sulphur as the other.

The combination between the atoms may of course take place in other
simple ratios. For example, two atoms of one element might combine with
three or with five of the other. In all such cases it is clear that the
law of multiple proportion must hold true. For on selecting such numbers
of the two kinds of molecules as have the same number of the one kind of
atoms, the numbers of the other kind of atoms will stand in some simple
ratio to each other, and their weights will therefore stand in the same
simple ratio.

~Testing the hypothesis.~ Efforts have been made to find compounds which
do not conform to these laws, but all such attempts have resulted in
failure. If such compounds should be found, the laws would be no longer
true, and the hypothesis of Dalton would cease to possess value. When an
hypothesis has been tested in every way in which experiment can test it,
and is still found to be in harmony with the facts in the case, it is
termed a _theory_. We now speak of the atomic theory rather than of the
atomic hypothesis.

~Value of a theory.~ The value of a theory is twofold. It aids in the
clear understanding of the laws of nature because it gives an
intelligent idea as to why these laws should be in operation.

A theory also leads to discoveries. It usually happens that in testing a
theory much valuable work is done, and many new facts are discovered.
Almost any theory in explaining given laws will involve a number of
consequences apart from the laws it seeks to explain. Experiment will
soon show whether these facts are as the theory predicts they will be.
Thus Dalton's atomic theory predicted many properties of gases which
experiment has since verified.

~Atomic weights.~ It would be of great advantage in the study of chemistry
if we could determine the weights of the different kinds of atoms. It is
evident that this cannot be done directly. They are so small that they
cannot be seen even with a most powerful microscope. It is calculated
that it would take 200,000,000 hydrogen atoms placed side by side to
make a row one centimeter long. No balance can weigh such minute
objects. It is possible, however, to determine their relative
weights,--that is, how much heavier one is than another. _These relative
weights of the atoms are spoken of as the atomic weights of the
elements._

If elements were able to combine in only one way,--one atom of one with
one atom of another,--the problem of d