place there was elaborated, chiefly through the efforts of
Laurent and Gerhardt, a conception of the molecule as a unitary
structure, built up through the aggregation of various atoms, in
accordance with "elective affinities" whose nature is not yet understood
A doctrine of "nuclei" and a doctrine of "types" of molecular structure
were much exploited, and, like the doctrine of compound radicals, became
useful as aids to memory and guides for the analyst, indicating some of
the plans of molecular construction, though by no means penetrating the
mysteries of chemical affinity. They are classifications rather than
explanations of chemical unions. But at least they served an important
purpose in giving definiteness to the idea of a molecular structure
built of atoms as the basis of all substances. Now at last the word
molecule came to have a distinct meaning, as distinct from "atom," in
the minds of the generality of chemists, as it had had for Avogadro a
third of a century 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 fo