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every respect fimilar to the common fort.'-But the Author's eafieft method of procuring empyreal air is the following:

I put one ounce of faltpetre, purified for diftilling, into a glafs retort, and employed a moistened bladder, emptied of air, in lieu of a receiver. As foon as the faltpetre became red hot, it began boiling; and at that period of time the bladder was expanded from the air that paffed over. I continued the diftillation, till the boiling in the retort ceafed; and the faltpetre was on the point of penetrating through the foftened glass retort. In the bladder I found the pure empyreal air, taking up the space of 50 ounces measure.-This is the best and cheapest method of obtaining empyreal air.'

Several of the Author's experiments, as well as those above alluded to, in which he confined hepar fulphuris and other fubftances in air, and experienced a lofs, amounting to a third part, of empyreal air, led him to form a very fingular hypothefis with respect to the nature of fire; and to conclude that this fubftance is a compound confifting of empyreal air, and of phlogifion, or the univerfal inflammable principle. In the process with the liver of fulphur, for inftance, he fuppofes that the (empyreal) air, which difappears in the experiment, is really converted into heat, or fire, by its union with the phlogiston of one of the ingredients; and that it escapes in that form, through the glass or otherwise, into the atmosphere.

It is true, the Author owns, that no heat is obferved during this process; but it must likewise be observed, that many days, or even fome weeks, pafs before it is completed: the heat therefore is diffipated as faft as it is generated. In many other proceffes however, conducted likewife without fire, and in which common air is diminished in a fimilar manner, an evident warmth is produced. Thus, in a mixture of a strong folution of liver of fulphur with pounded chalk, in which the decompofition of the air took place in a fhort time, the heat generated was foon very fenfibly indicated by a thermometer. We may add, to this obfervation of the Author's, that it is well known that heat is produced in various other phlogistic proceffes, as they are called by Dr. Priestley.

In the two experiments above recited, in which empyreal air is produced (in the distillation of nitrous acid, and of nitre alone), the Author fuppofes that the empyreal air comes from the chemift's fire, or the ignited charcoal.-This feems to be the fubftance of his hypothefis in a few words :-Empyreal air and phlogifton conflitute fire or heat: but it is well known that heat passes freely through glafs and all other veffels. Having got however into the cavity of the retort, it there meets with fubftances that decompound it, and refolve it into its two conflituent principles, neither of which, fingly, can pass through glass; viz. empyreal air and pblo

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gifton. In the former experiments, with hepar fulphuris, &c. the empyreal air that disappeared, together with a portion of phlogifton, is fuppofed to be converted into fire: in thefe two laft trials, the converse takes place; and the fire in the furnace is fuppofed to be converted into empyreal air, on its entering the retort, through its fides.-Such, at leaft, appears to us to be the Author's doctrine concerning this matter; which is however nowhere very clearly expreffed.

This hypothefis of Mr. Scheele's-that fire is compofed of two fuch fubftances as empyreal air and phlogifton is indeed Angular but the mere fingularity of a philofophical hypothefis or opinion ought not certainly to prejudice philofophers against it; efpecially in these days, when the face of philofophy, and particularly of chemistry, has undergone fuch wonderful changes as we have experienced within even these few years paft. Were Stabl or Boerhaave to revifit the earth, and be informed, only in general terms, of the new chemical and philofophical propofitions deducible from the writings of Priestley only, or from the difcoveries of the ingenious Author of the prefent work, without being informed of the facts on which they were founded; they would confider them as the dreams of two unenlightened vifionaries, or empyrics. Even Newton, with all his candour, would scarce condefcend to liften to Franklin, on his undertaking to knock down a bullock, on bringing a metal rod within a small distance of him; or on his pretending to extract lightning from a cloud, just then hanging over their heads, and to preferve it for future ufe in an empty vial which he had brought in his pocket. In fact, we, who have lived in the age when these and many other ftriking discoveries have come to our knowledge, in a gradual, though by no means flow, progreffion, can scarce properly eftimate the furprize into which the philofophers of the last age would be thrown, on fuddenly announcing to them fome of the discoveries of the prefent.

