The acid air seemed to make no impression upon a piece of Derbyshire _spar_, of a very dark colour, and which, therefore, seemed to contain a good deal of phlogiston. As the acid air has so near an affinity with phlogiston, I expected that the fumes of _liver of sulphur_, which chemists agree to be phlogistic, would have united with it, so as to form inflammable air; but I was disappointed in that expectation. This substance imbibed half of the acid air to which it was introduced: one fourth of the remainder, after standing one day in quicksilver, was imbibed by water, and what was left extinguished a candle. This experiment, however, seems to prove that acid air and phlogiston may form a permanent kind of air that is not inflammable. Perhaps it may be air in such a state as common air loaded with phlogiston, and from which the fixed air has been precipitated. Or rather, it may be the same thing with inflammable air, that has lost its inflammability by long standing in water. It well deserves a farther examination. The following experiments are those in which the _stronger acids_ were made use of, and therefore they may assist us farther to ascertain their affinities with certain substances, with respect to this marine acid in the form of air. I put a quantity of strong concentrated _oil of vitriol_ to acid air, but it was not at all affected by it in a day and a night. In order to try whether it would not have more power in a more condensed state, I compressed it with an additional atmosphere; but upon taking off this pressure, the air expanded again, and appeared to be not at all diminished. I also put a quantity of strong _spirit of nitre_ to it without any sensible effect. We may conclude, therefore, that the marine acid, in this form of air, is not able to dislodge the other acids from their union with water. _Blue vitriol_, which is formed by the union of the vitriolic acid with copper, turned to a dark green the moment that it was put to the acid air, which it absorbed, though slowly. Two pieces, as big as small nuts, absorbed three ounce measures of the air in about half an hour. The green colour was very superficial; for it was easily wiped or washed off. _Green copperas_ turned to a deeper green upon being put into acid air, which it absorbed slowly. _White copperas_ absorbed this air very fast, and was dissolved in it. _Sal ammoniac_, being the union of spirit of salt with volatile alkali, was no more affected with the acid air than, as I have observed before, common salt was. I also introduced to the acid air various other substances, without any particular expectation; and it may be worth while to give an account of the results, that the reader may draw from them such conclusions as he shall think reasonable. _Borax_ absorbed acid air about as fast as blue vitriol, but without any thing else that was observable.