r companionship, an
"affinity"--call it what you will--which is bound to be satisfied 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 hydrogen or two atoms of oxygen.

Evidently, then, one atom is not always equivalent to another atom of
a different kind in combining powers. A recognition of this fact by
Frankland about 1852, and its further investigation by others (notably
A. Kekule and A. S. Couper), led to the introduction of the word
equivalent into chemical terminology in a new sense, and in particular
to an understanding of the affinities or "valency" of different
elements, which proved of the most fundamental importance. Thus it
was shown that, of the four elements that enter most prominently i