Изображения страниц
PDF
EPUB

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

• I put one ounce of saltpetre, purified for distilling, into a glafs retort, and employed a moistened bladder, emptied of air, in lieu of a receiver. As soon as the saltpetre became red hot, it began boiling; and at that period of time the bladder was expanded from the air that passed over. I continued the distilla. tion, 'till the boiling in the retort ceased; and the faltpetre was on the point of penetrating through the softened 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 sulphuris and other sub{tances in air, and experienced a loss, amounting to a third part, of empyreal air, led him to form a very singular hypothesis with respect to the nature of fire; and to conclude that this substance is a compound consisting of empyreal air, and of phlogiston, or the universal infammable principle. In the process with the liver of sulphur, for instance, he supposes that the (empyreal) air, which disappears 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 observed during this process; but it must likewise be observed, that many days, or even some weeks, pass before it is completed : the heat therefore is diffipated as fast as it is generated. In many other proceffes however, conducted likewise without fire, and in which common air is diminished in a similar manner, an evident warmth is produced. Thus, in a mixture of a strong folution of liver of sulphur with pounded chałk, in which the decomposition of the air took place in a short time, the heat generated was soon very sensibly indicated by a thermometer. We may add, to this observation of the Author's, that it is well known that heat is produced in various other phlogistic processes, 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 supposes that the empyreal air comes from the chemist's fire, or the ignited charcoal. This seems to be the fubftance of his hypothesis in a few words :-Empyreal air and phlogiston constitute fire or heat: but it is well known that heat passes freely through glass and all other vessels. Having got bowever into the cavity of the retort, it there meets with substances that decompound it, and resolve it into its two constituent principles, neither of which, fingly, can pass through glass; viz, empyreal air and pblo

Eifion.

23

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

This hypothesis of Mr. Scheele's--that fire is composed of two such substances as empyreal air and phlogillon - is indeed fingular : but the mere fingularity of a philosophical hypothesis or opinion ought not certainly to prejudice philosophers against it; especially in these days, when the face of philosophy, and particularly of chemistry, has undergone such wonderful changes as we have experienced within even these few years paft. Were Stahl or Boerhaave to revisit the earth, and be informed, only in general terms, of the new chemical and philosophical propositions deducible from the writings of Priestley only, or from the discoveries of the ingenious Author of the present work, without being informed of the facts on which they were founded; they would consider them as the dreams of two unenlightened visionaries, or empyrics. Even Newton, with all his candour, would scarce condescend to listen 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 preserve 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 striking discoveries have come to our knowledge, in a gradual, though by no means slow, progression, can scarce properly estimate the surprize into which the philosophers of the last age would be thrown, on suddenly announcing to them fome of the discoveries of the present.

But though the fingularity of the Author's hypothesis does not justify the rejection of it; it wili naturally and juftly render his Readers more nice and difficult, with respect to his proofs. The Author relates a variety of experiments and observations, which he confiders as furnishing the most incontrovertible arguments' in support of his fyftem ; for which the philofophical Reader must study the work itself. We shall observe, however, that the greater part, if not the whole, of the phenomena produced by him, will admit of an easy solution 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 philosophical inquirer. We shall only make one observation on one of the trials above alluded to:

In the decomposition of liver of sulphur in a certain quantity of common air, in which, as well as in many other processes, a certain portion of air disappears; and the remainder is left in the state of foul air ; it would surely have been more natural for the Author to look for the empyreal air, which he had lost, in the remaining vitriolated tartar; where he might naturally suspect that it was lodged, and from which possibly he might alterwards have succeeded in expelling it: rather than have recourse to the fuppofition, that the mising empyreal air had been converted into heat, and had, in that form, passed through the glass. In ' the other cases, likewise, where the empyreal air appears to be generated, it was surely not only the most obvious, but the most probable conjecture, that it proceeded from some of the ingredients contained in the retort; and not that it was a constituent principle of the fire, which had passed through the sides of the vefsel, and had been decompounded on entering its cavity.

Mr. Scheele's hypothesis concerning light is equally fingular; and is founded on the preceding hypothefis respecting heat. He fupposes that each particle of light is nothing more than a subtle particle of empyreal air, which is more charged with phlogiston than an equally subtle particle of heat;' –or, in other words, that light is fire fuper-faturated with, or containing a superfluous quantity of, phlogiston. On this head he relates forme curious experiments, to prove that phlogiston is contained in light, and may be separated from it:

A calx of gold, precipitated from aqua regia by means of salt of tartar, is reduced to its metallic state, merely by the solar light, collected into a focus by a burning glass.- Red precipitate likewise, lying on a gold coin placed in the same focus, was reduced into running mercury, so as to make the gold white. In these two cases, as well as in various others, Mr. Scheele supposes that the metallic calces acquire the phlogiston, by which they are re.' duced, solely from the light of the fun.

