and that of the oxygen which it contains. Hence, to
find the weight of oxygen in 15 g. of mercuric oxide, we have the
proportion

216 : 16 :: 15 : x (grams of oxygen).

On the other hand, suppose we wish to prepare, say, 20 g. of oxygen. The
problem is to find out what weight of mercuric oxide will yield 20 g. of
oxygen. The following proportion evidently holds

216 : 16 :: x (grams of mercuric oxide) : 20;

from which we get x = 270.

In the preparation of hydrogen by the action of sulphuric acid upon
zinc, according to the equation,

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

suppose that 50 g. of zinc are available; let it be required to
calculate the weight of hydrogen which can be obtained. It will be seen
that 1 atom of zinc will liberate 2 atoms of hydrogen. The ratio by
weight of a zinc to an hydrogen atom is 65.4: 1.008; of 1 zinc atom to 2
hydrogen atoms, 65.4: 2.016. Zinc and hydrogen will be related in this
reaction in this same ratio, however many atoms of zinc are concerned.
Consequently in the proportion

65.4 : 2.016 :: 50 : x,

x will be the weight of hydrogen set free by 50 g. of zinc. The weight
of zinc sulphate produced at the same time can be found from the
proportion

65.4 : 161.46 :: 50 : x;

where 161.46 is the molecular weight of the zinc sulphate, and x the
weight of zinc sulphate formed. In like manner, the weight of sulphuric
acid used up can be calculated from the proportion

65.4 : 98.076 :: 50 : x.

These simple calculations are possible because the symbols and formulas
in the equations represent the relative weights of the substances
concerned in a chemical reaction. When once the relative weights of the
atoms have been determined, and it has been agreed to allow the symbols
to stand for these relative weights, an equation or formula making use
of the symbols becomes a statement of a definite numerical fact, and
calculations can be based on it.

~Chemical equations not algebraic.~ Although chemical equations are
quantitative, it must be clearly understood that they are not algebraic.
A glance at the equations

7 + 4 = 11, 8 + 5 = 9 + 4

will show at once that they are true. The equations

HgO = Hg + O, FeO = Fe + O

are equally true in an algebraic sense, but experiment shows that only
the first is true chemically, for iron oxide (FeO) cannot be directly
decomposed into iron and oxygen. Only such equations as have been found
by careful exp