magnesia (mag-ne-se'ah)

Originally, black oxid of manganese, so called from confusion with magnetic iron ore. This was subsequently called m. nigra, and the term m. alba was given to what is now known as magnesium carbonate, and is still used occasionally. In German medical literature the term m. usually refers to the carbonate, in English medical literature to the oxid, MgO, but blocks of magnesium carbonate are commonly called magnesia by the laity. M. is obtained by the calcination of magnesium carbonate; it is odorle , has a slightly alkaline taste, and shows a faint alkaline reaction when moistened; it is insoluble in alcohol and only slightly soluble in water. When occurring in the form of powder, it is designated according to its density as light m. and heavy m. The m. of the U. S. Ph., 1890 (magnes i i oxidum, U. S. Ph., viii) is light m.; a light powder, white and fine, devoid of odor and having an earthy taste. calcined m. Magnesium oxid obtained by freeing magnesium carbonate from its water and carbonic acid by heating to a low red heat.