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 or advisable in a work of this kind to pursue the many chemical reactions, which, from the various acids and bases, result ultimately in the many shades and gradations of colour which are to be seen in dress and other fabrics.  Many of them, beautiful in the extreme, are the outcome of much careful and well-planned study, and to print here the complicated chemical formulae which show the great changes taking place in compounds of complex molecules, or to mention even the names of these many-syllabled compounds, would be to destroy the purpose of this little book.  The Rosanilines, the Indulines, and Safranines; the Oxazines, the Thionines:  the