derivatives of diphenylamine, containing two nitrogen atoms
joined together in a particular way. The parent-substance from which these
compounds are derived is known at the present time as "azine" (French,
azote = nitrogen), and the dyes belong accordingly to the azine group. The
first coal-tar colouring-matter, Perkin's mauve, is a member of this
class.

The azine dyes are basic, and mostly of a red or pink shade; they are
somewhat fugitive when exposed to light, but possess a certain value on
account of their affinity for cotton, and the readiness with which they
can be used in admixture with other colouring-matters. Some of the best
known are made by oxidizing certain derivatives of aniline or toluidine,
in the presence of these or analogous bases. To make this intelligible a
little more chemistry is necessary. Aniline is a derivative of benzene in
which one atom of hydrogen is replaced by the residue of ammonia. Ammonia
is composed of one atom of nitrogen and three atoms of hydrogen; benzene
is composed of six atoms of carbon and six of hydrogen. If one atom of
hydrogen is supposed to be withdrawn from ammonia, there remains a residue
called the amido-group, and if we imagine this group to be substituted for
one of the hydrogen atoms in benzene, we have an amido-derivative, _i.e._
amidobenzene or aniline. Similarly, the toluidines are amidotoluenes. If
two hydrogen atoms in benzene or toluene are replaced by two amido-groups,
we have diamidobenzenes and diamidotoluenes, which are strongly basic
substances, capable of existing in several isomeric modifications. Certain
of these diamido-compounds when oxidized in the presence of a further
quantity of aniline, toluidine, and such amido-compounds, give rise to
unstable blue products, which readily become transformed into red dyes of
the azine group.

Some azine dyes are produced by another method, which is instructive
because it brings us into contact with a derivative of dimethylaniline
which figures largely in the coal-tar colour industry. By the action of
nitrous acid on this base, there is produced a compound known as
nitrosodimethylaniline, which was discovered by Baeyer and Caro in 1874,
and which contains the residue of nitrous acid in place of one atom of
hydrogen. The residue of nitric acid which replaces hydrogen in benzene is
the nitro-group, and the compound is nitrobenzene. The analogy with
nitrous acid will therefore be sufficiently understood--the residu