It is said that Epsom salts have been found in the Alps, and in Switzerland, under a powdery form, and sometimes even in masses, or a state of incrustation on stones and rocks. They are, however, chiefly found dissolved in mineral waters, and particularly in those at Epsom in Surrey, and Sedlitz in Bohemia. Their taste is bitter and unpleasant. So little are they affected by exposure to the air, that the Abbé Haüy kept some by him for more than twelve years without any sensible alteration. These salts are much used in medicine, and are sometimes manufactured from the waters of Epsom (290) and Sedlitz (289), but more frequently, and in much greater abundance, from sea─water. The _magnesia_ of the shops is prepared by dissolving Epsom salts in water, and adding to the solution half their weight of potash (205). The substance that sinks to the bottom is magnesia; and this, washed with a sufficient quantity of water and dried, has the appearance of a light, soft, and white powder, of insipid taste. Magnesia is used in medicine, both in a simple state and when calcined or burned. It is also employed in some chemical processes; and is in considerable request in the manufacture of enamel and porcelain. If putrid water be agitated with a small quantity of magnesia, it will lose a considerable portion of its bad taste and smell. 200. SODA FAMILY. Soda, like potash (205), is an extremely caustic alkali (42). It has a greyish white colour, and agrees exactly with potash (205) in taste, smell, and corrosive quality, but it is not so heavy. In a mineral state soda has hitherto been found only in combination with some acid. Common salt (202) is a compound of soda with muriatic acid (29). The soda of commerce is obtained from sea─water; and from the ashes of different kinds of plants that grow on the sea─shores, but particularly from that called _salsola soda_, which is found in great abundance on the coasts of the southern parts of Europe; and from which it has its name. It is sometimes called _barilla_, from the salsola soda being so denominated in Spain. This alkali is of essential use in the arts. When melted with flint or sand, it forms glass, and answers much better for this purpose than potash. In conjunction with oil and lime, it is employed in the manufacture of soap; and it is used as a substitute for soap in the cleaning and bleaching of linen, flannels, and worsted goods. If a weak solution of soda be poured into foul bottles, or casks in which wine has long been kept, it will cleanse them. It may also be successfully used for the cleansing of vessels in which milk has become acid. Saddles, bridles, or boot─tops, may be effectually cleaned by means of this liquor, and restored nearly to their original colour and appearance.