After a small history of the study of airs, he explained his own experiments in an open and sincere style: "whatever he knows or thinks he tells: doubts, perplexities, blunders are set down with the most refreshing candour." He also invented and described cheap and easy-to-assemble experimental apparatus. His colleagues therefore believed that they could easily reproduce Priestley's experiments to verify them or to answer the questions that had puzzled him. Although many of his results puzzled him, Priestley used phlogiston theory to resolve the difficulties. This theory, however, led him to conclude that there were only three types of "air": "fixed", "alkaline", and "acid". Priestley ignored the burgeoning chemistry of his day, indeed dismissing it in these volumes. Instead, he focused on gases and the "changes in their sensible properties", as had natural philosophers before him. He isolated carbon monoxide (CO) but seems not to have realised that it was a separate "air" from the others that he had discovered. 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, SO2). 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 (O2). As revised for "Experiments and Observations", his paper begins:  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. …