be 16: 32.7. If two atoms of oxygen combine with one
atom of zinc, the ratio by weight between the two atoms will be 8: 65.4.
It is evident, therefore, that the real atomic weight of an element must
be some multiple or submultiple of the equivalent; in other words, the
equivalent multiplied by 1/2, 1, 2, or 3 will give the atomic weight.

~Combining weights.~ A very interesting relation holds good between the
equivalents of the various elements. We have just seen that the figures
16.03, 65.4, 215.86, and 70.9 are the equivalents respectively of
sulphur, zinc, silver, and chlorine. These same figures represent the
ratios by weight in which these elements combine among themselves. Thus
215.86 g. of silver combine with 70.9 g. of chlorine and with 2 x 16.03
g. of sulphur. 65.4 g. of zinc combine with 70.9 g. of chlorine and 2 x
16.03 g. of sulphur.

By taking the equivalent or some multiple of it a value can be obtained
for each element which will represent its combining value, and for this
reason is called its _combining weight_. It is important to notice that
the fact that a combining weight can be obtained for each element is not
a part of a theory, but is the direct result of experiment.

~Elements with more than one equivalent.~ It will be remembered that
oxygen combines with hydrogen in two ratios. In one case 16 g. of oxygen
combine with 2.016 g. of hydrogen to form water; in the other 16 g. of
oxygen combine with 1.008 g. of hydrogen to form hydrogen dioxide. The
equivalents of hydrogen are therefore 2.016 and 1.008. Barium combines
with oxygen in two proportions: in barium oxide the proportion is 16 g.
of oxygen to 137.4 g. of barium; in barium dioxide the proportion is 16
g. of oxygen to 68.7 g. of barium.

In each case one equivalent is a simple multiple of the other, so the
fact that there may be two equivalents does not add to the uncertainty.
All we knew before was that the true atomic weight is some multiple of
the equivalent.

~2. The determination of molecular weights.~ To decide the question as to
which multiple of the equivalent correctly represents the atomic weight
of an element, it has been found necessary to devise a method of
determining the molecular weights of compounds containing the element in
question. Since the molecular weight of a compound is merely the sum of
the weights of all the atoms present in it, it would seem to be
impossible to determine the molecular weight of a compound withou