Epsom salts, common name for magnesium sulfate heptahydrate, MgSO₄·7H₂O, a water-soluble bitter-tasting compound that occurs as white or colorless needle-shaped crystals. It was first prepared from the waters of mineral springs at Epsom, England; it also occurs as the mineral epsomite. Epsom salts is used medicinally as a purgative; it is also used in leather tanning, mordant dyeing, and as a filler in cotton goods and paper.