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.
It is a product of chemical factories, but is frequently prepared in the laboratory of the mineral-water factory from magnesium carbonate by neutralization with hydrochloric (muriatic) acid. It crystallizes in needles, but with difficulty, liquifies readily when exposed to the air by absorbing moisture, and is, therefore, to be kept in well-stoppered bottles; readily soluble in water and spirits.
Commercial chloride of magnesium is usually kept in stock in a 10 per cent solution. Dissolve one part by weight in nine parts of distilled water. Filter. Specific gravity 1.085 at 15° C. Proportion 10 to 1. A solution may also be made by dissolving 10 parts by weight of dry carbonate of magnesium in 73 parts of hydrochloric acid of 1.048 specific gravity. Add distilled water to make it 95 parts of 1.085 to 1.086 specific gravity. The solution will contain 10 per cent, of chloride of magnesium, anhydrous. The proportions are the same.