first promulgated his binary theory he was careful to restrict its unmodified
application to the compounds of the inorganic world. At that time, and
for a long time thereafter, it was supposed that substances of organic
nature had some properties that kept them aloof from the domain of inorganic
chemistry. It was little doubted that a so-called "vital force" operated
here, replacing or modifying the action of ordinary "chemical affinity."
It was, indeed, admitted that organic compounds are composed of familiar
elements - chiefly carbon, oxygen, hydrogen, and nitrogen; but these elements
were supposed to be united in ways that could not be imitated in the domain
of the non-living. It was regarded almost as an axiom of chemistry that
no organic compound whatever could be put together from its elements -
synthesized - in the laboratory. To effect the synthesis of even the simplest
organic compound, it was thought that the "vital force" must be in operation.
Therefore a veritable sensation was created
in the chemical world when, in the year 1828, it was announced that the
young German chemist, Friedrich Wohler, formerly pupil of Berzelius, and
already known as a coming master, had actually synthesized the well-known
organic product urea in his laboratory at Sacrow. The "exception which
proves the rule" is something never heard of in the domain of logical science.
Natural law knows no exceptions. So the synthesis of a single organic compound
sufficed at a blow to break down the chemical barrier which the imagination
of the fathers of the science had erected between animate and inanimate
nature. Thenceforth the philosophical chemist would regard the plant and
animal organisms as chemical laboratories in which conditions are peculiarly
favorable for building up complex compounds of a few familiar elements,
under the operation of universal chemical laws. The chimera "vital force"
could no longer gain recognition in the domain of chemistry.
Now a wave of interest in organic chemistry
swept over the chemical world, and soon the study of carbon compounds became
as much the fashion as electrochemistry had been in the, preceding generation.
Foremost among the workers who rendered
this epoch of organic chemistry memorable were Justus Liebig in Germany
and Jean Baptiste Andre Dumas in France, and their respective pupils, Charles
Frederic Gerhardt and Augustus Laurent. Wohler, too, must be named in the
same breath, as also must Louis Pasteur, who, though somewhat younger than
the others, came upon the scene in time to take chief part in the most
important of the controversies that grew out of their labors.
Several years earlier than this the way
had been paved for the study of organic substances by Gay-Lussac's discovery,
made in 1815, that a certain compound of carbon and nitrogen, which he
named cyanogen, has a peculiar degree of stability which enables it to
retain its identity and enter into chemical relations after the manner
of a simple body. A year later Ampere discovered that nitrogen and hydrogen,
when combined in certain proportions to form what he called ammonium, have
the same property. Berzelius had seized upon this discovery of the compound
radical, as it was called, because it seemed to lend aid to his dualistic
theory. He conceived the idea that all organic compounds are binary unions
of various compound radicals with an atom of oxygen, announcing this theory
in 1818. Ten years later, Liebig and Wohler undertook a joint investigation
which resulted in proving that compound radicals are indeed very abundant
among organic substances. Thus the theory of Berzelius seemed to be substantiated,
and organic chemistry came to be defined as the chemistry of compound radicals.
But even in the day of its seeming triumph
the dualistic theory was destined to receive a rude shock. This came about
through 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 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.