Joseph Priestley

On August 1, 1774, English chemist Joseph Priestley discovered oxygen by isolating it in its gaseous state. An excerpt from the article:

"described the pneumatic trough for gathering gases over water, Priestley began examining all the “airs” that might be released from different substances. Many, following Aristotle’s teachings, still believed there was only one “air.” By clever design of apparatus and careful manipulation, Priestley isolated and characterized eight gases, including oxygen—a record not equaled before or since. In addition, he contributed to the understanding of photosynthesis and respiration.

Priestley fought a long-running battle with Lavoisier and his followers over how to interpret the results of experiments with gases. Priestley interpreted them in terms of phlogiston—the hypothetical principle of flammability that was thought to give metals their luster and ductility and was widely used in the early 18th century to explain combustion, calcination, smelting, respiration, and other chemical processes. Proponents of phlogiston did not consider it to be a material substance, and so it was therefore unweighable. Priestley gave qualitative explanations of these phenomena, talking, for example, about oxygen as 'dephlogisticated air.'”