Imágenes de páginas
PDF
EPUB

furnish the proper quantity of phlogiston requisite to its subsequent reduction : for when the experiment is performed in a clean glass retort, the air which is collected is, in general, pure or dephlogifticated.

According to this theory, as crude mercury is a metallic earth combined with phlogiston ; so mercurius calcinatus is the same earth, which has lost a part or the whole of its phlogiston, but has, in its room, attracted from the atmosphere air of a certain standard; that is, not pure or dephlogisticated, but containing such a portion of phlogiston as is fufficient to supply that which it had lost in the calcination. On applying the proper degree of heat, this new combination of mercurial earth, and air fomewhat impure, is destroyed. The earth retains the whole of the phlogiston contained in this air, and accordingly recovers its metallic state and fluidity ; and parts at the same time with the remaining air, thus completely dephlogisticated. In a fomewhat fimilar manner, plants are supposed to attract air, more or less pure, from the atmosphere, or from water; and, while they retain the phlogisticated, they are found to emit the dephlogisti

cated part.

It

We offer these reflections on this subject, as seeming at least to afford a simple and satisfactory solution of this great difficulty: nor do we, at present at least, recollect any phenomena that are irreconcileable with it,

The Author's observations on fulminating gold contain many curious novelties; and, in particular, cvince ftill more satisfactorily than even the experiments of the Chevalier Bergman, that the presence of nitrous acid is not a necessary circumstance towards the producing the fulmination of that power, is true, that the Chevalier Bergman made Aurum fulminans from precipitates which did not appear to contain any nitrous acid. He precipitated gold, for instance, from its common folvent, aqua regia, by a fixed alcali; and then diffolving this precipitate, (first well edulcorated) in vitriolic, or marine acid, or even in distilled vinegar, and again precipitating it from these solutions by a volatile akali, he itill obtained fulminating gold. Nevertheless, we think that a doubt may still remain, which we very lately expressed at some length [See Appendix to our 63d or last Volume, December 1780, pag. 505], that, as nitrous acid had been employed in the original solution of the gold in aqua regia, some of that acid might ftill be retained in the precipitate, and might obstinately adhere to it in all the subsequent operations

Mr. Scheele's experiments absolutely remove this doubt. His theory and discoveries respecting this curious subject deserve to be abridged, and explained.

Gold is a compound of an earth sui generis, and of phlogiston. It is insoluble in any acids, till it has parted with its phlogiston

ta

upon it.

to an acid rapacious of that principle. The marine acid has the greatest affinity to its earth, which however it cannot attack till the phlogiston has been detached from it. This laft office is performed by the nitrous acid in the aqua regia, which evidently acquires phlogiston in the process; and the marine acid is thus left at liberty to diffolve the earth.

But the Author having discovered a method, not here indicated, of dephlogisticating marine acid (by means, we believe, of manganese), found that the marine acid, thus deprived of its own phlogiston, was capable, singly, of diffolving gold in its metallie state; in consequence of the avidity which, by its own dephlor gistication, it had acquired for phlogifton. A folution of gold, thus effected, being distilled in a retort with a strong heat, the gold is reduced without addition : for it recovers back again the very phlogiston which, during its solution, it had before parted with to the marine acid; and the acid accordingly passes over into the receiver in its prior dephlogisticated state.

Gold, thus dissolved by the dephlogisticated marine acid alone, is actually precipitated, on the addition of a volatile alcali, in the form of fulminating gold. From this process it appears clearly that the peculiar qualities of this powder are not to be accounted for from the presence of any nitrous acid supposed to be contained in it.

That the earth of gold + attracts a volatile alcali more strongly than the latter is attracted even by the vitriolic acid, the Author fhews by digesting this calx in a solution of Glauber's sal ammoniac. In this cale, this neutral solution of volatile alcali and vitriolic acid became acidulous ; in consequence of its parting with a portion of its volatile alcali, which had left the vitriolic acid, and united itself with the earth of gold; conftituting with it a true aurum fulminans, which, from these and other experiments, appears evidently to be nothing more than a combination of the earth of gold and volatile alcali ;- particularly from the following:

The Author dissolved fulminating gold in marine acid, and put several pieces of copper into the solution. The gold was precipitated, on recovering its phlogiston from the copper; and the volatile alcali which had been a constituent part of the fulminating gold, and which had been expelled from it on its reduction, and had entered into the solution, was readily recognised, on adding some fixed alcali to expel it from the solution previously evaporated to dryness.

The Author's analysis of fulminating gold, particularly with

+ By this term the Author denotes the precipitate thrown down from a solution of gold, on the addition of a fixed alcali; and which is known not to possess a fulminating quality,

rer

refpect to its accenfion, furnishes us, as Mr. Kirwan well ob. ferves in his Notes, with a new and important discovery ;-the decomposition of pure volatile alcali. This calx of gold, at the instant of its explofion, is fuddenly reduced to a metallic state, by means of the phlogiston which it attracts from the volatile alcali * : while the other constituent part, or parts, of that saline substance appear, from subsequent experiments, to assume the form of a particular species of air ; which Mr. Scheele confiders as agreeing with phlogisticated air in its principal properties. At leaft, a candle was extinguished in it; it was not abforbed by water; nor did it make lime-water turbid. The Author refers on this occasion to his Memoir on Manganese, above alluded to; where he has thewn, that, whenever a body attracts t'e phlogifton which is an integrant part of the volatile alcali, fuch air is always produced :-particularly in the distillation of crocus martis and fal ammoniac; the precipitate thrown down from mercury sublimate by volatile alcali; and in the detonation of nitrum flammans, or nitrous ammoniac.

