after a long time, depofited a white matter, which I suppose to be the flowers of zinc.

Copper, treated in the fame manner, yielded air very freely, with about the fame degree of heat that quickfilver had required, and the air continued to be generated with very little application of more heat. The whole produce was vitriolic acid air, and no part of it inflammable. The oil of vitriol remained a long time turbid, but at length depofited a brownish matter.

The folution of filver in the fame manner, had the very fame refult, all the air being acid, and no part of it inflammable. The oil of vitriol acquired a kind of orange-colour, and depofited nothing.

With a very great degree of heat lead yielded a little air, which was wholly acid, and had nothing inflammable in it.

Gold yielded no air at all in this treatment; but the oil of vitriol acquired the fame orangecolour that it had when the filver had been heated in it.

Neither had this treatment of platina any fenfible effect. What I made ufe of was fome

which I had been favoured with from Dr. Irving carefully purged from iron.

In most of these proceffes, air feems to iffue from the fubftances immediately upon the application of heat, and fometimes without it: and this first produce of air forms bubbles, which continue some time on the furface of the liquor. But it feems to be nothing more than the common air which had adhered to the furfaces of thofe fubftances, or had been confined in the little cavities near the furface, when they happened to be rough. For this feeming production of air foon ceafes, and no more is produced without a much greater degree of heat; and when the genuine acid air begins to rife, bubbles formed by it break inftantly, like bubbles of air in fpirit of wine, and there is nothing like froth on the furface of the oil of vitriol.

As fulphur is formed by the union of phlogifton with oil of vitriol highly concentrated and very hot, I imagined that by heating subftances containing phlogifton in vitriolic acid air, I could not fail to produce fulphur; but I tried charcoal in this manner without the effect that I had expected from it. The heat of a burning lens thrown upon it in this acid air, only made it throw out that quantity of the air, which, as I have obferved before, is condensed

C 3

condenfed upon its furface, or imbibed by it. The air that was unabforbed after this operation was in part fixed, and in part inflammable, having come from the charcoal,

There was frequently, however, the appearance of fulphur produced upon the mixture of alkaline air with vitriolic acid air; for the infide of the tube would be covered

with a perfectly yellow matter. But this colour goes off in time, and nothing but a white faline substance remains. This yellow appearance I firft obferved when I had produced the vitriolic acid air from ether; but afterwards I found the fame effect when it was produced from charcoal, and still more remarkably when it had been produced from copper. Why this yellow colour should not be permanent, I do not underftand,

SEC

Having hit upon a method of exhibiting fome of the acids in the form of air, nothing could be easier than to extend this process to the reft. I had nothing to do but either to procure the acid in a liquid form, viz. combined with water, and then expel the air by heat; or to find fome folid fubftance in which it was combined, and, diflodging it by fome ftronger acid, to receive the generated air in quickfilver.

To procure the vegetable acid air, I was favoured by Dr. Higgins, with a quantity of exceedingly strong concentrated acid of vinegar, from which, by means of heat, and with the apparatus represented vol. I, plate 2. fig. 8. I could easily expel as much air, as from an I found, equal quantity of spirit of falt. however, that unless the apparatus was furnished with a small recipient, to intercept the liquor that might be thrown out of the veffel by boiling, I could not (except at the very first) procure this acid air free from moisture:

but

but with this provifion I eafily got the air as perfectly dry as I could wish.

This vegetable acid air extinguishes the flame of a candle, exactly like the vitriolic acid air, viz. without any particular colour of the flame in going out, or in lighting again.

Upon putting alkaline air to vegetable acid air, the white cloud obfervable in fimilar mixtures was inftantly formed, and rofe at once to the top of the veffel, as in the cafe of the other acid airs. The fides of the veffel in which this mixture was made, were tinged with yellow, as in the fame procefs with vitriolic acid air; which to me is a puzzling fact, as I do not know that fuch a fulphur (if the fubftance be fulphur) was ever known to be formed without the vitriolic acid. At first I imagined that this colour had come from fomething contained in the ingredients for making the alkaline air; but I prefently found, that when I put the alkaline air, from the very fame preparation, to marine acid air, the falt formed by them was perfectly white, without the least tinge of yellow.

The affinities both of the marine and of the vitriolic acids in the form of air, have been

feen