Distinguished pyrolusite as the calx of a new metal.
From the en.wikipedia.org
Calx was later replaced by oxyd.
From the en.wikipedia.org
One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx.
From the en.wikipedia.org
More examples
Calcium oxide: a white crystalline oxide used in the production of calcium hydroxide
Calx is a residual substance, sometimes in the form of a fine powder, that is left when a metal or mineral combusts or is calcinated due to heat.
The substance which remains after a metal or mineral has been thoroughly burnt, seen as being the essential substance left after the expulsion of phlogiston; the oxide left after calcination of a metal; calcium oxide
Any powder obtained by strongly heating a substance in air. Almost always an oxide.
It was believed to be an ash-like substance which is left when the phlogiston leaves a metal. Although we now believe that metals form metal oxides when they burn, we no longer believe the phlogiston theory.