has seemed to become even more deadly, so as to require
more time to restore it, by the methods which will be explained
hereafter, than air which has been lately made noxious. I have even
spent several hours in pouring this air from one glass vessel into
another, in water, sometimes as cold, and sometimes as warm, as my hands
could bear it, and have sometimes also wiped the vessels many times,
during the course of the experiment, in order to take off that part of
the noxious matter, which might adhere to the glass vessels, and which
evidently gave them an offensive smell; but all these methods were
generally without any sensible effect. The _motion_, also, which the air
received in these circumstances, it is very evident, was of no use for
this purpose. I had not then thought of the simple, but most effectual
method of agitating air in water, by putting it into a tall jar and
shaking it with my hand.

This kind of air is not restored by being exposed to the _light_, or by
any other influence to which it is exposed, when confined in a thin
phial, in the open air, for some months.

Among other experiments, I tried a great variety of different
_effluvia_, which are continually exhaling into the air, especially of
those substances which are known to resist putrefaction; but I could not
by these means effect any melioration of the noxious quality of this
kind of air.

Having read, in the memoirs of the Imperial Society, of a plague not
affecting a particular village, in which there was a large sulphur-work,
I immediately fumigated a quantity of this kind of air; or (which will
hereafter appear to be the very same thing) air tainted with
putrefaction, with the fumes of burning brimstone, but without any
effect.

I once imagined, that the _nitrous acid_ in the air might be the general
restorative which I was in quest of; and the conjecture was favoured, by
finding that candles would burn in air extracted from saltpetre. I
therefore spent a good deal of time in attempting, by a burning glass,
and other means, to impregnate this noxious air, with some effluvium of
saltpetre, and, with the same view, introduced into it the fumes of the
smoaking spirit of nitre; but both these methods were altogether
ineffectual.

In order to try the effect of _heat_, I put a quantity of air, in which
mice had died, into a bladder, tied to the end of the stem of a
tobacco-pipe, at the other end of which was another bladder, out of
which the