, and, I
believe, indelibly tinged with green; and if the water be put into
another vessel, it presently deposits a considerable quantity of matter,
which when dry appears to be the earth or ochre of iron; from which it
is evident, that the acid of the nitrous air dissolves the iron; while
the phlogiston, being set loose, diminishes nitrous air, as in the
process of the iron filings and brimstone.

Upon this hint, instead of using _iron_, I introduced a pot of _liver of
sulphur_ into a jar of nitrous air, and presently found, that what I had
before done by means of iron in six weeks, or two months, I could do by
liver of sulphur (in consequence, no doubt, of its giving its phlogiston
more freely) in less than twenty-four hours, especially when the process
was kept warm.

It is remarkable, however, that if the process with liver of sulphur be
suffered to proceed, the nitrous air will be diminished much farther.
At one time not more than one twentieth of the original quantity
remained, and how much farther it right have been diminished, I cannot
tell. In this great diminution, it does not admit a candle to burn in it
at all; and I generally found this to be the case whenever the
diminution had proceeded beyond three fourths of the original
quantity[13].

It is something remarkable, that though the diminution of nitrous air by
iron filings and brimstone very much resembles the diminution of it by
iron only, or by liver of sulphur, yet the iron filings and brimstone
never bring it to such a state as that a candle will burn in it; and
also that, after this process, it is never capable of diminishing common
air. But when it is considered that these properties are destroyed by
agitation in water, this difference in the result of processes, in other
respects similar, will appear less extraordinary; and they agree in
this, that long agitation in water makes both these kinds of nitrous air
equally fit for respiration, being equally diminished by fresh nitrous
air. It is possible that there would have been a more exact agreement
in the result of these processes, if they had been made in equal degrees
of _heat_; but the process with iron was made in the usual temperature
of the atmosphere, and that with liver of sulphur generally near a fire.

It may clearly, I think, be inferred from these experiments, that all
the difference between fresh nitrous air, that state of it in which it
is partially inflammable, or wholly so, that i