/ClO
Ca + H_{2}SO_{4} = CaSO_{4} + HCl + HClO,
\Cl

HCl + HClO = H_{2}O + 2Cl.

This reaction can be employed in the preparation of chlorine, or the
nascent chlorine may be used as a bleaching agent.

2. It is slowly decomposed by the carbon dioxide of the air, yielding
calcium carbonate and chlorine:

CaOCl_{2} + CO_{2} = CaCO_{3} + 2Cl.

Owing to this slow action the substance is a good disinfectant.

3. When its solution is boiled the substance breaks down into calcium
chloride and chlorate:

6CaOCl_{2} = 5CaCl_{2} + Ca(ClO_{3})_{2}.

This reaction is used in the preparation of potassium chlorate.

~Calcium fluoride~ (_fluorspar_) (CaF_{2}). Fluorspar has already been
mentioned as the chief natural compound of fluorine. It is found in
large quantities in a number of localities, and is often crystallized in
perfect cubes of a light green or amethyst color. It can be melted
easily in a furnace, and is sometimes used in the fused condition in
metallurgical operations to protect a metal from the action of the air
during its reduction. It is used as the chief source of fluorine
compounds, especially hydrofluoric acid.

~Calcium sulphate~ (_gypsum_) (CaSO_{4}.2H_{2}O). This abundant substance
occurs in very perfectly formed crystals or in massive deposits. It is
often found in solution in natural waters and in the sea water. Salts
deposited from sea water are therefore likely to contain this substance
(see Stassfurt salts).

It is very sparingly soluble in water, and is thrown down as a fine
white precipitate when any considerable amounts of a calcium salt and a
soluble sulphate (or sulphuric acid) are brought together in solution.
Its chief use is in the manufacture of plaster of Paris and of hollow
tiles for fireproof walls. Such material is called _gypsite_. It is also
used as a fertilizer.

Calcium sulphate, like the carbonate, occurs in many forms in nature.
Gypsum is a name given to all common varieties. Granular or massive
specimens are called alabaster, while all those which are well
crystallized are called selenite. Satin spar is still another variety
often seen in mineral collections.

~Plaster of Paris.~ When gypsum is heated to about 115 deg. it loses a portion
of its water of crystallization in accordance with the equation

2(CaSO_{4}.2H_{2}O) = 2CaSO_{4}.H_{2}O + 2H_{2}O.

The product is a fine white powder called _plaster of Paris_. On being
moistened it again