<P> After the publication of the first volume of Experiments and Observations, Priestley undertook another set of experiments . In August 1774 he isolated an "air" that appeared to be completely new, but he did not have an opportunity to pursue the matter because he was about to tour Europe with Shelburne . While in Paris, however, Priestley managed to replicate the experiment for others, including Antoine Lavoisier . After returning to Britain in January 1775, he continued his experiments and discovered vitriolic acid air (sulphur dioxide, SO). In March he wrote to several people regarding the new "air" that he had discovered several months earlier . One of these letters was read aloud to the Royal Society, and he published a paper in Philosophical Transactions titled "An Account of further Discoveries in Air ." Priestley called the new substance "dephlogisticated air" and described it as "five or six times better than common air for the purpose of respiration, inflammation, and, I believe, every other use of common atmospherical air ." He had discovered oxygen gas (O). As revised for Experiments and Observations, his paper begins: </P> <P> The contents of this section will furnish a very striking illustration of the truth of a remark which I have more than once made in my (natural) philosophical writings...that more is owing to what we call chance--that is, philosophically speaking, to the observations of events rising from unknown causes than to any proper design or preconceived theory in this business....For my own part, I will frankly acknowledge that at the commencement of my experiments recited in this section I was so far from having formed any hypothesis that led to the discoveries I made in pursuing them that they would have appeared very improbable to me had I been told of them; and when the decisive facts did at length obtrude themselves upon my notice it was very slowly, and with great hesitation, that I yielded to the evidence of my senses . (emphasis Priestley's) </P> <P> Priestley assembled his oxygen paper and several others into a second volume of Experiments and Observations on Air and published it in 1776 . He does not emphasise his discovery of "dephlogisticated air" (leaving it to Part III of the volume) but instead argues in the preface how important such discoveries are to rational religion . His paper narrates the discovery chronologically, relating the long delays between experiments and his initial puzzlements . Thus, it is difficult to determine when exactly Priestley "discovered" oxygen . Such dating is significant as Lavoisier and Swedish pharmacist Carl Wilhelm Scheele both have strong claims to the discovery of oxygen as well, Scheele having been first to isolate the gas (although he published after Priestley) and Lavoisier having been first to describe it as purified "air itself entire without alteration" (not "dephlogisticated air"). </P> <P> In this section, a list of all Priestley's scientific books on Airs has been compiled . The list doesn't include any of the several scientific papers, that he also wrote to various journals on the subject (see: List of works by Joseph Priestley). </P>

Who discovered the different gases in the air and when