Experiments and Observations on Different Kinds of Air

''Experiments and Observations on Different Kinds of Air'' (1774–86) is a six-volume work published by 18th-century British polymath Joseph Priestley which reports a series of his experiments on "airs" or gases, most notably his discovery of oxygen gas (which he called "dephlogisticated air").

Airs

While working as a companion for Lord Shelburne, Priestley had a great deal of free time to engage in scientific investigations. The Earl even set up a laboratory for him. Priestley's experiments during his years in Calne were almost entirely confined to "airs" and from this work emerged his most important scientific texts: the six volumes of ''Experiments and Observations on Different Kinds of Air''. These experiments helped repudiate the last vestiges of the theory of four elements; as one early biographer writes: "taken collectively, riestleydid more than those of any one of his contemporaries to uproot and destroy the only generalisation by which his immediate predecessors had sought to group and connect the phenomena of chemistry", however "he was wholly unable to perceive this fact." Priestley's work on "airs" is not easily classified. As historian of science Simon Schaffer points out, it "has been seen as a branch of physics, or chemistry, or natural philosophy, or some highly idiosyncratic version of Priestley's own invention." Also, the volumes were both a scientific and a political enterprise for Priestley; he argued in them that science could destroy "undue and usurped authority," writing that the government has "reason to tremble even at an air pump or an electrical machine."

Priestley's first volume of ''Experiments and Observations on Different Kinds of Air'' outlined several important discoveries: experiments that would eventually lead to the discovery of photosynthesis and the discovery of several airs: "nitrous air" (nitric oxide, NO), "vapor of spirit of salt" (later called "acid air" or "marine acid air"; anhydrous hydrochloric acid, HCl), "alkaline air" (ammonia, NH₃), "diminished" or "dephlogisticated nitrous air" (nitrous oxide, N₂O), and "dephlogisticated air" (oxygen, O₂). Priestley also developed the "nitrous air test", which tested for the "goodness of air": using a "pneumatic trough", he would mix nitrous air with a test sample, over water or mercury, and measure the decrease in volume—the principle of eudiometry.Fruton, 20; 29 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.

Discovery of oxygen

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