h a suitable mordant. This derivative of gallein is
used to a considerable extent under the name of "coerulein." These two
colouring-matters were the first practical outcome of v. Baeyer's
researches. There is another possible development in this direction which
chemistry may yet accomplish, and another natural colouring-matter may be
threatened, even as the indigo culture was threatened by the later work of
the same chemist. There is reason for believing that the colouring-matter
of logwood, known to chemists as haematein, is related to or derived in
some way from the phthaleins, and the synthesis of this compound may
ultimately be effected.

The dye introduced by Caro in 1874 is the brominated phthalein of
resorcinol. The phthalein itself is a yellow dye, and the solutions of its
salts show a splendid and most intense greenish yellow fluorescence, for
which reason it is called "fluorescein." When brominated, the latter
furnishes a beautiful red colouring-matter known as "eosin" (Gr. [Greek:
eos], dawn), and the introduction of this gave an industrial impetus to
the phthaleins which led to the discovery of many other related
colouring-matters now largely used under various trade designations.
About a dozen distinct compounds producing different shades of pink,
crimson and red, and all derived from fluorescein, are at present in the
market, and a few other phthaleins formed by heating phthalic anhydride
with other phenolic compounds instead of resorcinol or pyrogallol (_e.g._
diethylamidophenol), are also of industrial importance. By converting
nitrobenzene into a sulpho-acid, reducing to an amido-sulpho-acid, and
then fusing with alkali, an amido-phenol is produced, the ethers of which,
when heated with phthalic anhydride, give rise to red phthaleins of most
intense colouring power introduced by the Baden Aniline Company as
"rhodamines."

It remains to point out that the scientific spirit which prompted the
investigation of the phthaleins in the first instance has followed these
compounds throughout their technological career. The researches started by
v. Baeyer were taken up by various chemists, whose work together with that
of the original discoverer has led to the elucidation of the constitution
of these colouring-matters. The phthaleins are members of the
triphenylmethane group, and are therefore related to magenta, corallin,
malachite green, methyl violet, and the phosgene dyes.

It has been said that the phenol