rrived
at by heating dry aniline hydrochloride, _i.e._ the hydrochloric acid salt
of aniline, with methyl alcohol or wood-spirit in strong metallic boilers
under great pressure. This is the process carried on in most factories,
and it involves the use of pure methyl alcohol, a branch of manufacture
which has been called into existence to meet the requirements of the
coal-tar colour maker.[4] This alcohol or wood-spirit is obtained by the
destructive distillation of wood, and is purified by a series of
operations which do not at present concern us. It must be mentioned that
the product of the methylation of aniline, which it is the object of the
manufacturer to obtain, is an oily liquid called dimethylaniline, which,
by virtue of the chemical transformation, is quite different in its
properties to the aniline from which it is derived. By a similar
operation, using ethyl alcohol, or spirit of wine, diethylaniline can be
obtained, and by heating dry aniline hydrochloride with aniline under
similar conditions a crystalline base called diphenylamine is also
prepared.

Now these products--dimethylaniline, diethylaniline, and
diphenylamine--are derived from aniline, and they are all sources of
colouring-matters. Methyl-violet is obtained by the oxidation of
dimethylaniline by means of a gentle oxidizer; a mixture of bases is not
necessary as in the case of the magenta formation. Then in 1866
diphenylamine was shown by Girard and De Laire to be capable of yielding a
fine blue by heating it with oxalic acid, and this blue, on account of the
purity of its shade, is still an article of commerce. It can be made
soluble by the action of sulphuric acid in just the same way as the other
aniline blue. Furthermore, by acting with excess of methyl chloride on
methyl violet, a brilliant green colouring-matter was manufactured in
1878, which was obviously analogous to the iodine green already mentioned,
and which for some years held its own as the only good coal-tar green.
These are the dyes--methyl violet and green, and diphenylamine blue--which
were in commerce before the discovery of the Fischers, and which this
discovery enabled chemists to class with magenta, aniline blue, and
Hofmann violet in the triphenylmethane group.

Later developments bring us into contact with other dyes of the same
class, and with the industrial evolution of the purely scientific idea
concerning the constitution of the colouring-matters of this group.
Benze