Phlogiston

Phlogiston is an obsolete chemical theory that posited the existence of a fire-like element, phlogiston, contained within many combustible substances. The concept was developed in the 17th century by Johann Joachim Becher and refined by Georg Ernst Stahl. According to the theory, substances that burn release phlogiston into the surrounding air, leaving behind a calx, or oxide, as the burnt residue. The framework was used to explain a range of phenomena, including why metals form calxes and how air participates in combustion.

To account for observed mass changes during burning and calcination, phlogiston was thought to be transferred between the burning material and the air. In some formulations, air was described as a mixture that could be “dephlogisticated” or “phlogisticated” depending on whether it had absorbed or released phlogiston. This vocabulary reflected the attempt to reconcile burning with conservation of mass, though the explanations differed from modern understanding.

The phlogiston theory began to unravel in the late 18th century, especially through the work of Antoine Lavoisier and his collaborators. Experiments showed that combustion involves a gas from the atmosphere combining with the burning substance (oxygen), and that mass is conserved overall. Lavoisier’s oxidation theory, which stated that burning is a process of chemical combination with oxygen, ultimately replaced phlogiston. The term and concept remain of historical interest as an early step in the development of modern chemistry.