and facility; it is moved at
pleasure by a pinnion which plays in a toothed rack. The rod, with its
appendages, are represented Pl. XII. Fig. 3. It appeared to me, that the
combustion would be assisted by surrounding the flame of the lamp with a
small glass jar open at both ends, as represented in its place in Pl.
XI.

I shall not enter into a more detailed description of the construction
of this apparatus, which is still capable of being altered and modified
in many respects, but shall only add, that when it is to be used in
experiment, the lamp and reservoir with the contained oil must be
accurately weighed, after which it is placed as before directed, and
lighted; having then formed the connection between the air in the
gazometer and the lamp, the external jar A, Pl. XI. is fixed over all,
and secured by means of the board BC and two rods of iron which connect
this board with the lid, and are screwed to it. A small quantity of oil
is burnt while the jar is adjusting to the lid, and the product of that
combustion is lost; there is likewise a small portion of air from the
gaz