noun

• (now chiefly historical) The substance which remains after a metal or mineral has been thoroughly burnt, once seen as being the essential substance left after the expulsion of phlogiston, but now recognised as being the metallic oxide (or, in some cases, the metal in a state of sublimation).
  "[S]ome ladies apply to what are termed coſmetics under various names, which crowd the newspapers. Of theſe the white has deſtroyed the health of thouſands; a calx, or magiſtery, of biſmuth is ſuppoſed to be ſold in the ſhops for this purpoſe; but it is either, I am informed, in part or entirely white lead or ceruffa. […] The real calx of biſmuth would probably have the ſame ill effect."