Phlogiston

Phlogiston (Greek, phlogistos,”flammable”), hypothetical substance, representing flammability, postulated in the late 17th century by the German chemists Johann Becher and Georg Stahl to explain the phenomenon of combustion. According to the phlogiston theory, every substance capable of undergoing combustion contains phlogiston, and the process of combustion is essentially the process of losing phlogiston. Because it was known that a substance such as mercury becomes heavier during combustion, it was assumed that phlogiston had negative weight; that is, the substance became heavier when it lost phlogiston. Substances such as coal and sulphur were believed to be composed almost entirely of phlogiston. In experiments with the substance now known as oxygen, the English chemist Joseph Priestley discovered its properties of supporting combustion, but he described the gas as dephlogisticated air. The phlogiston theory was discounted by the French chemist Antoine Lavoisier, who held that combustion is a process in which oxygen combines with another substance. By 1800 virtually all chemists had recognized the validity of Lavoisier's experiment, and the phlogiston theory was discredited.