was fixed, and the remainder was not diminifhed by nitrous air.

All the air that I was ever able to get from faline fubftances was fixed air. I began with alum, and the first experiment that I made upon this fubftance was with the fun-beams, in quickfilver; when I got from it a little air, which appeared to be fixed air, by extinguishing a candle, and by being readily abforbed by water. I repeated the experiment with the fame refult. The quantity of air extracted from a piece of alum, was about one-third of its bulk; but I imagined that a little, though not much, more might have been extracted, by a longer continuance of the operation.

I obferved, upon this occafion, that I could calcine only a given quantity of alum in a given quantity of air; and that when this was faturated, I could only keep the alum in a fluid ftate by heat. But it was easily calcined in vacuo; and as the receivers in which the calcination was made became very moift, it is pretty evident that this operation is performed by the mere expulfion of the water which enters into the compofition of this falt; fo that when the surrounding air can take no more water, that calcination can proceed no farther. I also observed, upon this occafion, that when

I had calcined a quantity of alum in a given quantity of common air, the air was not diminished, or in the smallest degree injured by the operation.

After this, I endeavoured to get air from calcined alum, with a burning lens, and I did get a little but I made no other observation upon it, than that it was not diminished by nitrous air. But when I put a quantity of calcined alum into a gun-barrel, I got from it a confiderable quantity of air, part of which was fixed air, precipitating lime in lime-water, and the remainder did not differ from the refiduum of fixed air, extinguishing a candle, and neither affecting common air, nor being affected by nitrous air.

From half an ounce of vitriolated tartar, in a gun-barrel, I got about 1 ounce-measure of air, which was chiefly fixed air. The laft produce diminished common air a little; but this I attribute to the gun-barrel, not having been perfectly cleaned from the materials ufed in a former experiment.

Borax was only melted by the burning lens ; but calcined borax gave a little air, about its own bulk; and this air extinguished a candle, and was not diminished by nitrous air; fo that

it feems to be the fame thing with the refiduum of fixed air: and this is, in fact, much the fame thing, if not quite the fame thing, with common air phlogisticated. I was induced to make this experiment, in confequence of that which I had made on fedative falt, which is made from borax, and from which, as I have observed, I had extracted air, about as good as common air; being in hopes that this experiment would throw fome light upon the other; but I was difappointed in that expectation.

Having thrown the focus of the burning lens upon a piece of volatile fal ammoniac, in quickfilver, a great quantity of air was prefently expelled from it; but upon withdraw, ing the heat, a great part of it foon difap. peared, leaving the fides of the veffel covered with flender crystals, exactly like thofe which are produced by a mixture of fixed air, and alkaline air. The remainder was imbibed by water, being, no doubt, fixed air.

Among other things, I threw the focus of the lens upon a piece of fine white fugar, in quickfilver. It was readily melted, and converted into a brown fubftance, yielding about two-thirds of its bulk of air, one-third of which was readily abforbed by water, and the remainder

I 3

mainder extinguished a candle. I repeated the experiment with a brownish powdered fugar, with the fame refult, excepting that more air was generated from this than from the white fugar, in proportion to their bulks.

From common falt, confined by quick filver, I got no air at all.

There has been a good deal of difference of opinion among philofophers, about the quality of the air that is really contained in chalk. Dr. Black's opinion is, that it is properly fixed air; whereas others have thought that the acid by which the air is diflodged from the chalk, really enters into the air that is produced in the procefs, and accordingly, that the fixed air produced by different acids, has different properties. An Italian philofopher, who did me the honour to write to me upon the subject, informs me, that he has difcovered that air produced from chalk by heat, is of a different nature from that which is got from it by acids, and particularly that the former will not make water acidulous. For my own part, I must acknowledge that. I have not examined this fubject thoroughly, and have been fometimes inclining to one opinion, and fometimes to another. Sometimes I have thought fixed air to be an original acid, and therefore one uniform inva

riable thing, from whatever fubftance, and in whatever manner procured. At other times I have been inclined to think, that its acidity is derived from fome other acid, especially the nitrous, for reafons that may appear in a fubfequent fection.

At prefent, I cannot say that I am quite decided about this question; but that I am much inclined to Dr. Black's opinion, and that all my experiments on chalk are in favour of it. For though I could get but very little air from pure chalk, either in quicksilver, or in vacuo, it was always fixed air, though the refiduum was fometimes more confiderable than I have found it to be when the air was produced by the solution of chalk in an acid. Once, however, I got a finall quantity of very pure fixed air from chalk, by heat, in quickfilver, almost as much of it being abforbed by water, as when chalk is made to give air by means of an acid,

It is remarkable, however, that heat is able to expel but very little air from chalk. I kept a very fmall quantity of chalk, in the focus of my burning lens, which I have obferved to be twelve inches in diameter, and twenty inches focal distance, more than half an hour, when the fun was near its greatest altitude, on the