“Inflammable air,” he says, “seems now to consist of water and inflammable air, which however seems extraordinary, as the two substances are hereby made to involve each other, one of the constituent parts of water being inflammable air, and one of the constituent parts of inflammable air being water; and therefore, if the experiments would favour it (but I do not see that they do so) it would be more natural to suppose that water, like fixed air, consists of phlogiston and dephlogisticated air in some different mode of combination.” That Priestley to the last imagined that the various kinds of inflammable air known to him were at bottom one and the same substance, modified or affected by other substances, accidental and unessential, might be proved by a number of passages. He says with respect to inflammable air generally:— “There is an astonishing variety in the different kinds of inflammable air, the cause of which is very imperfectly known. The lightest, and therefore, probably, the purest kind seems to consist of phlogiston and water only. But it is probable that _oil_, and that of different kinds, may be held in solution in several of them, and be the reason of their burning with a lambent flame, and also of their being so readily resolved into fixed air when they are decomposed with dephlogisticated air; though _why_ this should be the case I cannot imagine.” _Nitrous air_ (nitric oxide) he conceives to be a combination of a dephlogisticated nitrous air and phlogiston, and that by adding to it dephlogisticated air and water it is converted into nitrous acid. _Dephlogisticated nitrous air_ (nitrous oxide) he conceives may, like dephlogisticated air, be an elementary substance and to be formed by depriving nitrous air of its phlogiston. The various _acid airs_ (_e.g._, marine acid air, vitriolic acid air, etc.) consist of the peculiar acids as vapours combined with phlogiston. The _Alkaline air_ (ammonia) he thought to consist of inflammable air and phlogisticated air (nitrogen), “or of something capable of being converted into phlogisticated air.... That water enters into the composition of alkaline air seems necessary to be admitted, because it is decomposed into inflammable air, which I cannot help thinking necessarily requires water. It seems, however, clearly to be inferred ... that there is no occasion to admit the _alkaline principle_ into the number of _elements_; the _alkalinity_, as I may say, some way or other, arising from phlogiston, or phlogisticated air, as _acidity_ arises from dephlogisticated air.”