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Left it might be fufpected that, in confequence of the break ing of the retort, this dephlogisticated air might poffibly have been imbibed from the atmosphere; the Author repeated the experiment with a view to this circumftance; and accordingly kept the neck of the retort conftantly immersed either in quickfilver or water, from the time of putting the materials together, to the end of the process; which neverthelefs was interrupted by the melting of the retort. Before this accident, however, 20 ounce meafures of dephlogisticated air had been expelled, which had evidently been produced from the materials, without the help of any thing that could have been communicated to them from the atmosphere.

From the preceding as well as many other experiments of a fimilar kind, it appears that the propofition relating to the conftitution of atmospherical air, which had been deduced from the Author's former experiments, will require fome modification, or extenfion; and that we fhould now fay that the acid, confidered as a component principle of atmospherical air, is not neceffarily the nitrous acid, as fuch, or the fpirit of nitre of the fhops; but is, in fome cafes, the vitriolic: or that the latter, in the abovementioned procefles for procuring dephlogisticated air, is converted into the former; or, in fhort, that the effects are produced by fome acid, or fubftance, that bears an equal relation, or is common, to both.

We shall only add further, on this fubject, that the marine acid seems to differ effentially from the other two mineral acids, with refpect to the production of dephlogisticated air: for though Signor Landriani had informed the Author that he had procured this kind of air from corrofive fublimate, or a combination of the marine acid with mercury, Dr. Priestley has not yet been able to procure any, either from this fubftance, or fea falt; or from iron or quicklime diffolved in the marine acid, and exposed to a red heat.

From the various experiments refpe&ting Vegetation, contained in this volume, we fhall felect only one very remarkable obfervation relating to a particular plant called the Willow Plant, [the epilobium hirfutum of Linnæus] which was found by the Author to poffefs the fingular quality of abforbing a very confiderable quantity of common air, or any other kind of air to which it was expofed, in a glass jar ftanding inverted in water. In the common phlogistic processes, such as the calcination of metals, refpiration of animals, &c. the diminution of air has not been found to exceed one-fourth of the whole quantity. Indeed the Abbé Fontana has lately found that ignited charcoal has the property of abforbing a very great quantity of any kind of air to which it is expofed; but this plant, in a growing 6

ftate,

ftate, and even when on the decay, feems to poffefs 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 1 inch to 2 in diameter. One of them which had been confined about a fortnight had confumed three-fourths of the common air included with it. Another, in the fpace of a month, had abforbed feven-eighths of the common air to which it was expofed. The plant was quite yellow and dead; but though it had been fo for fome time, it had still continued to absorb air: fo that the re maining air was afterwards reduced to one-tenth of the original quantity. The inflammable air, to which another plant had been expofed, was reduced to one-seventh of the whole. An accident prevented the Author from examining the quality of the refiduum; which, however, when only one-third part of the air had been confumed, was found, to all appearance, to be as inflammable as ever. Another plant confined in nitrous air, became yellow, and died; and had then confumed 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 fubject itself, to take particular notice of a most remarkable discovery, to which thofe experiments gave occafion. This is nothing less than the fpontaneous emiffion of the pureft dephlogisticated air from common well water, in certain circumftances. We cannot more properly commence our account of this difcovery, than by prefixing to it the Author's own edifying exordium :

Few perfons, fays he, I believe, have met with fo much unexpected good fuccefs as myself, in the course of my philofophical purfuits. My narrative will fhew that the firft hints, at leaft, 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 fometimes a thing of much more value than that which I was in queft of. But none of these unexpected discoveries appear to me to have been fo extraordinary as that which I am about to relate; and it may ferve to admonish all perfons who are engaged in fimilar purfuits, not to overlook any circumstance relating to an experiment; but to keep their eyes open to every new appearance, and to give due at-· tention to it, how inconfiderable foever it may feem.'

The Author having obferved bubbles of air that seemed to iffue fpontaneoufly from the roots of feveral plants growing in water, was first led to fufpect that this air had percolated through the plant; which had probably feized upon and retained the phlogifton of the air. which it had imbibed, and was now emitting the abforbed air, deprived of that principle, and

