Rochelle salt, colorless to blue-white orthorhombic crystalline salt with a saline, cooling taste. It is also called Seignette salt after Pierre Seignette, an apothecary of La Rochelle, France, who was the first to make it (c.1675). Chemically, it is potassium sodium tartrate, KNa (C₄H₄O₆)·4H₂O. It is soluble in water and slightly soluble in alcohol, melts at about 75°C;, has specific gravity 1.79, and exhibits double refraction. It is used in medicine as a mild purgative, often in the form of Seidlitz powders. It is an ingredient of Fehling's solution. It is used in silvering mirrors. Crystals of Rochelle salt are easily grown and are used in piezoelectric devices, e.g., crystal microphones and phonograph pickup cartridges (see piezoelectric effect).