Lavoisier’s next research, communicated to the Academy in 1775, and published in 1778, was entitled “On the Nature of the Principle which combines with Metals during their Calcination, and which increases their Weight.” In this he describes experiments showing that when metallic calces are converted into metals by heating with charcoal, a quantity of fixed air is expelled; and here for the first time he points out that _fixed air is a compound of carbon with the elastic fluid contained in the calx_. He then describes the preparation of oxygen by Priestley’s process of heating red oxide of mercury (_mercurius precipitatus per se_), and shows that the red oxide, when heated with charcoal, manifests the properties of a true calx, inasmuch as metallic mercury is formed, and a large quantity of fixed air is produced. His next paper, which appeared in 1777 in the _Mémoires_ of the Academy, deals with the combustion of phosphorus; and here he recapitulates Rutherford’s experiments, and shows that one-fifth of the air disappears, and that the residue, to which he gave the name “mouffette atmosphérique,” is incapable of supporting combustion. It will be remembered that Rutherford named this residue “phlogisticated air,” inasmuch as he imagined it to have absorbed phlogiston from the burning phosphorus; Scheele, too, had made a similar experiment with a similar result. From these observations, Lavoisier concluded that air consists of a mixture or compound of two gases, one capable of absorption by burning bodies, the other incapable of supporting combustion. This paper was immediately followed by another, also published in 1777. Its title is, “On the Combustion of Candles in Atmospheric Air, and in Air eminently respirable.” In this memoir he distinguishes between four kinds of air:--1, Atmospheric air, in which we live and which we breathe. 2, Pure air, alone fit for breathing, constituting about one-fourth of atmospheric air, and termed by Priestley “dephlogisticated air.” 3, Azotic gas, identical with Rutherford’s “mephitic air,” and of which the properties were then unknown. 4, Fixed air, which he proposed to call “acide crayeux,” or acid of chalk, discovered twenty-five years previously by Black.