But though the fingularity of the Author's hypothefis does not justify the rejection of it; it wiH naturally and justly render his Readers more nice and difficult, with refpect to his proofs. The Author relates a variety of experiments and obfervations, which he confiders as furnishing the moft incontrovertible arguments' in fupport of his fyftem; for which the philofophical Reader muft ftudy the work itself. We fhall obferve, however, that the greater part, if not the whole, of the phenomena produced by him, will admit of an eafy folution on other principles; without having recourse to the hypothesis which he has formed to account for them. Many of these are nevertheless curious, and merit the attention of the philofophical inquirer. We shall only make one obfervation on one of the trials above alluded

to:

In the decompofition of liver of fulphur in a certain quantity of common air, in which, as well as in many other proceffes, a certain portion of air difappears; and the remainder is left in the ftate of foul air; it would furely have been more natural for the Author to look for the empyreal air, which he had loft, in the remaining vitriolated tartar; where he might naturally fufpect that it was lodged, and from which poffibly he might afterwards have fucceeded in expelling it: rather than have recourfe to the fuppofition, that the miffing empyreal air had been converted into heat, and had, in that form, paffled through the glafs. In the other cafes, likewife, where the empyreal air appears to be generated, it was furely not only the most obvious, but the moft probable conjecture, that it proceeded from fome of the ingredients contained in the retort; and not that it was a conflituent principle of the fire, which had paffed through the fides of the veffel, and had been decompounded on entering its cavity.

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Mr. Scheele's hypothefis concerning light is equally fingular; and is founded on the preceding hypothefis refpecting heat. He fuppofes that each particle of light is nothing more than a fubtle particle of empyreal air, which is more charged with phlogifton than an equally fubtle particle of heat;'-or, in other words, that light is fire fuper-faturated with, or containing a fuperfluous quantity of, phlogifton. On this head he relates fome curious experiments, to prove that phlogifton is contained in light, and may be feparated from it:

A calx of gold, precipitated from aqua regia by means of falt of tartar, is reduced to its metallic ftate, merely by the folar light, collected into a focus by a burning glafs.- Red precipitate likewife, lying on a gold coin placed in the fame focus, was reduced into running mercury, fo as to make the gold white. In thefe two cafes, as well as in various others, Mr. Scheele fuppofes that the metallic calces acquire the phlogifton, by which they are reduced, folely from the light of the fun.

Luna cornea becomes black, or acquires phlogifton, on being expofed fimply to the rays of the fun; though the fame fubftance, wrapped up in paper, and kept two months on a warm earthen-ware ftove, had not its colour altered. Nay, Mr. Scheele affirms, that there is a diverfity in the action of the differently coloured rays on this fubftance. Placing fome luna cornea on a paper, and expofing it to the fun-beams refracted through a prifim, he found that part of it on which the violet ray fell became fooner black than any other part of the powder; and he hence infers, that the calx of filver feparates the phlogifton fooner from the violet ray than from any other.

In the Notes with which Mr. Kirwan has illuftrated many obfcure parts of this treatife, and corrected others, he accounts otherwife for thefe and many other reductions, by obferving,

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that the above-mentioned calces of gold, mercury, and filver, fill contained either fixed or nitrous air, or retained a certain quantity of the nitrous or marine acid; and that the fire only expelled the phlogifton adhering to thefe airs or acids, which uniting with the calces effected their reduction. We muft obferve, however, that the Author in a fubfequent paffage (pag. 96) affirms, that the luna cornea, for inftance, will not undergo any change, or become black, though expofed to the heat of the fun's rays during many days fucceffively; if his light be excluded, by covering the vial containing it with a thick coating of black paint. He makes a fimilar affertion with respect to the precipitate of gold above mentioned: but we do not fee how a fimilar experiment, made in a vial painted black, could be properly executed with this calx; which, in the trial above mentioned, was not expofed, like the luna cornea, to the fimple funfhine, but to the folar focus.

