SUBCARBONATE OF MAGNESIA.

363

Preparation. —Subcarbonate of magnesia is prepared by mixing solu-tions of sulphate of magnesia (or chloride of magnesium) and carbonateof soda (or carbonate of potash). The directions of the London Phar-macopoeia are as follow:—Dissolve separately four pounds of sulphateof magnesia, and four pounds and eight ounces of carbonate of soda, intwo gallons of water, and strain: then mix the liquors, and boil for aquarter of an hour, constantly stirring with a spatula: lastly, havingpoured off' the liquor, wash the precipitated powder with boiling distilledwater, and dry it.

Two kinds of subcarbonate of magnesia are known and kept in theshops, the light and the heavy. Heavy magnesia [magnesia ponderosa ofthe shops) is thus prepared:—Add one volume of a cold saturated solu-tion of carbonate of soda to a boiling mixture of one volume of a satu-rated solution of sulphate magnesia, and three volumes of water. Boiluntil effervescence has ceased, constantly stirring with a spatula. Thendilute with boiling water, set aside, pour off the supernatant liquor, andwash the precipitate with hot water on a linen cloth : afterwards dry itby heat in an iron pot. Light magnesia (magnesia of the shops) is pre-pared by employing dilute solutions of the sulphate of magnesia andcarbonate of soda. If no heat be used, it is apt to be gritty. A heavyand gritty magnesia is prepared by separately dissolving 12 parts ofsulphate magnesia and 13 parts of crystallized carbonate of soda in assmall a quantity of water as possible, mixing the hot solutions, andwashing the precipitate.

The theory of the process is as follows :—When sulphate of magnesiaand carbonate of soda are mixed, double decomposition takes place : thesulphuric acid combines with the soda to form sulphate of soda, and thecarbonic acid with the magnesia. The neutral carbonate of magnesia,which we presume to be first formed, is immediately resolved by thewater into a subcarbonate, which, with some water, precipitates, and thebicarbonate which remains is dissolved in the cold liquid. 5 eq. of theneutral carbonate, with 4 eq. of water, produce 1 eq. of bicarbonate;while the 4 eq. of magnesia, 3 eq. carbonic acid, and 4 eq. of water,precipitate. If we mix the boiling solutions together, 4 eq. of the neutralcarbonate give out 1 eq. of free carbonic acid, and a precipitate is formedconsisting of4 eq. of magnesia, 3 eq. of carbonic acid, and 4 eq. of water.The precipitate in both of these cases constitutes the subcarbonate ofmagnesia of the shops.

Properties. —Subcarbonate of magnesia, as usually met with, is inthe form of a white, inodorous, and tasteless powder. The light varietyoccurs as a very fine light powder, or in large rectangular masses withbevelled edges, or in smaller cubical cakes. The heavy carbonate is, asits name indicates, of greater specific gravity than the light. Both kindsmoistened with water have a feebly alkaline reaction on test paper, butwhen boiled in water do not communicate this property to water. Sub-carbonate of magnesia is nearly insoluble in water: it readily dissolvesin carbonic acid water.

Characteristics. —-It is distinguished from caustic or calcined mag-nesia by the effervescence which takes place on the addition of a dilutemineral acid. Its other characteristics are the same as for the lattersubstance (vide p. 361.)