ls. The salt is
extensively used in the manufacture of sodium carbonate and glass. Small
quantities are used in medicine.

~Sodium sulphite~ (Na_{2}SO_{3}.7H_{2}O). Sodium sulphite is prepared by
the action of sulphur dioxide upon solutions of sodium hydroxide, the
reaction being analogous to the action of carbon dioxide upon sodium
hydroxide. Like the carbonate, the sulphite is readily decomposed by
acids:

Na_{2}SO_{3} + 2HCl = 2NaCl + H_{2}O + SO_{2}.

Because of this reaction sodium sulphite is used as a convenient source
of sulphur dioxide. It is also used as a disinfectant and a
preservative.

~Sodium thiosulphate~ (_hyposulphite of soda or "hypo"_)
(Na_{2}S_{2}O_{3}.5H_{2}O). This salt, commonly called sodium
hyposulphite, or merely hypo, is made by boiling a solution of sodium
sulphite with sulphur:

Na_{2}SO_{3} + S = Na_{2}S_{2}O_{3}.

It is used in photography and in the bleaching industry, to absorb the
excess of chlorine which is left upon the bleached fabrics.

~Thio compounds.~ The prefix "thio" means sulphur. It is used to
designate substances which may be regarded as derived from
oxygen compounds by replacing the whole or a part of their
oxygen with sulphur. The thiosulphates may be regarded as
sulphates in which one atom of oxygen has been replaced by an
atom of sulphur. This may be seen by comparing the formula
Na_{2}SO_{4} (sodium sulphate) with the formula
Na_{2}S_{2}O_{3} (sodium thiosulphate).

~Sodium carbonate~ (_sal soda_)(Na_{2}CO_{3}.10H_{2}O). There are two
different methods now employed in the manufacture of this important
substance.

1. _Le Blanc process._ This older process involves several distinct
reactions, as shown in the following equations.

(a) Sodium chloride is first converted into sodium sulphate:

2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl.

(b) The sodium sulphate is next reduced to sulphide by heating it with
carbon:

Na_{2}SO_{4} + 2C = Na_{2}S + 2CO_{2}.

(c) The sodium sulphide is then heated with calcium carbonate, when
double decomposition takes place:

Na_{2}S + CaCO_{3} = CaS + Na_{2}CO_{3}.

~Technical preparation of sodium carbonate.~ In a manufacturing
plant the last two reactions take place in one process. Sodium
sulphate, coal, and powdered limestone are heated together to a
rather high temperature. The coal reduces the sulphate to
sulphide, which in turn rea