The world recalls Priestley best as the man who discovered oxygen, the active ingredient in our planet’s atmosphere. In the process, he helped dethrone an idea that dominated science for 23 uninterrupted centuries: Few concepts “have laid firmer hold upon the mind,” he wrote, than that air “is a simple elementary substance, indestructible and unalterable.”
Priestley fought with Antoine Lavoisier and his followers over how to interpret the results of experiments with gases. Priestley interpreted them in terms of phlogiston. That’s 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. Priestley gave qualitative explanations of these phenomena, talking, for example, about oxygen as “dephlogisticated air.” . In a series of experiments culminating in 1774, Priestley found that “air is not an elementary substance, but a composition,” or mixture, of gases. This includes the colorless and highly reactive gas he called “dephlogisticated air,” to which the great French chemist Antoine Lavoisier would soon give the name “oxygen.” It’s highly reactive due to the amount of electrons in its valence shell and plays a major part in chemical reactions required for daily life, like respiration, photosynthesis, and combustion.