Magnesia

1. A light earthy white substance, consisting of magnesium oxide (MgO), 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 (Med. Chem.),
a bulky white amorphous substance, consisting of a hydrous basic carbonate of magnesium, and used as a mild cathartic.