applied to the
preparation of a great variety of compounds, and illustrations of such
methods will very frequently be found in the subsequent pages.

4. _By fusion methods._ It sometimes happens that substances which are
insoluble in water and in acids, and which cannot therefore be brought
into double decomposition in the usual way, are soluble in other
liquids, and when dissolved in them can be decomposed and converted into
other desired compounds. Thus barium sulphate is not soluble in water,
and sulphuric acid, being less volatile than most other acids, cannot
easily be driven out from this salt When brought into contact with
melted sodium carbonate, however, it dissolves in it, and since barium
carbonate is insoluble in melted sodium carbonate, double decomposition
takes place:

Na_{2}CO_{3} + BaSO_{4} = BaCO_{3} + Na_{2}SO_{4}.

On dissolving the cooled mixture in water the sodium sulphate formed in
the reaction, together with any excess of sodium carbonate which may be
present, dissolves. The barium carbonate can then be filtered off and
converted into any desired salt by the processes already described.

5. _By the action of metals on salts of other metals._ When a strip of
zinc is placed in a solution of a copper salt the copper is precipitated
and an equivalent quantity of zinc passes into solution:

Zn + CuSO_{4} = Cu + ZnSO_{4}.

In like manner copper will precipitate silver from its salts:

Cu + Ag_{2}SO_{4} = 2Ag + CuSO_{4}.

It is possible to tabulate the metals in such a way that any one of them
in the table will precipitate any one following it from its salts. The
following is a list of some of the commoner metals arranged in this way:

Zinc
Iron
Tin
Lead
Copper
Bismuth
Mercury
Silver
Gold

According to this table copper will precipitate bismuth, mercury,
silver, or gold from their salts, and will in turn be precipitated by
zinc, iron, tin, or lead. Advantage is taken of this principle in the
purification of some of the metals, and occasionally in the preparation
of metals and their compounds.

~Important insoluble compounds.~ Since precipitates play so important a
part in the reactions which substances undergo, as well as in the
preparation of many chemical compounds, it is important to know what
substances are insoluble. Knowing this, we can in many cases predict
reactions under certain conditions, and are assisted in devising ways to