important: indigo, archil or orchil, safflower, turmeric, and annatto;
(_b_) the very large sub-group of the artificial or coal-tar colours. We
will briefly consider now the dyestuffs mentioned in Group (_a_).

_Natural Substantive Colours._--Indigo, one of the most valuable dyes,
is the product of a large number of plants, the most important being
different species of _indigofera_, which belong to the pea family. None
of the plants (of which _indigofera tinctoria_ is the chief) contain the
colouring matter in the free state, ready-made, so to say, but only as a
peculiar colourless compound called _indican_, first discovered by
Edward Schunck. When this body is treated with dilute mineral acids it
splits up into Indigo Blue and a kind of sugar. But so easily is this
change brought about that if the leaf of the plant be only bruised, the
decomposition ensues, and a blue mark is produced through separation of
the Indigo Blue. The possibility of dyeing with Indigo so readily and
easily is due to the fact that Indigo Blue absorbs hydrogen from bodies
that will yield it, and becomes, as we say, reduced to a body without
colour, called Indigo White, a body richer in hydrogen than Indigo Blue,
and a body that is soluble. If this white body (Indigo White) be exposed
to the air, the oxygen of the air undoes what the hydrogen did, and
oxidises that Indigo White to insoluble Indigo Blue. Textile fabrics
dipped in such reduced indigo solutions, and afterwards exposed to the
air, become blue through deposit in the fibres of the insoluble Indigo
Blue, and are so dyed. This is called the indigo-vat method. We can
reduce this indigo so as to prepare the indigo-vat by simply mixing
Indigo Blue, copperas (ferrous sulphate) solution, and milk of lime in a
closely-stoppered bottle with water, and letting the mixture stand. The
clear liquor only is used. A piece of cotton dipped in it, and exposed
to the air, quickly turns blue by absorbing oxygen, and is thus dyed.
The best proportions for the indigo-vat are, for cloth dyeing, 4000