The phlogiston theory is a now discredited 17th century hypothesis regarding combustion.

The theory was invented by J. J. Becher late in the 17th century and extended and popularized by Georg Ernst Stahl, who declared the rusting of metal to be a combustion process. The theory states that all flammable materials contain phlogiston (derived noun form of the Greek phlogistos, meaning "inflammable"), a substance without color, odor, taste, or weight that is liberated in burning. Once burned, the "dephlogisticated" substance was now in its "true" form, the calx.

The theory is somewhat similar to the notion of alchemy, that fire is one of the four elements (water, air and earth being the other three) which are locked into a substance.

"Phlogisticated" substances are those that contain phlogiston and are "dephlogisticated" when burned. Since any substance could be observed to burn for only a limited time with limited air (for instance in a sealed container), air was thought to have a specific capacity for phlogiston. For this reason, the residue of air left after burning (actually a mixture of nitrogen and carbon dioxide), was sometimes referred to as "phlogisticated air", having taken up all of the phlogiston. Likewise, when oxygen was first discovered it was thought to be "dephlogisticated air", capable of combining with more phlogiston and thus supporting combustion.

Eventually, experiments revealed problems, including the fact that some metals gained weight when they burned, even though they were supposed to have lost phlogiston. Still, phlogiston remained the dominant theory until Antoine Laurent Lavoisier showed that combustion requires oxygen, solving the weight paradox and setting the stage for a new theory of what happens when objects burn.

See also

• Science – Discoveries of the chemical elements, Combustion (Calx)
• People – Antoine Laurent Lavoisier Joseph Priestley, Georg Ernst Stahl, J. J. Becher, Daniel Rutherford, Mikhail Lomonosov
• Theories / Beliefs – Caloric theory, Caloric, Eliminative materialism, Obsolete scientific theory, List of alternative, speculative and disputed theories