ccess.

Once, I let a mouse live and die in a quantity of air which had been
noxious, but which had been restored by this process, and it lived
nearly as long as I conjectured it might have done in an equal quantity
of fresh air; but this is so exceedingly various, that it is not easy to
form any judgment from it; and in this case the symptom of _difficult
respiration_ seemed to begin earlier than it would have done in common
air.

Since the plants that I made use of manifestly grow and thrive in putrid
air; since putrid matter is well known to afford proper nourishment for
the roots of plants; and since it is likewise certain that they receive
nourishment by their leaves as well as by their roots, it seems to be
exceedingly probable, that the putrid effluvium is in some measure
extracted from the air, by means of the leaves of plants, and therefore
that they render the remainder more fit for respiration.

Towards the end of the year some experiments of this kind did not answer
so well as they had done before, and I had instances of the relapsing of
this restored air to its former noxious state. I therefore suspended my
judgment concerning the efficacy of plants to restore this kind of
noxious air, till I should have an opportunity of repeating my
experiments, and giving more attention to them. Accordingly I resumed
the experiments in the summer of the year 1772, when I presently had the
most indisputable proof of the restoration of putrid air by vegetation;
and as the fact is of some importance, and the subsequent variation in
the state of this kind of air is a little remarkable, I think it
necessary to relate some of the facts pretty circumstantially.

The air, on which I made the first experiments, was rendered exceedingly
noxious by mice dying in it on the 20th of June. Into a jar nearly
filled with one part of this air, I put a sprig of mint, while I kept
another part of it in a phial, in the same exposure; and on the 27th of
the same month, and not before, I made a trial of them, by introducing a
mouse into a glass vessel, containing 2-1/2 ounce measures filled with
each kind of air; and I noted the following facts.

When the vessel was filled with the air in which the mint had grown, a
very large mouse lived five minutes in it, before it began to shew any
sign of uneasiness. I then took it out, and found it to be as strong and
vigorous as when it was first put in; whereas in that air which had been
kept in t