_Of AIR infected with the FUMES of BURNING CHARCOAL._
Air infected with the fumes of burning charcoal is well known to be
noxious; and the Honourable Mr. Cavendish favoured me with an account of
some experiments of his, in which a quantity of common air was reduced
from 180 to 162 ounce measures, by passing through a red-hot iron tube
filled with the dust of charcoal. This diminution he ascribed to such a
_destruction_ of common air as Dr. Hales imagined to be the consequence
of burning. Mr. Cavendish also observed, that there had been a
generation of fixed air in this process, but that it was absorbed by
sope leys. This experiment I also repeated, with a small variation of
circumstances, and with nearly the same result.
Afterwards, I endeavoured to ascertain, by what appears to me to be an
easier and more certain method, in what manner air is affected with the
fumes of charcoal, viz. by suspending bits of charcoal within glass
vessels, filled to a certain height with water, and standing inverted
in another vessel of water, while I threw the focus of a burning mirror,
or lens, upon them. In this manner I diminished a given quantity of air
one fifth, which is nearly in the same proportion with other diminutions
of air.
If, instead of pure water, I used _lime-water_ in this process, it never
failed to become turbid by the precipitation of the lime, which could
only be occasioned by fixed air, either discharged from the charcoal, or
deposited by the common air. At first I concluded that it came from the
charcoal; but considering that it is not probable that fixed air,
confined in any substance, can bear so great a degree of heat as is
necessary to make charcoal, without being wholly expelled; and that in
other diminutions of common air, by phlogiston only, there appears to be
a deposition of fixed air, I have now no doubt but that, in this case
also, it is supplied from the same source.
This opinion is the more probable, from there being the same
precipitation of lime, in this process, with whatever degree of heat the
charcoal had been made. If, however, the charcoal had not been made with
a very considerable degree of heat, there never failed to be a permanent
addition of inflammable air produced; which agrees with what I observed
before, that, in converting dry wood into charcoal, the greatest part
is changed into inflammable air.
I have sometimes found, that charcoal which was made with the most
intense heat of a smith's fire, which vitrified part of a common
crucible in which the charcoal was confined, and which had been
continued above half an hour, did not diminish the air in which the
focus of a burning mirror was thrown upon it; a quantity of inflammable
air equal to the diminution of the common air being generated in the
process: whereas, at other times, I have not perceived that there was
any generation of inflammable air, but a simple diminution of common
air, when the charcoal had been made with a much less degree of heat.
This subject deserves to be farther investigated.
To make the preceding experiment with still more accuracy, I repeated it
in quicksilver; when I perceived that there was a small increase of the
quantity of air, probably from a generation of inflammable air. Thus it
stood without any alteration a whole night, and part of the following
day; when lime-water, being admitted to it, it presently became turbid,
and, after some time, the whole quantity of air, which was about four
ounce measures, was diminished one fifth, as before. In this case, I
carefully weighed the piece of charcoal, which was exactly two grains,
and could not find that it was sensibly diminished in weight by the
operation.
Air thus diminished by the fumes of burning charcoal not only
extinguishes flame, but is in the highest degree noxious to animals; it
makes no effervescence with nitrous air, and is incapable of being
diminished any farther by the fumes of more charcoal, by a mixture of
iron filings and brimstone, or by any other cause of the diminution of
air that I am acquainted with.
This observation, which respects all other kinds of diminished air,
proves that Dr. Hales was mistaken in his notion of the _absorption_ of
air in those circumstances in which he observed it. For he supposed that
the remainder was, in all cases, of the same nature with that which had
been absorbed, and that the operation of the same cause would not have
failed to produce a farther diminution; whereas all my observations shew
that air, which has once been fully diminished by any cause whatever, is
not only incapable of any farther diminution, either from the same or
from any other cause, but that it has likewise acquired _new
properties_, most remarkably different from those which it had before,
and that they are, in a great measure, the same in all the cases. These
circumstances give reason to suspect, that the cause of diminution is,
in reality, the same in all the cases. What this cause is, may, perhaps,
appear in the next course of observations.