Lavoisier left unexplained the dynamics of combustion; but in 1843,
before the chemical section of the association meeting at Cork, Dr.
Joule announced the discovery which was to revolutionize modern
science, namely, the determination of the mechanical equivalent of
heat. Every change in the arrangement of the particles he found was
accompanied by a definite evolution or an absorption of heat. Heat was
evolved by the clashing of the atoms, and this amount was fixed and
definite. Thus to Joule we owe the foundation of chemical dynamics and
the basis of thermal chemistry. It was upon a knowledge of the mode of
arrangement of atoms, and on a recognition of their distinctive
properties, that the superstructure of modern organic chemistry
rested. We now assumed on good grounds that the atom of each element
possessed distinct capabilities of combination. The knowledge of the
mode in which the atoms in the molecule are arranged had given to
organic chemistry an impetus which had overcome many experimental
obstacles, and organic chemistry had now become synthetic.
Liebig and Wohler, in 1837, foresaw the artificial production in the
laboratories of all organic substances so far as they did not
constitute a living organism. And after fifty years their prophecy had
been fulfilled, for at the present time we could prepare an artificial
sweetening principle, an artificial alkaloid, and salacine.