the investigations of Dumas, who proved that in a certain organic
substance an atom of hydrogen may be removed and an atom of
chlorine substituted in its place without destroying the
integrity of the original compound--much as a child might
substitute one block for another in its play-house. Such a
substitution would be quite consistent with the dualistic theory,
were it not for the very essential fact that hydrogen is a
powerfully electro-positive element, while chlorine is as
strongly electro-negative. Hence the compound radical which
united successively with these two elements must itself be at one
time electro-positive, at another electro-negative--a seeming
inconsistency which threw the entire Berzelian theory into
disfavor.

In its 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