f practically all coloring matter, the pulverized root of
the madder plant yielding the reds, the leaves and stems of the indigo
plant the blues, the heartwood of the tropical logwood tree the blacks
and grays, and the fruit of certain palm and locust trees yielding the
soft browns. So great was the commercial demand for dyestuffs that
large areas of land were given over to the exclusive cultivation of
the more important dye plants. Vegetable dyes are now, however, rarely
used because about the year 1856 it was discovered that dyes could be
obtained from coal tar, the thick sticky liquid formed as a by-product
in the manufacture of coal gas. These artificial coal-tar, or aniline,
dyes have practically undisputed sway to-day, and the vast areas of
land formerly used for the cultivation of vegetable dyes are now free
for other purposes.

226. Wool and Cotton Dyeing. If a piece of wool is soaked in a
solution of a coal-tar dye, such as magenta, the fiber of the cloth
draws some of the dye out of the solution and absorbs it, becoming in
consequence beautifully colored. The coloring matter becomes "part and
parcel," as it were, of the wool fiber, because repeated washing of
the fabric fails to remove the newly acquired color; the magenta
coloring matter unites chemically with the fiber of the wool, and
forms with it a compound insoluble in water, and hence fast to
washing.

But if cotton is used instead of wool, the acquired color is very
faint, and washes off readily. This is because cotton fibers possess
no chemical substance capable of uniting with the coloring matter to
form a compound insoluble in water.

If magenta is replaced by other artificial dyes,--for example,
scarlets,--the result is similar; in general, wool material absorbs
dye readily, and uniting with it is permanently dyed. Cotton material,
on the other hand, does not combine chemically with coloring matter
and therefore is only faintly tinged with color, and loses this when
washed. When silk and linen are tested, it is found that the former
behaves in a general way as did wool, while the linen has more
similarity to the cotton. That vegetable fibers, such as cotton and
linen, should act differently toward coloring matter from animal
fibers, such as silk and wool, is not surprising when we consider that
the chemical nature of the two groups is very different; vegetable
fibers contain only oxygen, carbon, and hydrogen, while animal fibers
always contain