Priestley tells us that at this time he knew very little chemistry, but he thinks that this was a good thing, else he might not have been led to make so many new discoveries as he did afterwards make.
Experimenting with fixed air, he found that water could be caused to dissolve some of the gas. In 1772 he published a pamphlet on the method of impregnating water with fixed air; this solution of fixed air in water was employed medicinally, and from this time we date the manufacture of artificial mineral waters.
The next six years of Priestley's life (1773-1779) are very important in the history of chemistry; it was during these years that much of his best work on various airs was performed. During this time he lived as a kind of literary companion (nominally as librarian) with the Earl of Shelburne (afterwards Marquis of Lansdowne.) His wife and family--he had now three children--lived at Calne, in Wiltshire, near Lord Shelburne's seat of Bowood. Priestley spent most of the summer months with his family, and the greater part of each winter with Lord Shelburne at his London residence; during this time he also travelled in Holland and Germany, and visited Paris in 1774.
In a paper published in November 1772, Priestley says that he examined a specimen of air which he had extracted from saltpetre above a year before this date. This air "had by some means or other become noxious, but," he supposed, "had been restored to its former wholesome state, so as to effervesce with nitrous air" (in modern language, to combine with nitric oxide) "and to admit a candle to burn in it, in consequence of agitation with water." He tells us, in his "Observations on Air" (1779), that at this time he was altogether in the dark as to the nature of this air obtained from saltpetre. In August 1774, he was amusing himself by observing the action of heat on various substances--"without any particular view," he says, "except that of extracting air from a variety of substances by means of a burning lens in quicksilver, which was then a new process with me, and which I was very proud of"--when he obtained from _red precipitate_ (oxide of mercury) an air in which a candle burned with a "remarkably vigorous flame." The production of this peculiar air "surprised me more than I can well express;" "I was utterly at a loss how to account for it." At first he thought that the specimen of _red precipitate_ from which the air had been obtained was not a proper preparation, but getting fresh specimens of this salt, he found that they all yielded the same kind of air. Having satisfied himself by experiment that this peculiar air had "all the properties of common air, only in much greater perfection," he gave to it the name of _dephlogisticated air_. Later experiments taught him that the same air might be obtained from red lead, from manganese oxide, etc., by the action of heat, and from various other salts by the action of acids.