loride is found in
many natural waters and in many salt deposits (see Stassfurt salts). It
is obtained as a by-product in the manufacture of potassium chloride
from carnallite. As there is no very important use for it, large
quantities annually go to waste. When heated to drive off the water of
crystallization the chloride is decomposed as shown in the equation

MgCl_{2}.6H_{2}O = MgO + 2HCl + 5H_{2}O.

Owing to the abundance of magnesium chloride, this reaction is being
used to some extent in the preparation of both magnesium oxide and
hydrochloric acid.

~Boiler scale.~ When water which contains certain salts in
solution is evaporated in steam boilers, a hard insoluble
material called _scale_ deposits in the boiler. The formation
of this scale may be due to several distinct causes.

1. _To the deposit of calcium sulphate._ This salt, while
sparingly soluble in cold water, is almost completely insoluble
in superheated water. Consequently it is precipitated when
water containing it is heated in a boiler.

2. _To decomposition of acid carbonates._ As we have seen,
calcium and magnesium acid carbonates are decomposed on
heating, forming insoluble normal carbonates:

Ca(HCO_{3})_{2} = CaCO_{3} + H_{2}O + CO_{2}.

3. _To hydrolysis of magnesium salts._ Magnesium chloride, and
to some extent magnesium sulphate, undergo hydrolysis when
superheated in solution, and the magnesium hydroxide, being
sparingly soluble, precipitates:

MgCl_{2} + 2H_{2}O <--> Mg(OH)_{2} + 2HCl.

This scale adheres tightly to the boiler in compact layers and,
being a non-conductor of heat, causes much waste of fuel. It is
very difficult to remove, owing to its hardness and resistance
to reagents. Thick scale sometimes cracks, and the water coming
in contact with the overheated iron occasions an explosion.
Moreover, the acids set free in the hydrolysis of the magnesium
salts attack the iron tubes and rapidly corrode them. These
causes combine to make the formation of scale a matter which
occasions much trouble in cases where hard water is used in
steam boilers. Water containing such salts should be softened,
therefore, before being used in boilers.

~Magnesium sulphate~ (_Epsom salt_) (MgSO_{4}.7H_{2}O). Like the chloride,
magnesium sulphate is found rather commonly in springs and in salt
deposits.