Burning substances release a fire-like material called phlogiston.

Phlogiston—a hypothetical fire-like substance thought to leave material as it burned—gave 18th-century chemists a common way to connect burning, calcination, smelting, and respiration. A combustible substance was thought to contain phlogiston and release it during burning; what remained was the dephlogisticated residue. The theory could accommodate a surprising range of laboratory observations, which is why it was not discarded simply because later textbooks find it strange.

Its hardest problem was quantitative. Metals often gained mass when heated into what chemists called calxes, awkward behavior for a process supposedly involving the loss of material. Antoine Lavoisier used sealed vessels and careful weighing to follow matter through reactions and argued that combustion instead involved combination with a component of air—oxygen. His framework tied burning to conservation of mass and a new chemical nomenclature.

The change was not instantaneous, and early oxygen chemistry was not yet modern reaction theory. But phlogiston ceased to be needed once gases and mass balance could explain the phenomena more coherently.