nd putting the plant into one of them, left
the other in the same exposure, contained, also, in a glass vessel
immersed in water, but without any plant; and never failed to find, that
a candle would burn in the former, but not in the latter.

I generally found that five or six days were sufficient to restore this
air, when the plant was in its vigour; whereas I have kept this kind of
air in glass vessels, immersed in water many months, without being able
to perceive that the least alteration had been made in it. I have also
tried a great variety of experiments upon it, as by condensing,
rarefying, exposing to the light and heat, &c. and throwing into it the
effluvia of many different substances, but without any effect.

Experiments made in the year 1772, abundantly confirmed my conclusion
concerning the restoration of air, in which candles had burned out by
plants growing in it. The first of these experiments was made in the
month of May; and they were frequently repeated in that and the two
following months, without a single failure.

For this purpose I used the flames of different substances, though I
generally used wax or tallow candles. On the 24th of June the experiment
succeeded perfectly well with air in which spirit of wine had burned
out, and on the 27th of the same month it succeeded equally well with
air in which brimstone matches had burned out, an effect of which I had
despaired the preceding year.

This restoration of air, I found, depended upon the _vegetating state_
of the plant; for though I kept a great number of the fresh leaves of
mint in a small quantity of air in which candles had burned out, and
changed them frequently, for a long space of time, I could perceive no
melioration in the state of the air.

This remarkable effect does not depend upon any thing peculiar to
_mint_, which was the plant that I always made use of till July 1772;
for on the 16th of that month, I found a quantity of this kind of air to
be perfectly restored by sprigs of _balm_, which had grown in it from
the 7th of the same month.

That this restoration of air was not owing to any _aromatic effluvia_ of
these two plants, not only appeared by the _essential oil of mint_
having no sensible effect of this kind; but from the equally complete
restoration of this vitiated air by the plant called _groundsel_, which
is usually ranked among the weeds, and has an offensive smell. This was
the result of an experiment made the 16th