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Webster 1828 Edition

Verb.

[Gr. Burnt, inflammable.] To deprive of phlogiston, or the supposed principle of inflammability.

dephlogisticate ‎(third-person singular simple present dephlogisticates, present participle dephlogisticating, simple past and past participle dephlogisticated)

To take away the phlogiston, i.e. ability to burn, which is now understood to be chemically impossible