“Fluctuating, however, as the present state of this branch of knowledge is, I do not think that I can, on this occasion, entirely decline giving some observations of a theoretical nature, and though I cannot pretend to perform the whole of my promise, I shall give a summary view of what appears to me to be the constituent parts of all the kinds of air with which we are acquainted, and a more particular account of the hypothesis concerning phlogiston, which is at present more an object of discussion than anything else of a theoretical nature.” Priestley then passes in review all the “airs” of which the chemistry of his time had any knowledge, giving the _elements_ or constituent principles of which he imagined them to be composed. The only kind of air that he thinks to be properly _elementary_, and to consist of a simple substance, is _dephlogisticated air_, with possibly the addition of the principle of heat, which, as it is not probable that it adds to the _weight_ of bodies, can hardly be called an _element_ in their composition. “Dephlogisticated air appears to be one of the elements of water, of fixed air, of all the acids, and of many other substances which, till lately, have been thought to be simple. The air of the atmosphere, exclusive of a great variety of foreign impregnations, appears to consist of dephlogisticated and phlogisticated air.” As regards _phlogisticated air_—the mephitic air of Rutherford, the azote of Lavoisier, the nitrogen of Chaptal—Priestley, reasoning from Cavendish’s work, concluded that it was probably not elementary, but “that it consists of nitrous acid and phlogiston; this acid having always been produced by decomposing it with ... dephlogisticated air.” He is conscious, however, of the insufficiency of this hypothesis, and suggests “that the _acid principle_ is supplied by the dephlogisticated air, while the nitrous air gives the base of the nitrous acid and phlogiston; and then this [phlogisticated] air may perhaps be considered as phlogiston combined not with all the necessary elements of nitrous acid, but only what may be called the base of it, _viz._, the dephlogisticated nitrous vapour, or something which when united to dephlogisticated air will constitute nitrous acid.” “_Fixed air_ (carbonic acid) seems to be a compound of phlogiston and dephlogisticated air.” In other words, carbonic acid and water have, according to Priestley, “the same elementary composition.” “It is something remarkable that two substances so different from each other as _fixed air_ and _water_ should be analysed into the same principles. But there is this difference between them, that water is the union not of pure phlogiston but of inflammable air and dephlogisticated air.” Of the true nature of _inflammable air_, Priestley, as we have more than once had occasion to point out, had only the vaguest notions.