l, though a candle will not burn in the
residuum of the purest fixed air that I can make; and I once made a very
large quantity for the sole purpose of this experiment. This, therefore,
seems to be one instance of the generation of genuine common air, though
vitiated in some degree. It is also another proof of the residuum of
fixed air being, in part at least, common air, that it becomes turbid,
and is diminished by the mixture of nitrous air, as will be explained
hereafter.

That fixed air only wants some addition to make it permanent, and
immiscible with water if not in all respects, common air, I have been
led to conclude, from several attempts which I once made to mix it with
air in which a quantity of iron filings and brimstone, made into a paste
with water, had stood; for, in several mixtures of this kind, I imagined
that not much more than half of the fixed air could be imbibed by water;
but, not being able to repeat the experiment, I conclude that I either
deceived myself in it, or that I overlooked some circumstance on which
the success of it depended.

These experiments, however, whether they were fallacious or otherwise,
induced me to try whether any alteration would be made in the
constitution of fixed air, by this mixture of iron filings and
brimstone. I therefore put a mixture of this kind into a quantity of as
pure fixed air as I could make, and confined the whole in quicksilver,
lest the water should absorb it before the effects of the mixture could
take place. The consequence was, that the fixed air was diminished, and
the quicksilver rose in the vessel, till about the fifth part was
occupied by it; and, as near as I could judge, the process went on, in
all respects, as if the air in the inside had been common air.

What is most remarkable, in the result of this experiment, is, that the
fixed air, into which this mixture had been put, and which had been in
part diminished by it, was in part also rendered insoluble in water by
this means. I made this experiment four times, with the greatest care,
and observed, that in two of them about one sixth, and in the other two
about one fourteenth, of the original quantity, was such as could not be
absorbed by water, but continued permanently elastic. Lest I should have
made any mistake with respect to the purity of the fixed air, the last
time that I made the experiment, I set part of the fixed air, which I
made use of, in a separate vessel, and found it to be