Crystallization of Saltpetre (Nitre, Potassium Nitrate, or KN03. Saltpetre is the principal ingredient in gunpowder, and is still used in the preservation of some foods.  In medicine it was used internally as a diuretic, but now is only used externally for a number of conditions, such as asthma. From 1683 English edition of  'Beschreibung allerfurnemisten mineralischen Ertzt', Lazarus Ercker, (Prague, 1574). Copperplate engraving.
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