air contiguous to it, and thereby generates fixed air, which is abforbed by the alkali. But, in reality, the only circumstance from which Mr. De Laffone judged the alkali to become mild, was its making fome effervefcence when faturated with acids; and this effervefcence is more likely to have proceeded from the expulfion of inflammable air than of fixed air, as it feems likely that the zinc might be more completely deprived of its phlogifton by the acid than by the alkali.

"In the above-mentioned paper, I fay, Dr. Priestley obferved, that quickfilver, fouled by the addition of lead or tin, depofits a powder by agitation and expofure to the air, which confifts in a great measure of the calx of the imperfect metal. He found too fome powder of this kind to contain fixed air; but it must be obferved, that the powder used in this experiment was not prepared on purpofe, but was procured from quickfilver fouled by having been ufed in various experiments, and may therefore have contained other impurities befides the metallic calces. On this Mr. Kirwan remarks, that Doctor Priestley did not at first prepare this powder on purpose, but he afterwards did fo prepare it (4 PR. p. 148 and 149) and obtained a powder exactly of the fame fort. It was natural to fuppofe from this remark, that Dr. Prieft ley muft have obtained fixed air from the powder prepared on purpofe, and that I had overlooked the paffage; but, on turning to the pages referred to, I was furprifed to find that it was other- wife, and that Dr. Prieftley not fo much as hints that he procured fixed air from the powder thus prepared.

"With regard to the calcination of metals, it may be proper to remark, that this operation is ufually performed over the fire, by methods in which they are expofed to the fumes of the burning fuel, and which are fo replete with fixed air, that it is not extraor dinary that the metallic calx fhould, in a short time, abforb a confiderable quantity of it; and in particular red lead, which is the calx on which moft experiments have been made, is always to prepared. There is another kind of

calcination, however, called which is perforined in the open but this is fo flow an operation. the ruft may eafily imbibe a f quantity of fixed air, notwit the fmall quantity of it usually tained in the atmosphere.

"Mr. Kirwan allows that water is not rendered cloudy by mixture of nitrous and conne but contends that this does not that fixed air is not generated union, as he thinks it may be ed by the nitrous felenite prod the union of the nitrous acid with lime. This induced me to tr fmall a quantity of fixed air we perceived in this experiment. in cordingly repeated it in the fame a ner as defcribed in my paper, that I purpofely added a little air to the common air, and fou when this addition was the bulk, orth of the weight di common air, the effect on the water was fuch as could not p have been overlooked in my of ments. But as those who fuppo ed air to be generated by the of nitrous and common air may to this manner of trying the ment, and fay that the quant”, fixed air abforbed by the line was really more thanth of the of the common air, being equal quantity over and above the airrated by the mixture, I made a experiment in a different m namely, I filled a bottle with i water, previoufly mixed with as nitrous acid as is contained in and bulk of nitrous air, and having verted it into a veffel of the fan. up into it, in the fame manner a the above-mentioned experiment" mixture of common air with its bulk of fixed air, until it was full. The event was the fame as " fore; namely, the cloudiness prod in the lime-water was fuch that I o not poffibly have overlooked. It be obferved, that in this experi no fixed air could be generated, ftill greater proportion of the h ter was turned into nitrous ke than in the above-mentioned experi

ats; fo that we may fafely conclude, t if any fixed air is generated by the cture of common and nitrous air, nuft be less than 3th of the bulk the common air.

As for the nitrous felenite, it ms not to make the effect of the ed air at all less fenfible, as I found 1 filling two bottles with common mixed with dth of its bulk of ed air, and pouring into each of m equal quantities of diluted limeter; one of thefe portions of limeter being previously diluted with an ial quantity of diftilled water, and other with the fame quantity of a uted folution of nitrous felenite, taining about dth of its weight calcareous earth; when I could not ceive that the latter portion of limeter was rendered at all lefs cloudy in the former. Though the nitrous enite, however, does not make the ect of the fixed air lefs fenfible, yet dilution of the lime-water, in conquence of fome of the lime being abbed by the acid, does; but, I beve, not in any remarkable degree. "There is an experiment mentionby Mr. Kirwan, which, though it not be confidered as an argument favour of the generation of fixed air, he only fuppofes, without any proof it fixed air is produced in it, does t deferve to be taken notice of as a rious experiment. It is, that, if nious and common air be mixed over y quickfilver, the common air is not all diminished, that is, the bulk of e mixture will be not lefs than that the common air employed, until ater is admitted, and the mixture itated for a few minutes. The rean of this in all probability is, that art of the phlogisticated nitrous acid, to which the nitrous air is converted, mains in the ftate of vapour until ondenfed by the addition of water. proof that this is the real cafe, is, at in this manner of performing the speriment, the red fumes produced n mixing the airs remain vifible for me hours, but immediately difappear 1 the addition of water and agitation. "The moft material experiment aldged by Mr. Kirwan is one of Dr.

