G lime-water, when it is coloured or dyed orange. In each little
tubular cotton fibre the same change goes on as went on in the glass
vessel, and as the tube or glass vessel
looks orange, so does the fabric, because the cotton fibres or tubes
are filled with the orange chromium compound. 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. 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