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