own by
Koerner to be a derivative of benzene, and from this hint the technical
process for the preparation of the compound on a large scale has been
developed. Resorcinol is a phenolic derivative of benzene containing two
hydroxyl groups; it is therefore related to phenol in the same way that
diamidobenzene is related to aniline or phthalic acid to benzoic acid. The
relationships can be expressed in a tabular form thus--

Amidobenzene or Aniline. Benzoic acid. Carbolic acid or Phenol.
Diamidobenzene. Phthalic acid. Resorcinol.

Resorcinol is now made by heating benzene with very strong sulphuric acid
so as to convert it into a disulpho-acid, and the sodium salt of the
latter is then fused with alkali. As a technical operation it is one of
great delicacy and skill, and the manufacture is confined to a few
Continental factories.

When phthalic acid is heated it loses water, and is transformed into a
white, magnificently crystalline substance known as phthalic anhydride,
_i.e._ the acid deprived of water. In 1871, A. v. Baeyer, the eminent
chemist who subsequently synthesised indigo, published the first of a
series of investigations describing the compounds produced by heating
phthalic anhydride with phenols. To these compounds he gave the name of
"phthaleins." Baeyer's work, like that of so many other chemists who have
contributed to the advancement of the coal-tar colour industry, was of a
purely scientific character at first, but it soon led to technological
developments. The phthaleins are all acid compounds possessing more or
less tinctorial power. One of the first discovered was produced by heating
phthalic anhydride with an acid known as gallic acid, which occurs in
vegetable galls, and in the form of tannin in many vegetable extracts
which are used by the tanner. The acid is a phenolic derivative of benzoic
acid, viz. trihydroxybenzoic acid, and on heating it readily passes into
trihydroxybenzene, which is the "pyrogallic acid" or pyrogallol familiar
as a photographic developer. The phthalein formed from gallic acid and
phthalic anhydride really results from the union of the latter with
pyrogallol. It is now manufactured under the name of "gallein," and is
largely used for imparting a bluish grey shade to cotton fabrics. By
heating gallein with strong sulphuric acid, it is transformed into another
colouring-matter which gives remarkably fast olive-green shades when dyed
on cotton fibre wit