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 nitrogen in addition, and in many cases sulphur as
well.
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