any judgment from it; and in this cafe the fymptom of difficult refpiration feemed to begin earlier than it would have done in common air.

Since the plants that I made ufe of manifeftly grow and thrive in putrid air; fince putrid matter is well known to afford proper nourishment for the roots of plants; and fince 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 fome 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 fome experiments of this kind did not answer fo well as they had done before, and I had inftances of the relapfing of this restored air to its former noxious ftate, I therefore fufpended my judgment concerning the efficacy of plants to restore this kind of noxious air, till I fhould have an opportunity of repeating my experiments, and giving more attention to them. Accordingly I refumed the experiments in the fummer 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 fome importance, and the fubfequent variation in the state of this

kind of air is a little remarkable, I thing it neceffary to relate fome of the facts pretty circumftantially.

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 fame exposure; and on the 27th of the fame month, and not before, I made a trial of them, by introducing a mouse into a glafs veffel, containing 2 ounce meafures filled with each kind of air; and I noted the following facts.

When the veffel was filled with the air in which the mint had grown, a very large mouse lived five minutes in it, before it began to fhew any fign of uneafinefs. I then took it out, and found it to be as ftrong and vigorous as when it was first put in; whereas in that air which had been kept in the phial only, without a plant growing in it, a younger moufe continued not longer than two or three feconds, and was taken out quite dead. It never breathed after, and was immediately motionlefs. After half an hour, in which time the larger mouse (which I had kept alive, that the experiment might be made on both the kinds of air with the very fame animal) would have been fufficiently

ciently recruited, fuppofing it to have received. any injury by the former experiment, was put into the fame veffel of air; but though it was withdrawn again, after being in it hardly one fecond, it was recovered with difficulty, not being able to ftir from the place for near a minute. After two days, I put the fame mouse into an equal quantity of common air, and obferved that it continued feven minutes without any fign of uneafiness; and being very uneasy after three minutes longer, I took it out. Upon the whole, I concluded that the reftored air wanted about one fourth of being as wholesome as common air. The fame thing also appeared when I applied the test of nitrous air.

In the seven days, in which the mint was growing in this jar of noxious air, three old fhoots had extended themfelves about three inches, and several new ones had made their appearance in the fame time. Dr. Franklin and Sir John Pringle happened to be with me, when the plant had been three or four days in this ftate, and took notice of its vigorous vegetation, and remarkably healthy appearance in that confinement.

On the 30th of the fame month, a mouse lived fourteen minutes, breathing naturally all the time, and without appearing to be much

uneasy,

uneafy, till the last two minutes, in the vessel containing two ounce measures and a half of air which had been rendered noxious by mice breathing in it almost a year before, and which I had found to be moft highly noxious on the 19th of this month, a plant having grown in it, but not exceedingly well, thefe eleven days; on which account I had deferred making the trial fo long. The reftored air was affected by a mixture of nitrous air, almost as much as common air.

As this putrid air was thus easily restored to a confiderable degree of fitnefs for refpiration, by plants growing in it, I was in hopes that by the fame means it might in time be fo much. more perfectly restored, that a candle would burn in it; and for this purpose I kept plants growing in the jars which contained this air till the middle of Auguft following, but did not take fufficient care to pull out all the old and rotten leaves. The plants, however, had grown, and looked fo well upon the whole, that I had no doubt but that the air muft conftantly have been in a mending ftate; when I was exceedingly furprized to find, on the 24th of that month, that though the air in one of the jars had not grown worse, it was no better; and that the air in the other jar was fo much worse than it had been, that a moufe would have died in it in a

few

few feconds.

It alfo made no effervefcence with

nitrous air, as it had done before.

Sufpecting that the fame plant might be capable of restoring putrid air to a certain degree only, or that plants might have a contrary tendency in fome ftages of their growth, I withdrew the old plant, and put a fresh one in its place; and found that, after feven days, the air was restored to its former wholesome state. This fact I confider as a very remarkable one, and well deferving of a farther investigation, as it may throw more light upon the principles of vegetation. It is not, however, a fingle fact; for I had feveral inftances of the fame kind in the preceding year; but it seemed so very extraordinary, that air fhould grow worse by the continuance of the fame treatment by which it had grown better, that, whenever I obferved it, I concluded that I had not taken fufficient care to fatisfy myself of its previous restoration.

That plants are capable of perfectly restoring air injured by refpiration, may, I think, be inferred with certainty from the perfect restora tion, by this means, of air which had paffed through my lungs, fo that a candle would burn in it again, though it had extinguished flame before, and a part of the fame original quantity

of