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

The Author, however, afterwards found that the plants had no fhare 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 obferved too that the vials and other veffels, in which this pure air had been emitted from the water, had their bottoms and fides more or lefs covered with a green matter, from which the air evidently feemed to proceed. It appeared to him that this green matter could neither be of an animal or vegetable nature; but that it was a fubftance fui 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 clofely corked, and in the middle veffel 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 confiderable quantity of fixed air, and water artificially impregnated with fixed air; he found, after they had stood fome time, that no green matter was depofited 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 eftimate from one of the Author's experiments; in which he used a tall conical receiver, about 18 inches high, and 5 wide at the bottom. This had been employed in former experiments, and was coated with this green matter, which in time pafies gradually to a kind of yellow or rather orange colour. On the 17th of September, 1778, the Author had taken all the air from this receiver, and had re-filled it: on the 14th of October following, he took from it about 9 ounce meafures 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 fubject, we fee reafon to infer that this green matter will not be depofited in vials clofely corked, unlefs fome air is included; and that the quantity of the depofit bears fome 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 intermediate communication with the atmosphere; and the process goes on, as is defcribed above: but if that communication be stopped, from the beginning, by inverting the vials in quickfilver (a fluid impermeable to air), no green matter, or pure air, is produced,

and two of nitrous air, occupied the fpace 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 moft remarkable circumftance attending the production of this green matter, and the emiffion of this pure air; and which the Author feems to have obferved too often to justify a fufpicion that he can have been mistaken with refpect to the fact. This is, the inftrumentality of the Sun's Light, qua light, and independent of his. mere heat, in the production of the green matter, and the confequent emiffion of dephlogifticated air.

After making fome obfervations on this green matter, and declaring that he never found it except in circumftances where the water had been exposed to the light; the Author goes on to fay, that he had not proceeded far in this inquiry till it was too late in the last fummer to make ufe of Sunshine; though he was affiduous enough to avail himself of the ftate of the weather, fuch as it was. He then draws thefe general conclufions from the whole that he had hitherto been able to obferve:

That whatever air is naturally contained in water, or in fubftances diffolved in water, as calcareous matter, &c. becomes, after long ftanding, but especially when expofed to the fun, depurated, fo as at length to become abfolutely dephlogisticated; and that this air being continually emitted by all water, exposed to the action of the fun's rays, muit contribute to the melioration of the state of the atmosphere in general.

'When I have kept water a long time in the fhade, 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 vefel 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 lefs time to the fun.

No degree of warmth will supply the place of the fun's light; and though, when the water is once prepared by exposure to the fun, warmth will fuffice to expel that air; yet, in this cafe, the air has never been fo pure, as that which has been yielded fpontaneously, without additional heat. The reafon of this may be that, befides 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 phlogifticated, and held in a clofer union with the water; which air, the action of light, whatever that be, would in time have depurated alfo.

The quantity of air, yielded by water fpontaneously, far exceeds that which can be expelled from it by heat. Indeed, I have frequently obferved, that whatever circumftance depraves air, leffens alfo the quantity of it; fince it requires a large quantity of dephlogisticated air to make a fmall 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 confequence of this exposure to the light, it is all diffipated, and the natural refiduum of it becomes pure dephlogisticated air. For no fixed air at all, but only the pureft dephlogisticated air, is at length procured from it; and water impregnated with fixed air yields, after this expofure, the greatest quantity of dephlogisticated air.'

In confirmation of these conclufions, the Author recites fuch of his experiments as appear fufficient to establish every thing that is of importance in them. Recommending the perufal of thefe to the philofophical reader, we fhall tranfcribe the Author's latest observation relative to this subject, inferted at the end of this volume:

On the Effect of Light on Water.

My obfervation that Light difpofes water, containing calcareous and other fubftances, to make a depofit of a greenish or brownish matter, and then to yield dephlogifticated air, feems 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 fame fouth window; one of them nearly covered from the fun with brown paper, and the other quite uncovered. In about ten days, the water in the uncovered jar had yielded about four ounce meafures of air, and the covered jar only a few bubbles. Taking. a journey I could make no farther obfervations 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, fo pure that one measure of it, and one of nitrous air, occupied the fpace 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. Alfo the uncovered jar had a fediment larger than the other in about the fame proportion, viz. of 10 to 1. Oil of vitriol expelled from this fediment a very great quantity of fixed air. N. B. The lowest part of the jar was not covered with the paper, left being moiftened with the water, in the difh in which the jar flood inverted, it fhould imbibe the water, and cause it to evaporate too soon

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It may not be amifs, on an obfervation fo very fingular and curious, to add our teftimony, fo far as it goes, to that of the Author. Two quart glafs retorts, and two 12 ounce vials, filled, at the fame time, with the fame well water (which naturally contained a moderate quantity of fixed air), were expofed, to the fun, in July laft, inverted in a bafon of the fame well water, itanding on a table placed

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