This air is not a “dry acid of nitre converted into elastic vapours,” for it does not produce nitre with alkalies; moreover, it can be prepared from substances which have nothing in common with nitre, no compound of nitre having been used during their preparation. Scheele next describes experiments proving that “fire-air” is produced by the distillation of black oxide of manganese with concentrated oil of vitriol, or with the “phosphorus acid of urine” (phosphoric acid), by distilling nitrate of magnesium, made by dissolving the “white magnesia employed in medicine” (magnesium carbonate) in _aquafortis_ (nitric acid), or by distilling “mercurial nitre” (mercuric nitrate). The cheapest and the best method of producing “fire-air” is to distil purified nitre in a glass retort. But Scheele also obtained it from “calx of silver” (silver carbonate) prepared from silver nitrate and “alkali of tartar” (potassium carbonate); during this process he got aerial acid, which had been present originally in the alkali of tartar; but it was easily removed by means of milk of lime. Similarly, “calx of gold,” obtained from a solution of gold with “alkali of tartar,” gave “fire-air” when heated; but no aerial acid, for that air escapes during the precipitation of the “calx.” The brown-red precipitate obtained by adding “alkali of tartar” to “corrosive sublimate” (potassium carbonate to mercuric chloride, giving a basic carbonate of mercury and potassium chloride) yielded a mixture of fire-air and aerial acid when heated. But if the “calx of mercury” had been prepared by means of the “acid of nitre,” or in modern language by heating mercuric nitrate, a pure “fire-air,” unmixed with “aerial acid,” was the product. And lastly, arsenic acid, when heated, gave ordinary white arsenic together with “fire-air.” This fire-air was completely absorbed by “liver of sulphur” (a polysulphide of potassium, formed by heating together potassium carbonate and sulphur); and a mixture of four parts of “fire-air” with fourteen parts of “vitiated air” lost the whole of its fire-air on standing for fourteen days in contact with liver of sulphur. Dippel’s animal oil, and burning phosphorus, charcoal, and sulphur, all absorbed “fire-air”--completely if it was pure, incompletely if it was mixed with “vitiated air”; in short, the identity of “fire-air” prepared from calces, etc., with that in ordinary air was completely established. As “vitiated air” is lighter than ordinary air, it follows that “fire-air” must be heavier; and experiment proved this to be the case.