It seems to consist of a peculiar acid vapour united to the stony substance of the fluor; for water being admitted to it absorbs the acid vapour, and the stony substance is deposited. By this means it exhibits an amusing appearance, whether water be admitted to a glass jar previously filled with that air, or the bubbles of air be admitted, as they are formed, to a quantity of water resting on mercury. LECTURE X. _Of Alkaline Air._ Alkaline air is produced by means of heat from caustic volatile alkali, and also from a mixture of sal-ammoniac and slaked lime, in the proportion of about one-fourth of the former to three-fourths of the latter. In this case the marine acid in the sal-ammoniac unites with the calcareous earth, and the volatile alkali (probably with the assistance of the water) takes the form of air. This species of air is heavier than inflammable air, but lighter than any of the acid airs. Like them, however, it dissolves ice, and deprives alum, and some other saline substances, of the water which they contain. United with fixed air, it makes the concrete volatile alkali; with marine acid air, the common sal-ammoniac; and with water, the caustic volatile alkali. The electric spark, or a red heat, converts alkaline air into three times its bulk of inflammable air; and the calces of metals are revived in alkaline, as well as in inflammable air; but there remains about one-fourth of its bulk of phlogisticated air. These facts shew that alkaline air consists chiefly of phlogiston. _Miscellaneous Observations relating to Air._ The _nitrous_ acid may be exhibited in the form of air, as well as the vitriolic, the marine, and the fluor acids. But it cannot be confined even by mercury, which it instantly dissolves. It may, however, in some measure, be confined in a dry glass vessel, from which it will in a great measure expel the common air. This nitrous acid air is that red vapour, which is produced by the rapid solution of bismuth, and some other metals in the nitrous acid. But the vegetable acid cannot be exhibited in the form of air. It is only capable of being converted into vapour, like water: and in the common temperature of our atmosphere, returns to a state of fluidity. Different kinds of air which have no affinity to each other, when once mixed together will not separate, notwithstanding any difference of specific gravity. Such is the case of a mixture of inflammable and dephlogisticated air, and even of inflammable and fixed air. Without this property also, the phlogisticated air, which constitutes the greatest part of our atmosphere, being specifically lighter than dephlogisticated air, of which the other part of it consists, would separate from it, and ascend into the higher regions of the atmosphere. Inflammable air, however, will not mix with acid or alkaline air. Different kinds of air are expanded differently by the same degrees of heat; dephlogisticated air the least, and alkaline air the most.