Phlogiston, in early chemical theory, hypothetical principle of fire, of which every combustible substance was in part composed. In this view, the phenomena of burning, now called oxidation, was caused by the liberation of phlogiston, with the dephlogisticated substance left as an ash or residue.

Read More on This Topic chemistry: Phlogiston theory This shift was partly simple self-promotion by chemists in the new environment of the Enlightenment, whose vanguard glorified rationalism,...

Johann Joachim Becher in 1669 set forth his view that substances contained three kinds of earth, which he called the vitrifiable, the mercurial, and the combustible. He supposed that, when a substance burned, combustible earth (Latin terra pinguis, meaning “fat earth”) was liberated. Thus, wood was a combination of phlogiston and wood ashes. To this hypothetical substance Georg Ernst Stahl, at about the beginning of the 18th century, applied the name phlogiston (from Greek, meaning “burned”). Stahl believed that the corrosion of metals in air (e.g., the rusting of iron) was also a form of combustion, so that when a metal was converted to its calx, or metallic ash (its oxide, in modern terms), phlogiston was lost. Therefore, metals were composed of calx and phlogiston. The function of air was merely to carry away the liberated phlogiston.

The major objection to the theory, that the ash of organic substances weighed less than the original while the calx was heavier than the metal, was of little significance to Stahl, who thought of phlogiston as an immaterial “principle” rather than as an actual substance. As chemistry advanced, phlogiston was considered a true substance, and much effort was expended in accounting for the weight changes observed. When hydrogen, very light in weight and extremely flammable, was discovered, some thought it was pure phlogiston.

The phlogiston theory was discredited by Antoine Lavoisier between 1770 and 1790. He studied the gain or loss of weight when tin, lead, phosphorus, and sulfur underwent reactions of oxidation or reduction (deoxidation); and he showed that the newly discovered element oxygen was always involved. Although a number of chemists—notably Joseph Priestley, one of the discoverers of oxygen—tried to retain some form of the phlogiston theory, by 1800 practically every chemist recognized the correctness of Lavoisier’s oxygen theory.