The phlogiston theory was a widely held scientific theory in the 17th and 18th centuries that postulated the existence of a substance called phlogiston that was released during combustion. The theory held that all combustible substances (e.g., wood, coal, oil) contained phlogiston and that when ignited, they released the phlogiston, resulting in fire.

Supporters of the phlogiston theory used it to explain various observations and phenomena related to combustion and chemical reactions. Some key beliefs associated with the phlogiston theory included:

- Combustion and Release of Phlogiston: When a combustible substance burned, it released phlogiston. As more phlogiston was released, the flame became brighter and the reaction faster.

- Conservation of Phlogiston: The total amount of phlogiston in a closed system is always conserved. It cannot be created or destroyed but can transfer between substances during chemical reactions.

- Inert Phlogiston: Phlogiston was considered weightless and did not chemically react with other substances. Instead, it acted as a passive element released during combustion.

- Calcination: The process of heating metals in air (often resulting in oxide formation) was also explained using the phlogiston theory. Metals were viewed as rich in phlogiston; upon calcination, phlogiston is lost, leaving behind the metal's "calx" (oxide).

- Respiration and Phlogiston: Some supporters extended the theory to life processes. They suggested that animals inhaled vital air (fresh air high in phlogiston content) and exhaled dephlogisticated air which lacked vitality but was heavier on account of phlogiston's weight being added to inhaled vital air.

The phlogiston theory was widespread until the late 1700s. However, experimental evidence accumulated contradicting and challenging the theory's tenets. Notably, experiments and theoretical works of scientists like Georg Stahl, Carl Wilhelm Scheele, Antoine Lavoisier, Joseph Priestley, and Henry Cavendish gradually led to the development of an improved and eventually replacement of the phlogiston theory by the caloric theory, and further, to more robust understanding through oxygen theory (proposed by Lavoisier) , which laid foundations for modern chemistry.