Rochelle Salt
Rochelle Salt, the common name of potassium sodium tartrate. It is a salty, slightly bitter substance that occurs as clear or bluish crystals and as a white powder. Rochelle salt is used chiefly as a laxative, particularly in Seidlitz powders. It is also used in microphones and loudspeakers, in silvering mirrors, and to make Fehling's solution (which is used in urinalysis).
Formula: KNaC4H4O6. 4H2O.