drogen and 88.82% oxygen
in it. Red oxide of mercury, from whatever source it is obtained,
contains 92.6% mercury and 7.4% oxygen. This truth is known as _the law
of definite composition_, and may be stated thus: _The composition of a
chemical compound never varies._

~Law of multiple proportion.~ It has already been noted, however, that
hydrogen and oxygen combine in two different ratios to form water and
hydrogen dioxide respectively. It will be observed that this fact does
not contradict the law of definite composition, for entirely different
substances are formed. These compounds differ from each other in
composition, but the composition of each one is always constant. This
ability of two elements to unite in more than one ratio is very
frequently observed. Carbon and oxygen combine in two different ratios;
nitrogen and oxygen combine to form as many as five distinct compounds,
each with its own precise composition.

In the first decade of the last century John Dalton, an English
school-teacher and philosopher, endeavored to find some rule which holds
between the ratios in which two given substances combine. His studies
brought to light a very simple relation, which the following examples
will make clear. In water the hydrogen and oxygen are combined in the
ratio of 1 part by weight of hydrogen to 7.94 parts by weight of oxygen.
In hydrogen dioxide the 1 part by weight of hydrogen is combined with
15.88 parts by weight of oxygen. The ratio between the amounts of oxygen
which combine with the same amount of hydrogen to form water and
hydrogen dioxide respectively is therefore 7.94: 15.88, or 1: 2.

[Illustration: JOHN DALTON (English) (1766-1844)

Developed the atomic theory; made many studies on the properties and the
composition of gases. His book entitled "A New System of Chemical
Philosophy" had a large influence on the development of chemistry]

Similarly, the element iron combines with oxygen to form two oxides, one
of which is black and the other red. By analysis it has been shown that
the former contains 1 part by weight of iron combined with 0.286 parts
by weight of oxygen, while the latter contains 1 part by weight of iron
combined with 0.429 parts by weight of oxygen. Here again we find that
the amounts of oxygen which combine with the same fixed amount of iron
to form the two compounds are in the ratio of small whole numbers, viz.,
2:3.

Many other examples of this simple relation might be given, si