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tinues to do fo, till it occupies twice as much fpace as it did at firft. The force with which it expands is great; but how great it is I have not endeavoured to determine.

When this mixture is immerfed in water, it generates no air, though it becomes black, and fwells.

SECTION VI.

Of NITROUS AIR,

Ever fince I firft read Dr. Hales's most excellent Statical Effays, I was particularly ftruck with that experiment of his, of which an account is given, VOL. I. p. 224. and VOL. II. p. 280. in which common air, and air generated from the Walton pyrites, by fpirit of nitre, made a turbid red mixture, and in which part of the common air was absorbed; but I never expected to have the fatisfaction of feeing this remarkable appearance, fuppofing it to be peculiar to that particular mineral. Happening to mention this fubject to the Hon. Mr. Cavendish, when I was in London, in the fpring of the year 1772, he faid that he did not imagine but that other kinds of pyrites, or the metals might anfwer as well, and that

probably the red appearance of the mixture depended upon the fpirit of nitre only. This encouraged me to attend to the fubject; and having no pyrites, I began with the solution of the different metals in fpirit of nitre, and catching the air which was generated in the folution, I presently found what I wanted, and a good deal more.

Beginning with the solution of brass, on the 4th of June 1772, I first found this remarkable species of air, only one effect of which, was cafually obferved by Dr. Hales; and he gave fo little attention to it, and it has been fo much unnoticed fince his time, that, as far as I know, no name has been given to it. I therefore found myself, contrary to my firft refolution, under an abfolute neceffity of giving a name to this kind of air myself. When I first began to fpeak and write of it to my friends, I happened to diftinguish it by the name of nitrous air, because I had procured it by means of 1pirit of nitre only; and though I cannot fay that I altogether like the term, neither myself nor any of my friends, to whom I have applied for the purpose, have) been able to hit upon, a better; fo that I am obliged, after all, to content myself with

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I have found that this kind of air is readily procured from iron, copper, brafs, tin, filver, quickfilver, bifmuth, and nickel, by the nitrous acid only, and from gold and the regulus of antimony by aqua regia. The circumstances at tending the folution of each of these metals are various, but hardly worth mentioning, in treating of the properties of the air which they yield; which, from what metal foever it is extracted, has, as far as I have been able to observe, the very fame properties.

One of the most confpicuous properties of this kind of air is the great diminution of any quantity of common air, with which it is mixed, attended with a turbid red, or deep orange colour, and a confiderable heat. The smell of it, alfo, is very ftrong, and remarkable, but very much resembling that of smoking spirit of nitre.

The diminution of a mixture of this and common air is not an equal diminution of both the kinds, which is all that Dr. Hales could obferve, but of about one-fifth of the common air, and as much of the nitrous air as is neceffary to produce that effect; which, as I have found by many trials, is about one half as much as the original quantity of common air. For if one measure of nitrous air be put to two

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measures of common air, in a few minutes (by which time the effervefcence will be over, and the mixture will have recovered its tranfparency) there will want about one-ninth of the original two measures; and if both the kinds of air be very pure, the diminution will ftill go on flowly, till in a day or two, the whole will be reduced to one-fifth lefs than the original quantity of common air. This farther diminution, by long ftanding, I had not observed at the time of the first publication of these papers.

I hardly know any experiment that is more adapted to amaze and furprize than this is, which exhibits a quantity of air, which, as it were, devours a quantity of another kind of air half as large as itself, and yet is fo far from gaining any addition to its bulk, that it is confiderably diminished by it. If, after this full faturation of common air with nitrous air, more nitrous air be put to it, it makes an addition equal to its own bulk, without producing the leaft redness, or any other visible effect.

If the smallest quantity of common air be put to any larger quantity of nitrous air, though the two together will not occupy fo much space as they did feparately, yet the quantity will still be larger than that of the

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nitrous air only. One ounce measure of com mon air being put to near twenty ounce meafures of nitrous air, made an addition to it of about half an ounce measure. This being a much greater proportion than the diminution of common air, in the former experiment, proves that part of the diminution in the former cafe is in the nitrous air. Befides, it will presently appear, that nitrous air is fubject to a most remarkable diminution; and as common air, in a variety of other cafes, fuffers a diminution from one-fifth to one-fourth, I conclude, that in this cafe alfo it does not exceed that proportion, and therefore that the remainder of the diminution respects the nitrous air.

In order to judge whether the water contributed to the diminution of this mixture of nitrous and common air, I made the whole procefs feveral times in quickfilver, ufing one-third of nitrous, and two-thirds of common air, as before. In this cafe the rednefs continued a very long time, and the diminution was not fo great as when the mixtures had been made in water, there remaining one-feventh more than the original quantity of common air.

This mixture ftood all night upon the quickfilver; and the next morning I obferved that it was no farther diminished upon the admiffion

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