Luna cornea becomes black, or acquires phlogiston, on being exposed fimply to the rays of the fun; though the fame fube stance, 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 diversity in the action of the differently coloured rays on this substance. Placing fome lura cornca on a paper, and exposing it to the fun-beams retracted through a prisim, he found that part of it on which the violet ray fell became sooner black than any other part of the powder; and he hence infers, that the calx of filver separates the phlogiston sooner from the violet

ray
than from

any

other. In the Notes with which Mr. Kirwan has illuftrated many obscure parts of this treatise, and corrected others, he accounts otherwise for these and many other reductions, by observing,

Z
4

that

that the above-mentioned calces of gold, mercury, and filver, Nill contained either fixed or nitrous air, or retained a certain quantity of the nitrous or marine acid; and that the fire only expelled the phlogiston adhering to these airs or acids, which uniting with the calces effected their reduction. We must observe, however, that the Author in a subsequent paffage (pag. 96) affirms, that the luna cornea, for instance, will not undergo any change, or become black, though exposed to the heat of the sun's rays during many days successively; if his light be excluded, by covering the vial containing it with a thick coating of black paint. He makes a similar allertion with respect to the precipitate of gold above mentioned : but we do not see how a similar experiment, made in a vial painted black, could be properly executed with this calx ; which, in the trial above mentioned, was not exposed, like the luna cornea, to the simple sunfhine, but to the solar focus.

Some curious experiments, made on manganese, are likewise produced by the Author, to demonstrate that the solar light contains phlogiston. Mr. Scheele has made some interesting discoveries respecting the true nature of this hitherto heteroclite substance ; which were published about five or six years ago, in the Transactions of the Swedish Academy. On the present occafion it will be fufficient merely to premise, that manganese cannot be dissolved in the nitrous acid without an addition of phlogiston. On adding, however, some inflammable substance, sugar for instance, it is readily diffolved, and the solution is colourless. On adding falt of tartar to this solution, a white precipitate falls down, which is manganese united with the phlogiston of the sugar. If this phlogiston be expelled from the white precipitate, by fire, or otherwile, the manganese is left, of its natural blaek colour

The Author added a little manganese, reduced to a subtle powder, to a quantity of nitrous acid ; and exposed the vial two hours to the sunshine. In that time, the mixture lost its black colour, and the manganese was diffolved, in the same manner as if sugar, or an inflammable fubftance, had been added. Adding more manganese, and again exposing the acid to the sun's light, this portion too was dissolved. Diluting the solution with water, and adding falt of tartar, a white precipitate fell down, This white precipitate, according to the Author, was manganese united with phlogiston, as in the preceding case; but with phlogiston attracted from the beams of the fun--with the addition however of some fixed air from the sale of tartar.

That the precipitate had actually thus acquired phlogiston, the Author ascertained -- by finding that it had the property of volatilifing vitriolic acid, or converting it into volatile sulphuTeous acid :-by its alcalising nitre; and at the same time re

affuming

assuming its original black colour :- and by keeping it red hot for a short time, in a vial loosely stopped (in order only to expel the fixed air from it, the presence of which would prevent or check its ignition afterwards), and then, when it was tolerably hot, throwing the still white powder on a paper; where it soon afterwards deflagrated or burned, and the manganese again became black as before :-or suffering the white precipitate to become quite cold, and throwing it on a hot, but not red hot, metal plate; when it deflagraces, or becomes red not, and is restored, as before, to its original state.

If we adopt Mr. Kirwan's manner of accounting for the reduction of the calces of gold and silver 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 phlogiston-the corner-stone of modern chemistry. We mean the complete reduction of mercurius calcinatus in close vessels ; apparently without the addition of any foreign phlogistic matter, and which had not, like the calces above mentioned, undergone the action either of acid solvents, or alcaline precipitants, from which it might be suppored, according to Mr. Kirwan's hypothefis, to have acquired the phlogiston requisite to its reduction. This case has appeared so difficult and important, that it has given occasion to varinus fyftems; and some respectable philosophers have even been driven to adopt the fuppofition, that the phlogiston which the mercury must neceffarily have lost, during its calcination, must, during its subsequent reduction, have been supplied to it ab extra, from the burning fuel, and have passed through the pores of the glass retort in which it was contained.

Though Mr. Scheele does not particularly treat of this subject, he would find a ready solution of this difficulty in his hypothesis. He would say that fire (that is empyreal air combined with phlogisłon) paffes through the retort. In its cavity, it is decompounded : its phlogiston restoring the calx to its metallic Hate; and leaving the empyreal air naked, or uncombined ; which is accordingly caught by the operator, in the apparatus constructed for that purpose.-But may not this difficult problem be satisfactorily solved in the following manner ?

It is, we believe, generally allowed, that the mercury, in the first part of this process, as it passes from its Auid or metallic itate into that of a calx, attracts a certain quantity of air from the atmosphere, in the room of the phlogiston which is gradually expelled from it by the heat. To account for its subsequent reduction to a metallic ftate, is it not sufficient to suppole, that, during its calcination, it does not attract from the atmosphere its empyreal (or dephlogisticated) part solely; but likewise such a portion of its phugisticated, or impure part, as is just sufficient to

furnish

« ПредыдущаяПродолжить »