MAGNESIA. A white, tasteless, earthy substance, usually obtained by ex posing its hydrated carbonate to a red heat. Its specific gravity is 2.3. It is almost insoluble ; but when moistened and put upon turmeric paper it reddens it : this sometimes depends upon a trace of lime. It is an oxide of a brilliant white metal, which has been called magnesium, and which may be obtained by heating chloride of magnesium with potassium : they act intensely upon each other, chlo ride of potassium is formed, and magne sium separates : it may be washed with water and dried. Heated to redness in the air, it burns with great brilliancy into magnesia, 12 parts of the metal combin with 8 of oxygen to form 20 of mag nesia. In commerce, pure magnesia is generally distinguished by the term cal cined magnesia i and the hydrated carbo nate of magnesia, obtained by precipitat ing a solution of sulphate of magnesia by carbonate of soda, and washing and dry ing the precipitate, goes by the name of magnesia, or magnesia alba. The chief use of magnesia and its carbonate is in medicine. Sulphate of magnesia is ob tained by evaporating the residue of sea water after the common salt has been se parated, or by adding sulphuric acid to bittern and evaporating, so as to obtain the resulting sulphate of magnesia. This salt is also obtained by the action of di lute sulphuric acid on magnesian lime stone, and it is not uncommon in mine ral waters': it was formerly procured from certain springs near Epsom, in Surrey, and was hence termed Epsom salt. It crystallizes in four-sided prisms with di hedral summits. Its crystals are soluble in their weight of water at 60°,_ and in three-fourths their weight at 212°. They melt when heated, and gradually lose their water of crystallization. They con
sist of 20 magnesia, 40 sulphuric acid, and 63 water. Ibis salt is a useful purgative in medicine, and is the chief source of the other forms of magnesia. All the mag nesian salts have a peculiar bitterish fla vor. Magnesia is found native in the state of hydrate and carbonate ; it exists as a compound part of several minerals, and many of them are soft or soapy to the touch.
The carbonate of magnesia may be made thus:— Dissolve four parts of sulphate of mag nesia, and three parts of subcarbonate of potash, separately, in twice their weight of water, and filter them. Then mix them with eight times their weight of boiling water. Boil and stir, and then stand till partly cool ; when, being strain ed through linen, the carbonate remains. Wash it and dry it gradually. It is the best anti-acid in the stomach, and the acid renders it purgative.
It is however liable to produce accumu lations in the bowels. Almost all the Epsom salts used in the United States is manufactured at Baltimore from the mag nesite and magnesian limestone found in Lancaster county, Pa. The annual a mount manufactured there is about 1,500, 000 lbs.
Hydrate of Magnesia or native Magnesia is found at Hoboken, N. J., in thin seams traversing serpentine. The Sili ceous hydrate is found in serpentine at Middlefield, Mass., and at Baltimore, Md. A variety of carbonate containing 4 per cent. of silex is called by the Germans Meerschaum, or Ecume de mer by the French. This is also found at Hoboken. Sulphate of magnesia in fine silky needles is found in caves in Kentucky, and efflo rescing in the earth in Tennessee.