utual adhesion of the
two constituent parts of the atmosphere for each other, and the elective
attraction which unites the base of vital air with caloric; in
consequence of these, when the calcination ends, or is at least carried
as far as is possible, in a determinate quantity of atmospheric air,
there still remains a portion of respirable air united to the mephitis,
which the mercury cannot separate. I shall afterwards show, that, at
least in our climate, the atmospheric air is composed of respirable and
mephitic airs, in the proportion of 27 and 73; and I shall then discuss
the causes of the uncertainty which still exists with respect to the
exactness of that proportion.

Since, during the calcination of mercury, air is decomposed, and the
base of its respirable part is fixed and combined with the mercury, it
follows, from the principles already established, that caloric and light
must be disengaged during the process: But the two following causes
prevent us from being sensible of this taking place: As the calcination
lasts during several days, the disengagement of caloric and light,
spread out in a considerable space of time, becomes extremely small for
each particular moment of that time, so as not to be perceptible; and,
in the next place, the operation being carried on by means of fire in a
furnace, the heat produced by the calcination itself becomes confounded
with that proceeding from the furnace. I might add the respirable part
of the air, or rather its base, in entering into combination with the
mercury, does not part with all the caloric which it contained, but
still retains a part of it after forming the new compound; but the
discussion of this point, and its proofs from experiment, do not belong
to this part of our subject.

It is, however, easy to render this disengagement of caloric and light
evident to the senses, by causing the decomposition of air to take place
in a more rapid manner. And for this purpose, iron is excellently
adapted, as it possesses a much stronger affinity for the base of
respirable air than mercury. The elegant experiment of Mr Ingenhouz,
upon the combustion of iron, is well known. Take a piece of fine iron
wire, twisted into a spiral, (BC, Plate IV. Fig. 17.) fix one of its
extremities B into the cork A, adapted to the neck of the bottle DEFG,
and fix to the other extremity of the wire C, a small morsel of tinder.
Matters being thus prepared, fill the bottle DEFG with air deprive