Some curious experiments, made on manganefe, are likewise produced by the Author, to demonftrate that the folar light contains phlogifton. Mr. Scheele has made fome interefting difcoveries refpecting the true nature of this hitherto heteroclite fubftance; which were published about five or fix years ago, in the Tranfactions of the Swedish Academy. On the present occafion it will be fufficient merely to premife, that manganefe cannot be diffolved in the nitrous acid without an addition of phlogifton. On adding, however, some inflammable substance, fugar for inftance, it is readily diffolved, and the folution is colourless. On adding falt of tartar to this folution, a white precipitate falls down, which is manganefe united with the phlogiston of the fugar. If this phlogiston be expelled from the white precipitate, by fire, or otherwife, the manganefe is left, of its natural blaek colour.

The Author added a little manganefe, reduced to a subtle powder, to a quantity of nitrous acid; and expofed the vial two hours to the funshine. In that time, the mixture loft its black colour, and the manganefe was diffolved, in the fame manner as if fugar, or an inflammable fubftance, had been added. Adding more manganefe, and again expofing the acid to the fun's light, this portion too was diffolved. Diluting the folution with water, and adding falt of tartar, a white precipitate fell down. This white precipitate, according to the Author, was manganefe united with phlogifton, as in the preceding cafe; but with phlogiston attracted from the beams of the fun-with the addition however of fome fixed air from the falt of tartar.

That the precipitate had actually thus acquired phlogifton, the Author afcertained--by finding that it had the property of volatilifing vitriolic acid, or converting it into volatile fulphureous acid-by its alcalifing nitre; and at the fame time re

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affuming its original black colour :- and by keeping it red hot for a fhort time, in a vial loosely ftopped (in order only to expel the fixed air from it, the prefence of which would prevent or check its ignition afterwards), and then, when it was tolerably hot, throwing the ftill white powder on a paper; where it foon afterwards deflagrated or burned, and the manganefe again became black as before: or fuffering the white precipitate to become quite cold, and throwing it on a hot, but not red hot, metal plate; when it deflagrates, or becomes red hot, and is reftored, as before, to its original state.

If we adopt Mr. Kirwan's manner of accounting for the reduction of the calces of gold and filver above mentioned, and of the red precipitate; yet a ftill more difficult problem remains to be solved, the subject of which has incited fome even to question the truth of the Stahlian doctrine of phlogifton-the corner-ftone of modern chemistry. We mean the complete reduction of mercurius calcinatus in clofe veffels; apparently without the addition of any foreign phlogistic matter, and which had not, like the calces above mentioned, undergone the action either of acid folvents, or alcaline precipitants, from which it might be fuppofed, according to Mr. Kirwan's hypothefis, to have acquired the phlogifton requifite to its reduction. This cafe has appeared fo difficult and important, that it has given occafion to various fyftems; and fome refpectable philofophers have even been driven to adopt the fuppofition, that the phlogifton which the mercury muft neceffarily have loft, during its calcination, muft, during its fubfequent reduction, have been supplied to it ab extra, from the burning fuel, and have paffed through the pores of the glafs retort in which it was contained.

Though Mr. Scheele does not particularly treat of this fubject, he would find a ready folution of this difficulty in his hypothefis. He would fay that fire (that is empyreal air combined with phlogiston) pafles through the retort. In its cavity, it is decompounded: its phlogifton reftoring the calx to its metallic fate; and leaving the empyreal air naked, or uncombined; which is accordingly caught by the operator, in the apparatus conftructed for that purpofe.-But may not this difficult problem be fatisfactorily folved in the following manner?

It is, we believe, generally allowed, that the mercury, in the first part of this procefs, as it paffes from its fluid or metallic ftate into that of a calx, attracts a certain quantity of air from the atmosphere, in the room of the phlogifton which is gradually expelled from it by the heat. To account for its fubfequent reduction to a metallic ftate, is it not fufficient to suppose, that, during its calcination, it does not attract from the atmosphere its empyreal (or dephlogifticated) part folely; but likewife fuch a portion of its phlogiflicated, or impure part, as is juft fufficient to

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