Magnesia, or Magnesia alia, a kind of absorbent earth, first discovered in the commencement of the 18th century. When pure, it is extremely white, loose, and light, being infusible, as it resists the heat of the most powerful burning lens. This earth, however, easily melts with borax ; and is divested of its fixed air by calcination : in this state, it is less soluble than before, and does not effervesce with acids, but it may be safely taken internally.
Native magnesia is not found in quantities sufficiently large and pure for general use. It is therefore prepared by dissolving separately equal portions of Epsom salt and pearl-ashes, in double their quantity of warm water, when the sediment is to be strained : the two liquids are now to bemixed; adding eight times their proportion of warm water. The whole is suffered to boil a few minutes, being carefully stirred with a wooden spatula, to prevent the powder from adhering to the bottom of the vessel. Now the liquid is to stand at rest, till the heat be somewhat diminished, when it should be filtered through a cloth, on which the magnesia will remain: lastly, it is to be washed till it be totallv divested of its saline taste.
White magnesia is of considerable service for preventing or re moving many disorders of children, especially of such as are troubled with a redundance of acid in the first passage-; for which purpose it is preferable to the calcareous absorbents. But even magnesia is frequently misapplied, and ought never to be given to infants disposed to flatulency, or where no symptoms of acidity can be discovered; as it is otherwise apt to lodge in the bowels, and produce obstinate costiveness; being in itself an inactive earth, unless combined with acids. Hence it is often, and very properly, conjoined with rhubarb ; so that children above one year old may take from five to ten grains of the former, and from one to two grains of the latter ; while adults generally require one or two scruples of magnesia, and from five to ten grains of rhubarb for a moderate dose.— See also Heart-burn.