Phlogiston theory

The phlogiston theory is a now discredited 17th century hypothesis regarding combustion. It states that all flammable materials contain phlogiston (derived noun form of the Greek for "to burn"), 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 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. "Phlogisticated" substances are those that contain phlogiston and are "dephlogisticated" when burned; for this reason, the residue of air left after burning (actually nitrogen), was sometimes referred to as "dephlogisticated air". The ash of the burned material is supposed to be the true material. The theory received strong and wide support throughout a large part of the 18th century. Quantitative measurements revealed problems with the phlogiston theory: when a metal burned, it was supposed to lose phlogiston. However, the metal ash could be shown to weigh more than the metal did before it had lost phlogiston: this implied that the removed phlogiston must have weighed not zero, but less than zero.

It was the work of Antoine Laurent Lavoisier, who revealed that combustion requires oxygen; this solved the weight problem, as the metal gained oxygen as it burned and so naturally gained mass. Joseph Priestley, however, defended phlogiston theory throughout his lifetime. Henry Cavendish remained doubtful, but most other chemists of the period, including C. L. Berthollet, rejected the theory.