those of sodium, potassium, and
magnesium being most abundant. Nearly all salt water contains these
substances, particularly sodium chloride, and very large salt beds
consisting of chlorides are found in many parts of the world.

~Preparation.~ Two general methods of preparing chlorine may be mentioned,
namely, the laboratory method and the electrolytic method.

1. _Laboratory method._ In the laboratory chlorine is made by warming
the mineral pyrolusite (manganese dioxide, MnO_{2}) with concentrated
hydrochloric acid. The first reaction, which seems to be similar to the
action of acids upon oxides in general, is expressed in the equation

MnO_{2} + 4HCl = MnCl_{4} + 2H_{2}O.

The manganese compound so formed is very unstable, however, and breaks
clown according to the equation

MnCl_{4} = MnCl_{2} + 2Cl.

Instead of using hydrochloric acid in the preparation of chlorine it
will serve just as well to use a mixture of sodium chloride and
sulphuric acid, since these two react to form hydrochloric acid. The
following equations will then express the changes:

(1) 2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl.

(2) MnO_{2} + 4 HCl = MnCl_{2} + 2Cl + 2H_{2}O.

(3) MnCl_{2} + H_{2}SO_{4} = MnSO_{4} + 2HCl.

Combining these equations, the following equation expressing the
complete reaction is obtained:

2NaCl + MnO_{2} + 2H_{2}SO_{4} = MnSO_{4} + Na_{2}SO_{4} + 2H_{2}O + 2Cl.

Since the hydrochloric acid liberated in the third equation is free to
act upon manganese dioxide, it will be seen that all of the chlorine
originally present in the sodium chloride is set free.

The manganese dioxide and the hydrochloric acid are brought
together in a flask, as represented in Fig. 53, and a gentle
heat is applied. The rate of evolution of the gas is regulated
by the amount of heat applied, and the gas is collected by
displacement of air. As the equations show, only half of the
chlorine present in the hydrochloric acid is liberated.

[Illustration: Fig. 53]

2. _Electrolytic method._ Under the discussion of electrolysis (p. 102)
it was shown that when a solution of sodium chloride is electrolyzed
chlorine is evolved at the anode, while the sodium set free at the
cathode reacts with the water to form hydrogen, which is evolved, and
sodium hydroxide, which remains in solution. A great deal of the
chlorine required in the chemical industries is now made in this way in
connect