Phlogiston theory
"Phlogiston": mysterious something.
Yielded up by combustible substances on burning.
The more phlogiston the more combustible.
Ordinary air contained some phlogiston.
Placed there by countless past combustions.
Mercuric oxide residue (calx) of previously heated mercury.
Contained no phlogiston.
Mercuric oxide heated in a closed vessel.
Absorbed all surrounding air's phlogiston.
Remaining air became "dephlogisticated".
Hence purer than ordinary air.
Theory completely backwards from truth.
Phlogisticated: de-oxygenated.
De-phlogisticated: oxygenated.
Engaged greatest scientific minds of 18th century.