energy.
Niagara Falls is the most important locality in this country for such
industries, and many different electro-chemical products are
manufactured there. Some industries depend upon electrolytic processes,
while in others the electrical energy is used merely as a source of heat
in electric furnaces.

~Preparation of compounds of the metals.~ Since the compounds of the
metals are so numerous and varied in character, there are many ways of
preparing them. In many cases the properties of the substance to be
prepared, or the material available for its preparation, suggest a
rather unusual way. There are, however, a number of general principles
which are constantly applied in the preparation of the compounds of the
metals, and a clear understanding of them will save much time and effort
in remembering the details in any given case. The most important of
these general methods for the preparation of compounds are the
following:

1. _By direct union of two elements._ This is usually accomplished by
heating the two elements together. Thus the sulphides, chlorides, and
oxides of a metal can generally be obtained in this way. The following
equations serve as examples of this method:

Fe + S = FeS,

Mg + O = MgO,

Cu + 2Cl = CuCl_{2}.

2. _By the decomposition of a compound._ This decomposition may be
brought about either by heat alone or by the combined action of heat and
a reducing agent. Thus when the nitrate of a metal is heated the oxide
of the metal is usually obtained. Copper nitrate, for example,
decomposes as follows:

Cu(NO_{3})_{2} = CuO + 2NO_{2} + O.

Similarly the carbonates of the metals yield oxides, thus:

CaCO_{3} = CaO + CO_{2}.

Most of the hydroxides form an oxide and water when heated:

2Al(OH)_{3} = Al_{2}O_{3} + 3H_{2}O.

When heated with carbon, sulphates are reduced to sulphides, thus:

BaSO_{4} + 2C = BaS + 2CO_{2}.

3. _Methods based on equilibrium in solution._ In the preparation of
compounds the first requisite is that the reactions chosen shall be of
such a kind as will go on to completion. In the chapter on chemical
equilibrium it was shown that reactions in solution may become complete
in either of three ways: (1) a gas may be formed which escapes from
solution; (2) an insoluble solid may be formed which precipitates; (3)
two different ions may combine to form undissociated molecules. By the
judicious selection of materials these principles may be