September 23, 1882.

In 1825 Wöhler became acquainted with Liebig, and an intimate friendship
resulted, which continued until the death of Liebig, a few years ago.
Though they lived far apart, they met during the vacations at their
homes, or traveled together. Many important investigations were
conceived by them as they talked over the problems of chemistry, and
many papers appeared under both their names, containing the results of
their joint work. Among such papers may be mentioned: "On Cyanic Acid"
(1830); "On Mellithic Acid" (1830); "On Sulphotartaric Acid" (1831); "On
Oil of Bitter Almonds, Benzoic Acid, and Related Compounds" (1832); "On
the Formation of Oil of Bitter Almonds from Amygdalin" (1837); and "On
Uric Acid" (1837).

Of the papers included in the above list, the two which most attract
attention are those "On the Oil of Bitter Almonds" and "On Uric Acid."
In the former it was shown for the first time that in analogous carbon
compounds there are groups which remain unchanged, though the compounds
containing them may, in other respects, undergo a variety of changes.
This is the conception of radicals or residues as we use it at the
present day. It cannot be denied that this conception has done very much
to simplify the study of organic compounds. The full value of the
discovery was recognized at once by Berzelius, who, in a letter to the
authors of the paper, proposed that they should call their radical proin
or orthrin (the dawn of day), for the reason that the assumption of its
existence might be likened to the dawn of a new day in chemistry. The
study of this paper should form a part of the work of every advanced
student of chemistry. It is a model of all that is desirable in a
scientific memoir. The paper on uric acid is remarkable for the number
of interesting transformation products described in it, and the skill