Saltpetre

saltpetre, also spelled Saltpeter, also called Nitre, or Niter, any of three naturally occurring nitrates, distinguished as (1) ordinary saltpetre, or potassium nitrate, KNO₃; (2) Chile saltpetre, cubic nitre, or sodium nitrate, NaNO₃; and (3) lime saltpetre, wall saltpetre, or calcium nitrate, Ca(NO₃)₂. These three nitrates generally occur as efflorescences caused by the oxidation of nitrogenous matter in the presence of the alkalis and alkaline earths.