_Calcined Magnesia._
QUALITIES. _Form_, a white, very light, soft powder; _Specific gravity_,
2·3; it turns to green the more delicate vegetable blues. SOLUBILITY.
Although it requires 2000 times its weight of water to hold it in
solution, yet it has the property of considerably increasing the
solubility of camphor, opium, and resins in the same fluid; it is
soluble in solutions of the alkaline carbonates, but not in those of
caustic alkalies. CHEMICAL COMPOSITION. It is an oxide of a peculiar
metal. MEDICINAL USES. Antacid, and when acidity prevails, purgative; it
is preferable to the carbonate whenever the bowels are distended with
flatus, (_Form: 150_): in other respects, its virtues are the same. See
_Magnes. Carb_. The Medicinal Dynameter will shew the equivalent doses
of the pure earth and its carbonate; it will be seen for instance that
12 grains of the former will be as efficient, as an antacid, as 25
grains of the latter. INCIDENTAL IMPURITIES. It ought not to effervesce
with acids, and if magnesia and muriatic acid be placed at one time in
separate cups, in a scale of a balance, no diminution of weight should
take place on mixing them. Lime is detected by its solution in dilute
sulphuric acid affording a precipitate with oxalate of ammonia; the
_sulphuret of lime_ betrays itself by yielding, when moistened, the
smell of sulphuretted hydrogen.[559]