Arcanum duplicatum

Arcanum duplicatum (potassium sulfate), also known as panacea duplicata, in pre-modern medicine, is a preparation of the caput mortuum, or the remaining residue from distillation or sublimation, of aqua fortis, by dissolving it in hot water, filtering, and evaporating it to a cuticle. It is then left to shoot. It was used as a diuretic and sudorific.

The recipe was purchased for 500 dollars by the Duke of Holstein. Schroder, the prince's physician, wrote wonders of its great uses in hypochondriacal cases, continued and intermitting fevers, stone, scurvy, etc.