To remove phlogiston from a substance; used in 18th-century chemistry based on an incorrect theory about combustion.
From de- + phlogiston (from Greek phlogistos, 'inflammable') + -ate (verb suffix). Used during the phlogiston theory era (1660s-1780s) before the oxygen theory replaced it.
Phlogiston was a ghost element that scientists believed was released during burning—they literally couldn't see it but were convinced it existed, so dephlogistication sounded like serious science until oxygen was discovered.
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