the simplest kind of chemical action. It consists
in the union of two or more substances to produce a new substance. The
combination of iron with sulphur is an example:

Fe + S = FeS.

2. _Decomposition._ This is the reverse of addition, the substance
undergoing reaction being parted into its constituents. The
decomposition of mercuric oxide is an example: HgO = Hg + O.

3. _Substitution._ It is sometimes possible for an element in the free
state to act upon a compound in such a way that it takes the place of
one of the elements of the compound, liberating it in turn. In the study
of the element hydrogen it was pointed out that hydrogen is most
conveniently prepared by the action of sulphuric or hydrochloric acid
upon zinc. When sulphuric acid is used a substance called zinc sulphate,
having the composition represented by the formula ZnSO_{4}, is formed
together with hydrogen. The equation is

Zn + H_{2}SO_{4} = ZnSO_{4} + 2H.

When hydrochloric acid is used zinc chloride and hydrogen are the
products of reaction:

Zn + 2HCl = ZnCl_{2} + 2H.

When iron is used in place of zinc the equation is

Fe + H_{2}SO_{4} = FeSO_{4} + 2H.

These reactions are quite similar, as is apparent from an examination of
the equations. In each case 1 atom of the metal replaces 2 atoms of
hydrogen in the acid, and the hydrogen escapes as a gas. When an element
in the free state, such as the zinc in the equations just given, takes
the place of some one element in a compound, setting it free from
chemical combination, the act is called _substitution_.

Other reactions illustrating substitution are the action of sodium on
water,

Na + H_{2}O = NaOH + H;

and the action of heated iron upon water,

3Fe + 4H_{2}O = Fe_{3}O_{4} + 8H.

4. _Double decomposition._ When barium dioxide (BaO_{2}) is treated with
sulphuric acid two compounds are formed, namely, hydrogen dioxide
(H_{2}O_{2}) and barium sulphate (BaSO_{4}). The equation is

BaO_{2} + H_{2}SO_{4} = BaSO_{4} + H_{2}O_{2}.

In this reaction it will be seen that the two elements barium and
hydrogen simply exchange places. Such a reaction is called a _double
decomposition_. We shall meet with many examples of this kind of
chemical reactions.

~Chemical equations are quantitative.~ The use of symbols and formulas in
expressing chemical changes has another great advantage. Thus, according
to the equation

H_{2}O = 2H + O,

1 molecule of wat