632. Coloring Matters. — ^There is a great variety of these
in the vegetable world. A portion of them are composed
of C, H, O, and N, but some of them contain no N. The
former are called nitrogenous because they contain nitro-
gen, and the latter non-nitrogenous. The principal of the
latter class are madder, hematoxyline, which is the color-
ing principle in logwood^ Brazil-wood, and camwood, gam-
boge, etc. Indigo is the most important of the nitrogenous
class. This is derived from the juice of several species of
the plants called indigofera. The indigo is not, however,
of a blue color in the plants, but is almost colorless. It ac-
quires the brilliant blue color that we see it have by a fer-
mentation, to which the leaves of the plants. are subjected
in the extraction of the indigo. Blue indigo is, therefore,
oxidized indigo, and by depriving it of its oxygen we can
restore it to its colorless state. When thus deoxidized it
is soluble, as it is in its natural state in the plant; but by
exposure to the air it absorbs oxygen rapidly, and so be-
comes blue and insoluble. The blue litmus, so much used
by the chemist in testing acids and bases, is a nitrogenous
coloring matter which is derived from certain lichens. Al-
most the only coloring matter of animal origin is cochineal
— an insect. The color from this insect is called carmine.
633. Mordants. — Some coloring matters have such an
affinity for the substance of cloth that they will unite inti-
mately and firmly with the fibres, and so make fast colors.
But some have not this power. In order to fix the colors



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878 • CHEMISTBY.
