the glass
tube, so it is now precipitated in the little tubes of the cotton fibre
(see Lecture I.). Let us see if we can now change our chrome yellow to
chrome orange, just as we did in the glass vessel by boiling in
lime-water. I place the yellow fabric in boiling 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 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