e
oil used, and for this reason it is subject to one or more processes of
rectification. This is performed by distilling, the distillates at the
various temperatures being separately collected.

When pure, aniline is a colourless oily liquid, but on exposure rapidly
turns brown. It has strong refracting powers and an agreeable aromatic
smell. It is very poisonous when taken internally; its sulphate is,
however, sometimes used medicinally. It is by the action upon aniline of
certain oxidising agents, that the various colouring matters so well
known as aniline dyes are obtained.

Commercial aniline oil is not, as we have seen, the purest form of
rectified aniline. The aniline oils of commerce are very variable in
character, the principal constituents being pure aniline, para- and
meta-toluidine, xylidines, and cumidines. They are best known to the
colour manufacturer in four qualities--

(_a_) Aniline oil for blue and black.

(_b_) Aniline oil for magenta.

(_c_) Aniline oil for safranine.

(_d_) _Liquid toluidine.

From the first of these, which is almost pure aniline, aniline black is
derived, and a number of organic compounds which are further used for the
production of dyes. The hydrochloride of aniline is important and is
known commercially as "aniline salt."

The distillation and rectification of aniline oil is practised on a
similar principle to the fractional distillation which we have noticed as
being used for the distillation of the naphthas. First, light aniline
oils pass over, followed by others, and finally by the heavy oils, or
"aniline-tailings." It is a matter of great necessity to those engaged in
colour manufacture to apply that quality oil which is best for the
production of the colour required. This is not always an easy matter, and
there is great divergence of opinion and in practice on these points.

The so-called aniline colours are not all derived from aniline, such
colouring matters being in some cases derived from other coal-tar
products, such as benzene and toluene, phenol, naphthalene, and
anthracene, and it is remarkable that although the earlier dyes were
produced from the lighter and more easily distilled products of
coal-tar, yet now some of the heaviest and most stubborn of the
distillates are brought under requisition for colouring matters, those
which not many years ago were regarded as fit only to be used as
lubricants or to be regarded as waste.

It is scarcely necessary o