This section is from the book "Materia Medica: Pharmacology: Therapeutics Prescription Writing For Students and Practitioners", by Walter A. Bastedo. Also available from Amazon: Materia Medica: Pharmacology: Therapeutics: Prescription Writing for Students and Practitioners.
The magnesium antacids are the oxide, the hydroxide, and the carbonate. Magnesium perhydrol is the magnesium peroxide. The magnesium oxide (magnesii oxidum) of the Pharmacopoeia, or burnt magnesia, is a very light, odorless white powder, which, when exposed to air, slowly changes to carbonate. One part of it, on being mixed with 15 parts of water and allowed to stand half an hour, hydrates and forms a gelatinous mass. The heavy oxide (magnesii oxidum ponderosum) is 3 1/2 times as heavy and does not readily hydrate. Magnesium hydroxide comes in the form of a thick white liquid or magma (magma magnesiae), commonly called "milk of magnesia." This is formed by precipitating a solution of magnesium sulphate with sodium hydroxide, which leaves the magnesium hydroxide suspended in the water in a fine state of subdivision. It contains about 4 grains (0.25 gm.) of magnesium hydroxide in each dram (4 c.c.). Magnesium carbonate (magnesii carbonas), (MgCo3)4.-Mg(OH)2+5H2O, is a white, insoluble powder, capable of neutralizing acids with the liberation of carbon dioxide gas.
These magnesium salts are all very weak alkalies without any caustic action, but they have considerable combining power for acid. The oxide in the hydrated gelatinous form will neutralize 1 1/2 times its weight of hydrochloric acid (U. S. P.). Benedict states that magnesium forces calcium from the system and hinders the calcium retention necessary for bone building. This might be a highly undesirable effect from the repeated administration of milk of magnesia to infants. They all act as cathartics, and will be considered further under that heading. (See Magnesium Sulphate under Cathartics and Anesthetics.)