nty-one years
after that a further discovery was made, which may indeed be said to have
achieved a world-wide reputation. It was found that, by adding bichromate
of potash to a solution of aniline and sulphuric acid, a powder was
obtained from which the dye was afterwards extracted, which is known as
_mauve_. Since that time dyes in all shades and colours have been
obtained from the same source. _Magenta_ was the next dye to make its
appearance, and in the fickle history of fashion, probably no colours
have had such extraordinary runs of popularity as those of mauve and
magenta. Every conceivable colour was obtained in due course from the
same source, and chemists began to suspect that, in the course of time,
the colouring matter of dyer's madder, which was known as _alizarin_,
would also be obtained therefrom. Hitherto this had been obtained from
the root of the madder-plant, but by dint of careful and well-reasoned
research, it was obtained by Dr Groebe, from a solid crystalline coal-tar
product, known as _anthracene_, (C_{12}H_{14}). This artificial alizarin
yields colours which are purer than those of natural madder, and being
derived from what was originally regarded as a waste product, its cost of
production is considerably cheaper.

We have endeavoured thus far to deal with (1) gas, and (2) tar, the two
principal products in the distillation of coal. We have yet to say a few
words concerning the useful ammoniacal liquor, and the final residue in
the retorts, _i.e._, coke.

The ammoniacal liquor which has been passing over during distillation of
the coal, and which has been collecting in the hydraulic main and in
other parts of the gas-making apparatus, is set aside to be treated to a
variety of chemical reactions, in order to wrench from it its useful
constituents. Amongst these, of course, _ammonia_ stands in the first
rank, the others being comparatively unimportant. In order to obtain
this, the liquor is first of all neutralised by being treated with a
quantity of acid, which converts the principal constituent of the liquor,
viz., carbonate of ammonia (smelling salts), into either sulphate of
ammonia, or chloride of ammonia, familiarly known as sal-ammoniac,
according as sulphuric acid or hydrochloric acid is the acid used. Thus
carbonate of ammonia with sulphuric acid will give sulphate of ammonia,
but carbonate of ammonia with hydrochloric acid will give sal-ammoniac
(chloride of ammonia). By a further trea