What effect this circumstance may have had upon the air I
have not considered.

November 6, 1772, I had the curiosity to examine the state of a quantity
of this air which had been extracted from saltpetre above a year, and
which at first was perfectly wholesome; when, to my very great surprize,
I found that it was become, in the highest degree, noxious. It made no
effervescence with nitrous air, and a mouse died the moment it was put
into it. I had not, however, washed it in rain-water quite ten minutes
(and perhaps less time would have been sufficient) when I found, upon
trial, that it was restored to its former perfectly wholesome state. It
effervesced with nitrous air as much as the best common air ever does;
and even a candle burned in it very well, which I had never before
observed of any kind of noxious air meliorated by agitation in water.
This series of facts, relating to air extracted from nitre, appear to me
to be very extraordinary and important, and, in able hands, may lead to
considerable discoveries.

3. There are many substances which impregnate common air in a very
remarkable manner, but without making it noxious to animals. Among other
things I tried volatile alkaline salts, and camphor; the latter of which
I melted with a burning-glass, in air inclosed in a phial. The mouse,
which was put into this air, sneezed and coughed very much, especially
after it was taken out; but it presently recovered, and did not appear
to have been sensibly injured.

4. Having made several experiments with a mixture of iron filings and
brimstone, kneaded to a paste with water, I had the curiosity to try
what would be the effect of substituting _brass dust_ in the place of
the iron filings. The result was, that when this mixture had stood about
three weeks, in a given quantity of air, it had turned black, but was
not increased in bulk. The air also was neither sensibly increased nor
decreased, but the nature of it was changed; for it extinguished flame,
it would have killed a mouse presently, and was not restored by fixed
air, which had been mixed with it several days.

5. I have frequently mentioned my having, at one time, exposed equal
quantities of different kinds of air in jars standing in boiled water.
_Common air_ in this experiment was diminished four sevenths, and the
remainder extinguished flame. This experiment demonstrates that water
does not absorb air equally, but that it decomposes it, taking one part,
and