th water
the fibre shrinks very much. This shrinkage is more particularly to be
observed in the case of cotton. As John Mercer was the first to point
out the action of the alkaline solutions on cotton, the process has
become known as "mercerisation".

Solutions of caustic soda of 1.000 or 20 deg. Tw. in strength have very
little mercerising action, and it is only by prolonged treatment that
mercerisation can be effected. It is interesting to observe that the
addition of zinc oxide to the caustic solution increases its mercerising
powers. Solutions of 1.225 to 1.275 (that is from 45 deg. to 55 deg. Tw. in
strength) effect the mercerisation almost immediately in the cold, and
this is the best strength at which to use caustic soda solutions for
this purpose. In addition to the change brought about by the shrinking
and thickening of the material, the mercerised fibres are stronger than
the untreated fibres, and at the same time they have a stronger affinity
for dyes, a piece of cloth mercerised taking up three times as much
colouring matter as a piece of unmercerised cloth from the same
dye-bath.

The shrinkage of the cotton, which takes place during the operation of
washing with water, was for a long time a bar to any practical
application of the "mercerising" process, but some years ago Lowe
ascertained that by conducting the operation while the cotton was
stretched or in a state of tension this shrinkage did not take place;
further, Thomas and Prevost found that the cotton so treated gained a
silky lustre, and it has since been ascertained that this lustre is
most highly developed with the long-stapled Egyptian and Sea Island
cottons. This mercerising under tension is now applied on a large scale
to produce silkified cotton. When viewed under the microscope, the
silkified cotton fibres have the appearance shown in Fig. 3, long
rod-like fibres nearly if not quite cylindrical; the cross section of
those fibres has the appearance shown in Fig. 3A. This structure fully
accounts for the silky lustre possessed by the mercerised fibres. Silky
mercerised cotton has very considerable affinity for dye-stuffs, taking
them up much more readily from dye-baths, and it is dyed in very
brilliant shades.

[Illustration: FIG. 3.--Silkified Cotton Fibre.]

[Illustration: FIG 3A.]

In the chapter on Scouring and Bleaching of Cotton, some reference will
be made to the action of alkalies on cotton.

ACTION OF ACIDS ON CELLULOSE.