Palling over many of the Author's experiments and reason. ings, more or less connected with his peculiar theory, we shall describe a fimple and easy method proposed by him, of discovering whether water contains empyreal, i. e. dephlogisticated air, or agt. In a preceding experiment, when he was inquiring into the nature of atmospherical air, he found that the green precipitate, formed on adding fixed alcali to a solution of martial or green vitriol, Mut up in a vial containing common air, during a fortnight, had decompounded the air, and had itself acquired the colour of crocus martis: and at the same time, out of 40 parts of common air, 12 were loft. The air which disappeared was the Author's empyreal air; and the remaining 28 parts were bis foul air, in which a candle would not burn. By a mode of trial founded on this experiment, he discovers whether any particular water contains dephlogisticated air.

To an ounce of the water, for instance, he adds about four drops of a solution of vitriol of iron, and adds two drops of a folution of falt of tartar somewhat diluted with water. The dark er precipitate thus formed changes into a yellow colour, in a couple of minutes, if the water contain empyreal air : but if the water has been very lately boiled, and cooled without having had free access to the air, or has been lately distilled; the precipitate retains its green colour, and does not turn yel

* There are fome peculiarities respecting heat, in the Author's mode of explication, which differ fomewhat from this account: buc they cannot be here eafily explained, as they are founded on

particular theory of fire; and the subject is certainly rendered clearer by the omission of thein.

low,

low, unless about an hour afterwards; and it never acquires a yellow colour, if it be kept in full glasses, so that the air have no access to it.

Before we conclude our Review of the present interesting performance, we shall gratify our philosophical Readers by taking notice of a late discovery of the Author's; though it be here only incidentally mentioned, with a view to corroborate his particular system respecting the conftitution of empyreal air. It has indeed likewise induced him to propose this singular opinion that metallic and earthy substances consist only of acids combined with phlogiston. We allude to his discoveries relating to chat singular mineral, arsenic; which at once possesses the characters of a faline and of a metallic substance.

M. Macquer, whose analysis of this mineral is well known to our chemical Readers, had indeed detected an acid in it; as forming a conftituent part of it; or rather had discovered that it contained a certain principle, capable of constituting a neutral salt, when combined with a fixed clcali: but he entertained no suspicion that this heavy metallic fubftance had not an earth for its bafis. In a Memoir printed in the Transactions of the Swedish Academy, the Author has shewn, that the common arsenic of the shops consists only of an acid, sui generis, combined with a certain quantity of phlogiston ; and that even the regulus of arsenic, in which this heteroclite substance affumes the form of a femi-metal, consists only of the fame acid, combined with a ftill greater portion of phlogiston.

We did not speak at random when, at the beginning of this article, we observed that chemistry had, in our times, assumed a new face. In the instance before us, we find a sober and or thodox chemist inferring from his experiments, that all earths are a species of acids; and hinting, that even metals in general consist of acid and phlogiiton only. Mr. Scheele founds this opinion not merely on his analysis of arsenic, but likewise on other facts and observations; particularly on his former discoveries relative to the sparry acid. He still confiders the fluor cruft or earth, as consisting of this acid fixed by an union with water.

This conclusion, however, is liable to objections: nevertheless, that great chemist, the Chevalier Bergman, in an excellent prefatory introduction to this work, seems to hold the same language with the Author, as well with respect to this subject, as to his extension of the principle to metals, &c. These are his words:

• Since the acid of fuor and water, meeting one another in the form of vapours t, coagulate into filiceous earth; and acid

of

+ This is expressed too generally. From Dr. Priestley's mode of experimenting,, is appears that the water need not be in a late of va

pour;

of arsenic with phlogiston coagulates into solid white arsenic and with still more phlogiston (he might have added), into a true regulus or semi-metal; there is some indication that terreous substances, as well as metallic calces, may be considered in their first principles as acids; which, in the first case, are become fixed by water, and, in the lait, by phlogiston.'

The Reader will meet with many obscurities in this work, befides those incidental to a translation ; and which arise froin the Author's great conciseness on many subjects, where, it will naturally be wished that he had been more explicit and circum{tantial; and from the want of proper arrangement of his materials. Though the philosophical Reader may probably reject the Author's theory of fire; he will here meet with many curious facts, dispersed in various parts of the work, which may, in future, lead to important consequences.' In chemistry, as the Chevalier Bergman well observes, " there are none of the Veritates Otiofæ. The least phenomenon, when examined in all its causes, is always connected with others of the greatest importance; in such a manner, that every thing is thewn connected in the great economy of nature.'

pour; as the filiceous earth appears as soon as the

fluor acid vapour, or air, reaches the under surface of the water resting on mercury: and from many of his experiments, on o:her occasions as well as the present, ic feems probable that the fiuor acid air holds the fluor crust or earth in solution, previcu;ly to its meeting with the water ; which only decompounds shis air by condensing, and uniting with, its acid: while the earth, deprived of its former aerial folvent, is precipitated, and becomes visible, under the form of fluor cruft.

Art. IV. An Examination of Dr. Crawford's Theory of Heat and Combustion : By William Morgan. Svo. 16. 6 d. Cadell, 1781.

we have, in those parts of our work which are specified below *, already given a pretty full account of Dr. Crawford's theory of fire ; yet, as the subject is rather of an intricate nature, we fhall endeavour to throw a little more light upon it, by availing ourselves of the present Author's general exposition of that hypothesis: the refutation of which, at least in some of its most effential parts, is the professed object of this publication. In short, we fall only, except perhaps in a few instances, take upon us the office of Relaters; and confine

ourselves to such parts of the performance as are not much involved in calculation.

* See M. Review, Vol. Ixi. November 1779, pag. 378; and Mr. Magellan's explanation of Dr. Crawford's theory of fire, in the Appendix to our 63d volume [1780] pag. 499.

That

« AnteriorContinuar »