before. Avogadro's hypothesis that there are equal numbers of
these molecules in equal volumes of gases, under fixed
conditions, was revived by Gerhardt, and a little later, under
the championship of Cannizzaro, was exalted to the plane of a
fixed law. Thenceforth the conception of the molecule was to be
as dominant a thought in chemistry as the idea of the atom had
become in a previous epoch.


CHEMICAL AFFINITY

Of course the atom itself was in no sense displaced, but
Avogadro's law soon made it plain that the atom had often usurped
territory that did not really belong to it. In many cases the
chemists had supposed themselves dealing with atoms as units
where the true unit was the molecule. In the case of elementary
gases, such as hydrogen and oxygen, for example, the law of equal
numbers of molecules in equal spaces made it clear that the atoms
do not exist isolated, as had been supposed. Since two volumes
of hydrogen unite with one volume of oxygen to form two volumes
of water vapor, the simplest mathematics show, in the light of
Avogadro's law, not only that each molecule of water must contain
two hydrogen atoms (a point previously in dispute), but that the
original molecules of hydrogen and oxygen must have been composed
in each case of two atoms---else how could one volume of oxygen
supply an atom for every molecule of two volumes of water?

What, then, does this imply? Why, that the elementary atom has
an avidity for other atoms, a longing for companionship, an
"affinity"--call it what you will--which is bound to be satisfied