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Left it might be suspected that, in consequence of the break. ing of the retort, this dephlogisticated air might possibly have been imbibed from the atmosphere; the Author repeated the experiment with a view to this circumstance ; and accordingly kept the neck of the retort constantly immersed either in quickfilver or water, from the time of putting the materials together, to the end of the process; which nevertheless was interrupted by the melting of the retort. Before this accident, however, 20 ounce measures 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 proposition relating to the conftitution of atmospherical air, which had been deduced from the Author's former experiments, will require some modification, or extension; and that we thould now say that the acid, confidered as a component principle of atmospherical air, is not neceffarily the nitrous acid, as such, or the spirit of nitre of the fhops'; but is, in some cases, the vitriolic : or that the latter, in the abovementioned processes for procuring dephlogisticated air, is converted into the former; or, in short, that the effects are produced by some acid, or substance, 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 essentially from the other two mineral acids, with respect to the production of dephlogisticated air : for though Signor Landriani had informed the Author that he had procured this kind of air from corrosive sublimate, or a combination of the marine acid with mercury, Dr. Priestley has not yet been able to procure any, cither from this substance, or fea falt; or from iron or quicklime dissolved in the marine acid, and exposed to a red heat.
From the various experiments respecting Vegetation, contained in this volume, we shall select only one very remarkable observation relating to a particular plant called the Willow Plant, (the epilobium hirsutum of Linnæus] which was found by the Author to possess the fingular quality of absorbing a very confiderable quantity of common air, or any other kind of air to which it was exposed, in a glass jar standing inverted in water. In the common phlogistic processes, such as the calcination of metals, respiration 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 absorbing a very great quantity of any kind of air to which it is exposed; but this plant, in a growing
Itate, 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 io inches long, and from 1 inch to 21 in 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 seven-eighths of the common air to which it was exposed. 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 red maining 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 residuum ; 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 remarkable discovery, to which those experiments gave occasion. This is nothing less than the spontaneous emission of the pureft 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 fo much unexpected good success as myself, in the course of my philofophical pursuits. My narrative will few that the first hints, ac 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 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 inconsiderable foever it may seem.'
The Author having observed bubbles of air that seemed to issue spontaneously 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
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, afterwards 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 plenti, fully as when the plants were growing in it. He observed too that the vials and other vessels, in which this pure air had been emitted from the water, had their bottoms and fides more or less 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 closely corked, and in the middle veslel 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 cons fiderable quantity of fixed air, and water artificially impregnated with fixed air; he found, after they had stood some 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 high, and 5 wide at the bottom. This had been employed in former experiments, and was coated with this green matter, which in time pafles 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 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 the quantity of the deposit bears some proportion to that of the air lest 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 described above : but if that communication be stopped, from the beginning, by inverting the vials in quicksilver (a fuid impermeable to air), no green masa ter, or purc 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 circumstance attending the production of this green matter, and the emission of this pure air; and which the Author seems to have observed too often to justify a suspicion that he can have been mistaken with respect to the fact. This is, the instrumentality of the Sun's Light, qua light, and independent of his . mere heat, in the production of the green matter, and the consequent emiffion 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 Sunfine; though he was affiduous enough to avail himself of the ftate 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 diffolved in water, as calcareous matter, &c. becomes, after long standing, but especially when exposed to the fun, depurated, so as at length to become absolutely dephlogisticated ; and that this air being continually emitted by all water, exposed to the action of the fun'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 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 vessel the air has been better, it has never been so good as the air in the same kind of water that has been exposed a mucha less time to the fun.
• 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 sun, warmth will suffice to expel that air ; yet, in this case, 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 closer union with the water ; which air, the action of light, whatever that be, would in time have depurated also.
• The quantity of air, yielded by water spontaneously, far exceeds that which can be expelled from it by heat. Indeed, I have frequently observed, that whatever circumstance depraves air, lessens also the quantity of it; fince it requires a large quantity of dephlogisticated air to make a small quantity of
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 diffipated, and the natural refiduum 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 depblogisticated air.'
In confirmation of these conclusions, the Author recites such of his experiments as appear fufficient to establish every thing that is of importance in them. Recommending che perusal of these to the philosophical reader, we shall transcribe the Author's latest observation relative to this subject, inserted at the end of this volume :
On the Effect of Light on Water. • 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, fo 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 lo pure as the former. Alo 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 food inverted, it should imbibe the water, and cause it to evaporate too soon *.!!
It may not be amiss, on an observasion so very singular and cua rious, to add our testimony, so far as it goes, to that of the Author. Two quart glass retoris, and cwo 12 ounce vials, filled, at the same time, with the same well water (which naturally contained a ma. derate quantity of fixed air), were exposed to the fun, in July laft, inverted in a bason of the fame well water, standing on a cable placed