\Mag*ne"si*a\ (?; 277), n. [L. Magnesia, fem. of
Magnesius of the country Magnesia, Gr. ? ? ? a magnet. Cf.
{Magnet}.] (Chem.)
A light earthy white substance, consisting of magnesium
oxide, and obtained by heating magnesium hydrate or
carbonate, or by burning magnesium. It has a slightly
alkaline reaction, and is used in medicine as a mild antacid
laxative. See {Magnesium}.
{Magnesia alba} [L.] (Med. Chem.), a bulky white amorphous
   substance, consisting of a hydrous basic carbonate of
   magnesium, and used as a mild cathartic.