Magnesium sulfate (or sulphate) is a chemical compound containing magnesium and sulfate, with the formula MgSO4. It is often encountered as the heptahydrate, MgSO4·7H2O, commonly called Epsom salts. Anhydrous magnesium sulfate is used as a drying agent. Since the anhydrous form is hygroscopic (readily absorbs water from the air) and therefore harder to weigh accurately, the hydrate is often preferred when preparing solutions, for example in medical preparations. Epsom salts have traditionally been used as a component of bath salts.
Another question. You have a water softener?