The phlogiston theory is a superseded scientific theory that postulated that a fire-like element called phlogiston is contained within combustible bodies and released during combustion. The name comes from the Ancient Greek φλογιστόν phlogistón (burning up), from φλόξ phlóx (flame). It was first stated in 1667 by Johann Joachim Becher and then put together more formally by Georg Ernst Stahl. The theory attempted to explain processes such as combustion and rusting, which are now collectively known as oxidation.

Phlogiston legacy

In the 1770s Carl Scheele was one of the first to identify hydrogen and oxygen (as well as molybdenum, tungsten, barium and chlorine), but because he followed the phlogiston theory, assumed that hydrogen was composed of phlogiston (a reducing principle lost when objects were burned) plus heat. Scheele speculated that his fire air or oxygen (which he found the active part of air, estimating it to compose one quarter of air) combined with the phlogiston in objects to produce either light or heat (light and heat were presumed to be composed of differing proportions of phlogiston and oxygen).

Joseph Priestley independently isolated oxygen from mercury oxide, which he called dephlogisticated air, rather than believing it to be a new element.

In France Antoine Lavoisier (1743-1794) performed similar experiments with the same substances. He got the same results as Priestley, but he was seeking a new explanation of combustion, so he saw his results from a different perspective. Lavoisier suggested that rather than phlogiston being given off when a metal rusted, or a substance burned, a more simple explanation was that Priestley's new gas, which he called oxygen, was being absorbed from the air.

Also during this time Henry Cavendish recognised the elemental nature of hydrogen. In the 1780s he accurately measured the composition of air where he concluded that "common air consists of one part of dephlogisticated air [oxygen], mixed with four of phlogisticated [nitrogen]".

It was not until the twentieth century that the last legacy of phlogiston was explained away with the new theory of combustion where heat was revealed to be a form of energy.