parent-compound is diphenylamine in which
sulphur replaces hydrogen, and is therefore known as thiodiphenylamine. It
can be prepared by heating diphenylamine with sulphur, and is sometimes
called thiazine, because it is somewhat analogous in type to azine. We
must therefore credit dimethylaniline with being the industrial generator
of the thiazines. The blue is largely used for cotton dyeing, producing on
this fibre when properly mordanted an indigo shade. By the action of
nitrous acid the blue is converted into a green known as "methylene
green."

Although the scope of this work admits of our dealing with only a few of
the more important groups of colouring-matters, it will already be evident
that the chemist has turned benzene and toluene to good account. But great
as is the demand for these hydrocarbons for the foregoing purposes, there
are other branches of the coal-tar industry which are dependent upon them.
It will serve as an answer to those who are continually raising the cry of
brilliancy as an offence to aesthetic taste if we consider in the next
place a most valuable and important black obtained from aniline. All
chemists who studied the action of oxidizing agents, such as chromic acid,
on aniline, from Runge in 1834 to Perkin in 1856, observed the formation
of greenish or bluish-black compounds. After many attempts to utilize
these as colouring-matters, success was achieved by John Lightfoot of
Accrington near Manchester in 1863. By using as an oxidizing agent a
mixture of potassium chlorate and a copper salt, Lightfoot devised a
method for printing and dyeing cotton fabrics, the use of which spread
rapidly and created an increased demand for the hydrochloride of aniline,
this salt being now manufactured in enormous quantities under the
technical designation of "aniline salt." Lightfoot's process was improved
for printing purposes by Lauth in 1864, and many different oxidizing
mixtures have been subsequently introduced, notably the salts of vanadium,
which are far more effective than the salts of copper, and which were
first employed by Lightfoot in 1872. In 1875-76 Coquillion and
Goppelsroeder showed that aniline black is produced when an electric
current is made to decompose a solution of an aniline salt, the oxidizing
agent here being the nascent oxygen resulting from the electrolysis. In
these days when the generation of electricity is so economically effected,
this process may become more generally used