takes up this water, and in so doing first forms a
plastic mass, which soon becomes very firm and hard and regains its
crystalline structure. These properties make it very valuable as a
material for forming casts and stucco work, for cementing glass to
metals, and for other similar purposes. If overheated so that all water
is driven off, the process of taking up water is so slow that the
material is worthless. Such material is said to be dead burned. Plaster
of Paris is very extensively used as the finishing coat for plastered
walls.

~Hard water.~ Waters containing compounds of calcium and magnesium in
solution are called hard waters because they feel harsh to the touch.
The hardness of water may be of two kinds,--(1) temporary hardness and
(2) permanent hardness.

1. _Temporary hardness._ We have seen that when water charged with
carbon dioxide comes in contact with limestone a certain amount of the
latter dissolves, owing to the formation of the soluble acid carbonate
of calcium. The hardness of such waters is said to be temporary, since
it may be removed by boiling. The heat changes the acid carbonate into
the insoluble normal carbonate which then precipitates, rendering the
water soft:

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

Such waters may also be softened by the addition of sufficient lime or
calcium hydroxide to convert the acid carbonate of calcium into the
normal carbonate. The equation representing the reaction is

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

2. _Permanent hardness._ The hardness of water may also be due to the
presence of calcium and magnesium sulphates or chlorides. Boiling the
water does not affect these salts; hence such waters are said to have
permanent hardness. They may be softened, however, by the addition of
sodium carbonate, which precipitates the calcium and magnesium as
insoluble carbonates:

CaSO_{4} + Na_{2}CO_{3} = CaCO_{3} + Na_{2}SO_{4}.

This process is sometimes called "breaking" the water.

~Commercial methods for softening water.~ The average water of a
city supply contains not only the acid carbonates of calcium
and magnesium but also the sulphates and chlorides of these
metals, together with other salts in smaller quantities. Such
waters are softened on a commercial scale by the addition of
the proper quantities of calcium hydroxide and sodium
carbonate. The calcium hydroxide is added first to pr