The
colour is fully developed in them, and to dye a fabric they only require
fixing in their unchanged state upon that fabric. Such dyes are termed
_monogenetic_, because they can only generate or yield different shades
of but one colour. Indigo is such a dye, and so are Magenta, Aniline
Black, Aniline Violet, picric acid, Ultramarine Blue, and so on.
Ultramarine is not, it is true, confined to blue; you can get
Ultramarine Green, and even rose-coloured Ultramarine; but still, in
the hands of the dyer, each shade remains as it came from the
colour-maker, and so Ultramarine is a monogenetic colour. Monogenetic
means capable of generating one. Turning to the other class, which
comprises, as we have shown, Alizarin, and, besides, the colouring
principle of logwood (Haematein), Gallein, and Cochineal, etc., we have
bodies usually possessed of some colour, it is true, but such colour is
of no consequence, and, indeed, is of no use to dyers. These bodies
require a special treatment to bring out or develop the colours, for
there may be several that each is capable of yielding.
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