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fat?, and even when on the decay, seems to possess this quality in an equal degree.

The plants on which the Author made his experiments were confined in jars about 9 or 10 inches long, and from inch to 2in diameter. One of them which had been confined about a fortnight had consumed three-fourths of the common air included with it. Another, in the space of a month, had absorbed feven-eighths of the common air to which it was expofed. The plant was quite yellow and dead; but though it had been so for some time, it had still continued to absorb air : so that the remaining air was afterwards reduced to one-tenth of the original quantity. The inflammable air, to which another plant had been exposed, was reduced to one-seventh of the whole. An accident prevented the Author from examining the quality of the reiduum; which, however, when only one-third part of the air had been consumed, was found, to all appearance, to be as inflammable as ever. Another plant confined in nitrous air, became yellow, and died; and had then consumed one-third of the air.

The mention of the Author's experiments relative to vegetation naturally leads us, as well as the fingularity and importance of the subject itself, to take particular notice of a most re. markable discovery, to which those experiments gave occasion. This is nothing less than the spontaneous emission of the purest dephlogisticated air from common well water, in certain circumstances. We cannot more properly commence our account of this discovery, than by prefixing to it the Author's own edifying exordium:

Few persons, says he, I believe, have met with so much unexpected good success as myself, in the course of my philofophical pursuits. My narrative will fhew that the first hints, at least, of almost every thing that I have discovered, of much importance, have occurred to me in this manner. In looking for one thing, I have generally found another, and sometimes a thing of much more value than that which I was in quest of. But none of these unexpected discoveries appear to me to have been so extraordinary as that which I am about to relate; and it may serve to admonish all persons who are engaged in similar purfuits, not to overlook any circumstance relating to an experiment; but to keep their eyes open to every new appearance, and to give due attention to it, how inconfiderable foever it may seem.'

The Author having observed bubbles of air that seemed to issue spontaneoully from the roots of several plants growing in water, was first led to suspect that this air had percolated through the plant; which had probably seized upon and retained the phlogiston of the air which it had imbibed, and was now emitting the absorbed air, deprived of that principle, and

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consequently in a state of greater purity. In fact, on collecting and examining some of this air, he found it so pure, that one measure of it, and one of nitrous air, occupied the space of only one measure.

The Author, however, asterwards found that the plants had no share in this production of air : for on taking them out of the vials, the remaining water continued to emit air as plentifully as when the plants were growing in it. He observed too that the vials and other vessels, in which this pure air bad been emitted from the water, had their bottoms and fides more or Jcfs covered with a green matter, from which the air evidently seemed to proceed. It appeared to him that this green matter could neither be of an animal or vegetable nature; but that it was a substance sui generis, and that neither the external air or animalcules could have any thing to do in the formation of it: for it was produced in vials clolely corked, and in the middle yellel of Mr. Parker's apparatus *.

On filling a number of vials with different kinds of water, as river water, rain water, pump water which contained a considerable quantity of fixed air, and water artificially impregnated with fixed air; he found, after they had stood fome time, that no green matter was deposited in any of them, except in those which contained the pump water. He afterwards however found that much of this green matter, as well as of the pure air that rises from it, was produced from the water that had been strongly impregnated with fixed air.

Of the purity of the air emitted from pump water, under these circumstances, the Reader may form a general estimate from one of the Author's experiments; in which he used a tall conical receiver, about 18 inches bigh, and 5 wide at the bottom. This had been employed in former experiments, and was coated with this green matter, which in time pasles gradually to a kind of yellow or rather orange colour. On the

7th of September, 1778, the Author had taken all the air from this receiver, and had re-filled it: on the 14th of Odober foliowing, he took from it about 9 ounce measures of air-the purest he had ever got in this method : for one measure of it,

* From fome experiments that we have made on this subject, we see reason to infer that this green matter will not be deposited in vials closely corked, unless some air is included; and that one quantity of the deposit bears fone proportion to that of the air left in the vial. In open vials completely filled, and inverted in water, the water contained in the vials has an iniermediate communication with the atmosphere; and the proces goes on, as is described above: bat if that cu...munication be tłopped, from the beginning, by inverting the vials in quick lilver (a fuid impermeable to ais no green mais ter, or pure air, is produced.

and two of nitrous air, occupied the space of only 0.44 of a measure—which is quite as pure as dephlogisticated air is at a medium.'

We have hitherto kept out of fight a most remarkable cir-' cumstance attending the production of this green matter, and the emiffion of this pure air; and which the Author leems to have observed too often to justify a fuspicion that he can have been mistaken with respect to the fact. This is, the instrumentality of the Sun's Light, qra light, and independent of his mere heat, in the production of the green matter, and the consequent emillion of dephlogisticated air.

After making some observations on this green matter, and declaring that he never found it except in circumstances where the water had been exposed to the light; the Author goes on to say, that he had not proceeded far in this inquiry till it was too late in the last summer to make use of Sunshine ; though he was affiduous enough to avail himself of the state of the weather, such as it was. He then draws these general conclusions from the whole that he had hitherto been able to observe:

"That whatever air is naturally contained in water, or in substances dissolved in water, as calcareous matter, &c. becomes, after long standing, but especially when exposed to the fun, depurated, ro as at length to become absolutely dephlogisticated; and that this air being continually emitted by all water, exposed to the action of the sun's rays, must contribute to the melioration of the state of the atmosphere in general.

