efractory substance is needed. It combines with
water to form magnesium hydroxide, but much more slowly and with the
production of much less heat than in the case of calcium oxide.

~Magnesium hydroxide~ (Mg(OH)_{2}). The hydroxide formed in this way is
very slightly soluble in water, but enough dissolves to give the water
an alkaline reaction. Magnesium hydroxide is therefore a fairly strong
base. It is an amorphous white substance. Neither magnesia nor magnesium
salts have a very marked effect upon the system; and for this reason
magnesia is a very suitable antidote for poisoning by strong acids,
since any excess introduced into the system will have no injurious
effect.

~Magnesium cement.~ A paste of magnesium hydroxide and water
slowly absorbs carbon dioxide from the air and becomes very
hard. The hardness of the product is increased by the presence
of a considerable amount of magnesium chloride in the paste.
The hydroxide, with or without the chloride, is used in the
preparation of cements for some purposes.

~Magnesium carbonate~ (MgCO_{3}). Magnesium carbonate is a very abundant
mineral. It occurs in a number of localities as magnesite, which is
usually amorphous, but sometimes forms pure crystals resembling calcite.
More commonly it is found associated with calcium carbonate. The
mineral dolomite has the composition CaCO_{3}.MgCO_{3}. Limestone
containing smaller amounts of magnesium carbonate is known as dolomitic
limestone. Dolomite is one of the most common rocks, forming whole
mountain masses. It is harder and less readily attacked by acids than
limestone. It is valuable as a building stone and as ballast for
roadbeds and foundations. Like calcium carbonate, magnesium carbonate is
insoluble in water, though easily dissolved by acids.

~Basic carbonate of magnesium.~ We should expect to find magnesium
carbonate precipitated when a soluble magnesium salt and a soluble
carbonate are brought together:

Na_{2}CO_{3} + MgCl_{2} = MgCO_{3} + 2NaCl.

Instead of this, some carbon dioxide escapes and the product is found to
be a basic carbonate. The most common basic carbonate of magnesium has
the formula 4MgCO_{3}.Mg(OH)_{2}, and is sometimes called magnesia alba.
This compound is formed by the partial hydrolysis of the normal
carbonate at first precipitated:

5MgCO_{3} + 2H_{2}O = 4MgCO_{3}.Mg(OH)_{2} + H_{2}CO_{3}.

~Magnesium chloride~ (MgCl_{2}.6H_{2}O). Magnesium ch