a soluble white crystalline substance, MgSO4, which occurs in various minerals such as epsomite and kieserite, and is used medicinally as a laxative and anticonvulsant, and as a fertilizer, a white soluble compound, MgSO4, existing as the anhydrous compound (rhombic; r.d. 2.66; decomposes at 1124°C) and in hydrated crystalline forms