d which derives its name from
cinnamon, because the acid was prepared by the oxidation of oil of
cinnamon by Dumas and Peligot in 1834. The acid and its ethers occur also
in many balsams, so that we have here another instance of the synthesis of
a natural vegetable product from a coal-tar hydrocarbon. The subsequent
steps are--(1) the nitration of the acid to produce nitrocinnamic acid;
(2) the addition of bromine to form a dibromide of the nitro-acid; (3) the
action of alkali on the dibromide to produce what is known as "propiolic
acid." The latter, under the influence of mild alkaline reducing agents,
is transformed into indigo-blue. The process depending on the use of
nitrobenzoic aldehyde is much simpler; but the particular nitro-derivative
of the aldehyde which is required is at present difficult to make, and
therefore expensive. If the production of this compound could be
cheapened, the competition between artificial and natural indigo would
assume a much more serious aspect.[5]

The light oil of the tar-distiller has now been sufficiently dealt with so
far as regards colouring-matters; let us pass on to the next fraction of
the tar, the carbolic oil. The important constituents of this portion are
carbolic acid and naphthalene. The carbolic oil is in the first place
separated into two distinct portions by washing with an alkaline solution.
Carbolic acid or phenol belongs to a class of compounds derived from
hydrocarbons of the benzene and related series by the substitution of the
residue of water for hydrogen. This water-residue is known to chemists as
"hydroxyl"--it is water less one atom of hydrogen. Carbolic acid or phenol
is hydroxybenzene; and all analogous compounds are spoken of as "phenols."
It will be understood in future that a phenol is a hydroxy-derivative of a
benzenoid hydrocarbon. Now these phenols are all more or less acid in
character by virtue of the hydroxyl-group which they contain. For this
reason they dissolve in aqueous alkaline solutions, and are precipitated
therefrom by acids. This will enable us to understand the purification of
the carbolic oil.

The two layers into which this oil separates after washing with alkali are
(1) the aqueous alkaline solution of the carbolic acid and other phenols,
and (2) the undissolved naphthalene contaminated with oily hydrocarbons
and other impurities. Each of these portions has its industrial history.
The alkaline solution, on being drawn off and made a