ass. By diazotising this amidoazo-sulpho-acid, and combining it
with beta-naphthol, a fine scarlet dye was discovered by Nietzki in 1879,
and introduced under the name of "Biebrich scarlet." Two years later a new
sulpho-acid of beta-naphthol was discovered by Bayer & Co. of Elberfeld,
and this gave rise, when combined with diazotised acid yellow and
analogous compounds, to another series of brilliant dyes introduced as
"Crocein scarlets."

From these beginnings the development of the azo-dyes has been steadily
carried on to the present time--year by year new diazotisable
amido-compounds or new sulpho-acids of the naphthols and naphthylamines
are being discovered, and this branch of the colour industry has already
assumed colossal dimensions. An important departure was made in 1884 by
Boettiger, who introduced the first secondary azo-colours derived from
benzidine. As already explained in connection with salicylic acid, this
base and its homologue tolidine form tetrazo-salts, which combine with
phenols and amines or their sulpho-acids. One of the first
colouring-matters of this group was obtained by combining diazotised
benzidine with the sulpho-acid of alpha-naphthylamine (naphthionic acid),
and was introduced under the name of "Congo red." Then came the discovery
(Pfaff, 1885), that the tetrazo-salts of benzidine and tolidine combine
with phenols, amines, &c., in two stages, one of the diazo-groups first
combining with one-half of the whole quantity of phenol to form an
intermediate compound, which then combines with the other half of the
phenol to form the secondary azo-dye. In the hands of the
"Actiengesellschaft fuer Anilinfabrikation" of Berlin this discovery has
been utilized for the production of a number of such azo-colours
containing two distinct phenols, or amines, or sulpho-acids. Tolidine has
been found to give better colouring-matters in most cases than benzidine,
and it is scarcely necessary to point out that an increased demand for the
nitrotoluene from which this base is made is the necessary consequence of
this discovery.

It is impossible to attempt to specify by name any of these recent
benzidine and tolidine dyes. Their introduction has been the means of
finding new uses for the naphthylamines and naphthols and their
sulpho-acids, and has thus contributed largely to the utilization of
naphthalene. An impetus has been given to the investigation of these
sulpho-acids, and chemical science has profited