Of the true nature of inflammable air, Priestley, as we have more than once had occasion to point out, had only the vaguest notions.
“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.”
After these theoretical speculations, “in which,” he says, “I fear I have not communicated much light, though it is as much as I have been able to get,” Priestley proceeds to make some observations relating to phlogiston, “the existence of which is at present a great subject of discussion with philosophers; some maintaining that there is no such thing, and others holding the doctrine of Stahl on the subject.”