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
stoo