Priestley's, in which he obtained fixed air from a mixture of red precipitate and iron filings. This at first seems really a ftrong argument in favour of the generation of fixed air; for though plumbago, which is known to confift chiefly of that fubftance, has lately been found to be contained in iron, yet one would not have expected it to be decompounded by the red precipitate, efpecially when the quantity of pure iron in the filings was much more than fufficient to fupply the precipitate with phlogifton. The following experiment, however, fhews that it was really decompounded; and that the fixed air obtained was not generated, but only feparated by means of this decompofition.

Five hundred grains of red precipitate mixed with 1000 of iron filings yielded, by the affiftance of heat, 7800 grain measures of fixed air, befides 2400 of a mixture of dephlogifticated and inflammable air, but chiefly the latter. The fame quantity of iron filings, taken from the fame parcel, was then diffolved in diluted oil of vitriol, fo as to leave only the plumbago and other impurities. These mixed with 500 grains of the fame red precipitate, and treated as before, yielded 9200 grain measures of fixed air, and 4200 of dephlogisticated air, of an indifferent quality, but without any fenfible mixture of inflammable air. It appears, therefore, that lefs fixed air was produced when the red precipitate was mixed with the iron filings in fubftance, than when mixed only with the plumbago and other impurities; which fhews that its production was not owing to the iron itfelf, which feems to contain no fixed air, but to the plumbago, which contains a great deal. The reafon, in all probability, why lefs fixed air was produced in the first cafe than the latter is, that in the former more of the plumbago efcaped being decompounded by the red precipitate than in the other. It must be obferved, however, that the filings ufed in this experiment were mixed with about th of their weight of brafs, which was not difcovered till they were diffolved in the acid, and 3 H 2

which

which makes the experiment lefs decifive than it would otherwife be. The quantity of fixed air obtained is alfo much greater than (according to Mr. Bergman's experiment) could be yielded by the plumbago ufually contained in 1000 grains of iron; fo that though the experiment feems to fhew that the fixed air was only produced by the decompofition of the impurities in the filings, yet it certainly ought to be repeated in a more accurate manner. "Before I conclude this paper, it may be proper to fum up the ftate of the argument on this fubject. There are five methods of phlogistication confidered by me in my paper on air; namely, first, the calcination of metals, either by themfelves or when amalgamated with quickfilver; fecondly, the burning of fulphur or phofphorus; thirdly, the mixture of nitrous air; fourthly, the explofion of inflammable air; and, fifthly, the electric fpark; and Mr. Kirwan has not pointed out any other which he confiders as unexceptionable. Now, the laft of thefe I by no means confider as unexceptionable, as it feems much moft likely, that the phlogiftication of the air in that experiment is owing to the burning or calcination of fome fubftance contained in the apparatus*. It is true, that I have no proof of it; but there is fo much probability in the opinion, that till it is proved to be erroneous, no conclufion can be drawn from fuch experiments in favour of the

generation of fixed air. As to the method, or the calcination of there is not the leaft proof that fixed air is generated, though weird tainly have no direct proof of the trary; nor did I in my paper that we had. The fame thing mat faid of the burning of fulphur phofphorus. As to the mixture of trous air, and the combuftion of ti flammable air, it is proved, that ifr fixed air is generated, it is fo fml to elude the niceft teft we have. is certain too, that if it had bee much as th of the bulk of the mon air employed, it would have be perceived in the first of thefe me and would have been fenfible in fecond, though ftill lefs. So that, of the five methods enumerated, it been fhewn, that in two no fei quantity is generated, and not the f proof has been affigned that any two of the others; and as to the good reafons have been affigne thinking it inconclufive; and, i fore, the conclufion drawn by 4 the above-mentioned paper feem ciently juftified; namely, that the it is not impoffible that fixed air be generated in fome chemical p fes, yet it feems certain, that it s the general effect of phlogisticating and that the diminution of cen air by phlogistication is by no owing to the generation or fepart, of fixed air from it.