termining atomic weights.~ When Dalton first
advanced the atomic theory he attempted to solve this problem by very
simple methods. He thought that when only one compound of two elements
is known it is reasonable to suppose that it contains one atom of each
element. He therefore gave the formula HO to water, and HN to ammonia.
When more than two compounds were known he assumed that the most
familiar or the most stable one had the simple formula. He then
determined the atomic weight as explained above. The results he
obtained were contradictory and very far from satisfactory, and it was
soon seen that some other method, resting on much more scientific
grounds, must be found to decide what compounds, if any, have a single
atom of each element present.

~Determination of atomic weights.~ Three distinct steps are involved in
the determination of the atomic weight of an element: (1) determination
of the equivalent, (2) determination of molecular weights of its
compounds, and (3) deduction of the exact atomic weight from the
equivalent and molecular weights.

~1. Determination of the equivalent.~ By the equivalent of an element is
meant the weight of the element which will combine with a fixed weight
of some other element chosen as a standard. It has already been
explained that oxygen has been selected as the standard element for
atomic weights, with a weight of 16. This same standard will serve very
well as a standard for equivalents. _The equivalent of an element is the
weight of the element which will combine with 16 g. of oxygen._ Thus 16
g. of oxygen combines with 16.03 g. of sulphur, 65.4 g. of zinc, 215.86
g. of silver, 70.9 g. of chlorine. These figures, therefore, represent
the equivalent weights of these elements.

~Relation of atomic weights to equivalents.~ According to the atomic
theory combination always takes place between whole numbers of atoms.
Thus one atom unites with one other, or with two or three; or two atoms
may unite with three, or three with five, and so on.

When oxygen combines with zinc the combination must be between definite
numbers of the two kinds of atoms. Experiment shows that these two
elements combine in the ratio of 16 g. of oxygen to 65.4 g. of zinc. If
one atom of oxygen combines with one atom of zinc, then this ratio must
be the ratio between the weights of the two atoms. If one atom of oxygen
combines with two atoms of zinc, then the ratio between the weights of
the two atoms will