forms a red azo-dye, introduced by Dahl & Co. in 1885 as
"thiorubin." By modifying the conditions of reaction between the sulphur
and the base, it was found in 1887 by Arthur Green, that a complicated
thio-derivative of toluidine could be produced which possessed very
remarkable properties. The sulpho-acid of the thio-base is a yellow dye,
which was named by its discoverer "primuline." Not only is primuline a
dye, but it contains an amido-group which can be diazotised. If therefore
the fabric dyed with primuline is passed through a nitrite bath, a
diazo-salt is formed in the fibre, and on immersing the latter in a second
bath containing naphthol or other phenol or an amine, an azo-dye is
precipitated in the fibre. By this means there are produced valuable
"ingrain colours" of various shades of red, orange, purple, &c.

So much for the azo-dyes, one of the most prolific fields of industrial
enterprise connected with coal-tar technology. From the introduction of
aniline yellow in 1863 to the present time, about 150 distinct compounds
of this group have been given to the tinctorial industry. Of these over
thirty are cotton dyes containing two azo-groups. Sombre shades, rivalling
logwood black, bright yellows, orange-reds, browns, violets, and brilliant
scarlets equalling cochineal, have been evolved from the refuse of the
gas-works. The artificial colouring-matters have in this last case once
again threatened a natural product, and with greater success than the
indigo synthesis, for the introduction of the azo-scarlets has caused a
marked decline in the cochineal culture.

In addition to the azo-colours, there are certain other products which
claim naphthalene as a raw material. In 1879 it was found that one of the
sulpho-acids of beta-naphthol when treated with nitrous acid readily gave
a nitroso-sulpho-acid. A salt of this last acid, containing sodium and
iron as metallic bases, was introduced in 1884, under the name of
"naphthol green." It is used both as a dye for wool and as a pigment. It
may be mentioned here that other nitroso-derivatives of phenols, such as
those of resorcinol and the naphthols, under the name of "gambines," are
largely used for dyeing purposes, owing to the facility with which they
combine with metallic mordants to form coloured salts in the fibre. In
this same year, 1879, it was found that by heating nitrosodimethylaniline
with beta-naphthol in an appropriate solvent, a violet colouring-matter
w