of the oxygen atom. In
assigning a value to it, however, it is convenient to choose a whole
number, and as small a number as possible without making the atomic
weight of any other element less than unity. For these reasons the
number 16 has been chosen as the atomic weight of oxygen. This makes
the atomic weight of hydrogen equal to 1.008, so that there is but
little difference between taking oxygen as 16 and hydrogen as 1 for the
unit.

The atomic weights of the elements are given in the Appendix.

EXERCISES

1. Two compounds were found to have the following compositions: (a)
oxygen = 69.53%, nitrogen = 30.47%; (b) oxygen = 53.27%, nitrogen =
46.73%. Show that the law of multiple proportion holds in this case.

2. Two compounds were found to have the following compositions: (a)
oxygen = 43.64%, phosphorus = 56.36%; (b) oxygen = 56.35%, phosphorus
= 43.65%. Show that the law of multiple proportion holds in this case.

3. Why did Dalton assume that all the atoms of a given element have the
same weight?

CHAPTER VI

CHEMICAL EQUATIONS AND CALCULATIONS

~Formulas.~ Since the molecule of any chemical compound consists of a
definite number of atoms, and this number never changes without
destroying the identity of the compound, it is very convenient to
represent the composition of a compound by indicating the composition of
its molecules. This can be done very easily by using the symbols of the
atoms to indicate the number and the kind of the atoms which constitute
the molecule. HgO will in this way represent mercuric oxide, a molecule
of which has been found to contain 1 atom each of mercury and oxygen.
H_{2}O will represent water, the molecules of which consist of 1 atom of
oxygen and 2 of hydrogen, the subscript figure indicating the number of
the atoms of the element whose symbol precedes it. H_{2}SO_{4} will
stand for sulphuric acid, the molecules of which contain 2 atoms of
hydrogen, 1 of sulphur, and 4 of oxygen. The combination of symbols
which represents the molecule of a substance is called its _formula_.

~Equations.~ When a given substance undergoes a chemical change it is
possible to represent this change by the use of such symbols and
formulas. In a former chapter it was shown that mercuric oxide
decomposes when heated to form mercury and oxygen. This may be expressed
very briefly in the form of the equation

(1) HgO = Hg + O.

When water is electrolyzed two new substances, hydrogen