The case is just the same with logwood black dyeing: without the
presence of iron ("copperas," etc.), sulphate of copper ("bluestone"),
or bichrome, you would get no black at all. We will now try similar
experiments with woollen fabrics, taking three simple pieces of flannel,
and also two pieces, the one having been first treated with a hot
solution of alum and cream of tartar, and the other with copperas or
sulphate of iron solution, and then washed. Turmeric dyes the first
yellow, like it did the cotton. Magenta, however, permanently dyes the
woollen as it did not the cotton. Alizarin only stains the untreated
woollen, whilst the piece treated with alumina is dyed red, and that
with iron, purple. If, however, the pieces treated with iron and alumina
had been dyed in the Magenta solution, only one colour would have been
the result, and that a Magenta-red in each case. Here we have, as proved
by our experiments, two distinct classes of colouring matters. The one
class comprises those which are of themselves the actual colour. 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 (Hæmatein), 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