After buying the Alchemy Machine from the Fellow Merchant npc, just add any potion to any of the bottles and pull the lever. Any recipe that isn't one resulting in a reagent (e.g. Nigredo/Albedo) or a salt (e.g. Void Salt/Moon Salt) will result in an "Unknown Substance" when the lever's pulled. It's useless, though. Should only do this for the achievement.