The former experiment had been performed by Scheele and Priestley, who had named the gas " phlogisticated air "; Lavoisier subsequently named it oxygen, regarding it as the " acid producer " (OE, sour).
Chemical literature was full of the phlogistic modes of expression - oxygen was '" phlogisticated air," nitrogen " dephlogisticated air," &c. - and this tended to retard its promotion.
In his memoir of 1785 he writes: "As far as the experiments hitherto published extend, we scarcely know more of the phlogisticated part of our atmosphere than that it is not diminished by lime-water, caustic alkalies, or nitrous air; that it is unfit to support fire or maintain life in animals; and that its specific gravity is not much less than that of common air; so that, though the nitrous acid, by being united to phlogiston, is converted into air possessed of these properties, and consequently, though it was reasonable to suppose, that part at least of the phlogisticated air of the atmosphere consists of this acid united to phlogiston, yet it may fairly be doubted whether the whole is of this kind, or whether there are not in reality many different substances confounded together by us under the name of phlogisticated air.
I therefore made an experiment to determine whether the whole of a given portion of the phlogisticated air of the atmosphere could be reduced to nitrous acid, or whether there was not a part of a different nature to the rest which would refuse to undergo that change.
I then, in order to decompound as much as I could of the phlogisticated air [nitrogen] which remained in the tube, added some dephlogisticated air to it and continued the spark until no further diminution took place.
Having by these means condensed as much as I could of the phlogisticated air, I let up some solution of liver of sulphur to absorb the dephlogisticated air; after which only a small bubble of air remained unabsorbed, which certainly was not more than of the bulk of the dephlogisticated air let up into the tube; so that, if there be any part of the dephlogisticated air of our atmosphere which differs from the rest, and cannot be reduced to nitrous acid, we may safely conclude that it is not more than 7a part of the whole."
Although, as was natural, Cavendish was satisfied with his result, and does not decide whether the small residue was genuine, it is probable that his residue was really of a different kind from the main bulk of the "phlogisticated air," and contained the gas afterwards named argon.
In another experiment he fired, by the electric spark, a mixture of hydrogen and oxygen (dephlogisticated air), and found that the resulting water contained nitric acid, which he argued must be due to the nitrogen present as an impurity in the oxygen ("phlogisticated air with which it [the dephlogisticated air] is debased").
Further, remarking that little was known of the phlogisticated part of our atmosphere, and thinking it might fairly be doubted "whether there are not in reality many different substances confounded together by us under the name of phlogisticated air," he made an experiment to determine whether the whole of a given portion of nitrogen (phlogisticated air) of the atmosphere could be reduced to nitric acid.
Thus oxygen gas, at the end of the 18th century, was known as dephlogisticated air, nitrogen or azote as phlogisticated air, hydrogen as inflammable air, carbonic acid gas as fixed air.
His last chemical paper, published in 1788, on the "Conversion of a mixture of dephlogisticated and phlogisticated air into nitrous acid by the electric spark," describes measures he took to authenticate the truth of the experiment described in the 1785 paper, which had "since been tried by persons of distinguished ability in such pursuits without success."