Athanor or 'Slow Harry', a self-feeding furnace maintaining a constant temperature. Centre: 1) Athanor or 'Slow Harry': 2) side chambers containing reagents: 3) glass receivers. Back: 5) furnace heating retort. Bottom: 7) Long furnace. This plate shows the distillation of nitirc acid (also known as Aqua Fortis or Parting Acid) which was used in the refining and assaying of metals. From 1683 English edition of Beschreibung allerfurnemisten mineralischen Ertzt, Lazarus Ercker, (Prague,1574). Copperplate engraving.