if other atoms are in the neighborhood. Placed solely among
atoms of its own kind, the oxygen atom seizes on a fellow oxygen
atom, and in all their mad dancings these two mates cling
together--possibly revolving about each other in miniature
planetary orbits. Precisely the same thing occurs among the
hydrogen atoms. But now suppose the various pairs of oxygen atoms
come near other pairs of hydrogen atoms (under proper conditions
which need not detain us here), then each oxygen atom loses its
attachment for its fellow, and flings itself madly into the
circuit of one of the hydrogen couplets, and--presto!--there are
only two molecules for every three there were before, and free
oxygen and hydrogen have become water. The whole process, stated
in chemical phraseology, is summed up in the statement that under
the given conditions the oxygen atoms had a greater affinity for
the hydrogen atoms than for one another.

As chemists studied the actions of various kinds of atoms, in
regard to their unions with one another to form molecules, it
gradually dawned upon them that not all elements are satisfied
with the same number of companions. Some elements ask only one,
and refuse to take more; while others link themselves, when
occasion offers, with two, three, four, or more. Thus we saw that
oxygen forsook a single atom of its own kind and linked itself
with two atoms of hydrogen. Clearly, then, the oxygen atom, like
a creature with two hands, is able to clutch two other atoms.
But we have no proof that under any circumstances it could hold
more than two. Its affinities seem satisfied when it has two
bonds. But, on the other hand, the atom of nitrogen is able to
hold three atoms of hydrogen, and does so in the molecule of
ammonium (NH3); while the carbon atom can hold four atoms of