Soft water forms abundant lather with soap, but hard water does not. Hard water contains compounds of calcium or magnesium. Rainwater caught off a clean roof is soft, being almost entirely free from chemical impurities. But soft water found in clean roofs has in it some carbonic acid gas, which though does not make it hard, enables the water to dissolve calcium and magnesium easily. This process accounts for the hardness of water drawn from deep wells, which is rainwater that has penetrated the ground, dissolving chemicals on the way. Hard water can be softened to a certain extent by boiling, which deposits some of the dissolved matter on the inside of the vessel; and more thoroughly by treating with chemicals.