This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
The commercial carbonate of magnesium may, if desired, be prepared after the following directions: Dissolve 3 parts by weight of crystallized sulphate of magnesium in 10 parts of distilled water, filter into a glass flask, large enough to hold double this quantity of liquid, add 2 parts of finely powdered bicarbonate of soda, and slightly warm the whole in lukewarm, not hot, water, until 40° 0. is reached, which temperature should not be exaggerated. Let stand 3 or 4 days in a warm room, occasionally shaking the mixture, then filter; wash out the precipitate on the filter, press out the remaining water and dry. This salt is a white, coarse powder, decomposing if exposed to dry air, therefore to be kept in well-stoppered bottles ready for use. Magnesium carbonate hydrate, whether bought or thus prepared, requires prolonged agitation under pressure to become entirely dissolved. Magnesium bicarbonate is in all cases substituted by the proper equivalents of magnesium carbonate, as an abundance of free carbonic acid is present to convert it into bicarbonate.