You see this is
quite a different process of pigment colouring from that of rubbing or
working a colour mechanically on to the fibre.
Let us now turn to the substantive colours (Group I.), and see if we can
further sub-divide this large group for the sake of convenience. We can
divide the group into two--(_a_) such colours as exist ready formed in
nature, and chiefly occur in plants, of which the following are the most
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.
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