• When I have kept water a long time in the shade, it has not generally yielded any other kind of air than it would have yielded at the first; and though, when it has been kept in an open vefsel the air has been better, it has never been so good as the air in the fame kind of water that has been exposed a much less time to the sun.

'No degree of warmth will supply the place of the sun's light; and though, when the water is once prepared by exposure to the fun, warmth will suffice to expel that air; yet, in this cale, the air has never been so pure, as that which has been yielded spontaneously, without additional heat. The reason of this may be that, besides the air already depurated, and on that account ready to quit its union with the water, heat expels, together with it, the air that was phlogisticated, and held in a clofer union with the water ; which air, the action of iight, whatever that be, would in time have depurated allo.

The quantity of air, yielded by water spontaneous, far exceeds that which can be expelled from it by heat. In id, I have frequently observed, that whatever circumstance depraves air, lefsens also the quantity of it; since it requires a large quantity of dephlogisticated air to make a small quantity of

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phlogisticated air, or even of common air, which is air partially phlogisticated.

• If the water naturally contains fixed air, yet, in consequence of this exposure to the light, it is all disipated, and the natural residuum of it becomes pure dephlogisticated air. For no fixed air at all, but only the purest dephlogisticated air, is at length procured from it; and water impregnated with fixed air yields, after this exposure, the greatest quantity of dephlogisticated air.'

In confirmation of these conclusions, the Author recites such of his experiments as appear sufficient to establish every thing that is of importance in them. Recommending the perusal of these to the philosophical reader, we shall transcribe the Au. thor's latest observation relative to this subject, inserted at the end of this yolume :

On the Effect of Light on IVater. My observation that Light disposes water, containing calcareous and other substances, to make a deposit of a greenish or brownish matter, and then to yield dephlogisticated air, seems to be confirmed by the following experiment.

On the 19th of February, 1779, I placed two jars of pump water, each containing about 170 ounces, in the same south window; one of them nearly covered from the sun with brown paper, and the other quite uncovered. In about ten days, the water in the uncovered jar had yielded about four ounce measures of air, and the covered jar only a few bubbles. Taking a journey I could make no farther observations on these jars till my return; but on the 2d of April I found that the uncovered jar had yielded 10 ounce measures of air, so pure that one measure of it, and one of nitrous air, occupied the space of .84 measures ; whereas the covered jar had very little more than one ounce measure, and with this the measures of the test were 1.55 measures; i. e. by no means fo pure as the former. Also the uncovered jar had a sediment larger than the other in about the same proportion, viz. of 10 to 1. Oil of vitriol expelled from this sediment a very great quantity of fixed air. N. B. The lowest part of the jar was not covered with the paper, left being moistened with the water, in the dish in which the jar itood inverted, it Mould imbibe the water, and caule it to evaporate too foon *.'

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It may not be amiss, on an observation fo very fingular and curious, to add our testimony, so far as it goes, to that of the Author. Two quart glass retoris, and two 12 ounce vials, filled, at the same time, with ine same well water (which naturally contained a mo. derare quantity of fixed air), were exposed to the sun, in July lait, inverted in a balon of the same well water, itanding on a cable place

We must not omit to take notice of an objection, which Dr. Priestley here anticipates and answers, that may be made to the results which he deduced from some of his former experiments on air. From these he inferred that vegetation was one of the means employed by nature in purifying the atmosphere, by depriving it of that noxious principle with which it is charged by animal respiration, combustion of inflammable substances, putrefaction, &c. From the foregoing experiments a suspicion may arise that, as his trials were made with plants growing in water, the observed melioration of the included air might be principally effected by the emillion of dephlogisticated air from the water in which they grew. We shall give what he says on this subject :

• It will probably be imagined that the result of the experiments recited in this section throws some uncertainty on the re. fult of former experiments, from which I have concluded that air is meliorated by the vegetation of plants ; especially as the water by which they were confined was exposed to the open air, and the fun, in a garden. To this I can only say, that I was not then aware of the effect of these circumstances, and that I have represented the naked facts, as I observed them; and having no great attachment to any particular hypothesis, I am very willing that my reader fhould draw his own conclusions for himself.

· I must inform him, however, that my experiments at Leeds were made in a northeast window of the house, where the infuence of the light on the water could not be very considerable; that some of the processes were completed in two days, and generally in about a week; and that the water within the jars was lo small, in proportion to the quantity of air, that I do not at present imagine that the mclioration of the air at that time could have been owing to it. Besides, as I have observed, I frequently kept air in the same exposure, with respect to water, light, and every other circumstance that occurred to me to attend to, and the same space of time, but without any plant vegetating in it, when there was no sensible melioration of it.'

In an Appendix to this volume are contained several papers communicated to the Author by his Correspondents. No i and 2, contain observations, by Sir William Lee, on the efficacy of

in the middle of a garden. One retort, and one vial, were covered with black silk; which, at the same time that it excluded the sun's lighe, communicated more heat to the water than was transmitted to the other retort and vial. After they had stood a month, a large quantity of pure air, amounting to about one-ninth part of the bulk of the water in each, had been separated in the naked retort and vial : whereas on taking off the filk from the other two vesels, only a bubble of air, not exceeding the size of a small nut, was found to have been separated from the water contained in them.

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