The Dyeing of Cotton Fabrics | Page 3

is soluble in boiling water, and
the solution has a faint acid reaction. Acids and many metallic salts,
such as mercury, chloride and lead acetate, precipitate pectic acid from
its solutions. Alkalies combine with it, and these compounds form
brown substances, are but sparingly soluble in water, and many of them
can be precipitated out by addition of neutral salts, like sodium and
ammonium chlorides.
=Albumens.=--A small quantity of albuminous matter is found among
the impurities of cotton.
=Structure of the Cotton Fibre.=--The cotton fibre varies in length from
1 to 2 inches, not only in fibres of the same class but also in fibres from
different localities--Indian fibres varying from 0.8 in the shortest to 1.4
in the longest stapled varieties; Egyptian cotton fibres range from 1.1 to
1.6 inches long; American cotton ranges from 0.8 in the shortest to 2
inches in the longest fibres. The diameter is about 1/1260 of an inch.
When seen under the microscope fully ripe cotton presents the
appearance of irregularly twisted ribbons, with thick rounded edges.

The thickest part is the root end, or point of attachment to the seed. The
free end terminates in a point. The diameter is fairly uniform through
3/4 to 7/8 of its length, the rest is taper. In Fig. 1 is given some
illustrations of the cotton fibre, showing this twisted and ribbon-like
structure, while in Fig. 1A is given some transverse sections of the fibre.
These show that it is a collapsed cylinder, the walls being of
considerable thickness when compared with the internal bore or canal.
Perfectly developed, well-formed cotton fibres always present this
appearance. But all commercial cottons contain more or less of fibres
which are not perfectly developed or are unripe. These are known as
"dead fibres"; they do not spin well and they do not dye well. On
examination under the microscope it is seen that these fibres have not
the flattened, twisted appearance of the ripe fibres, but are flatter, and
the central canal is almost obliterated and the fibres are but little
twisted. Dead fibres are thin, brittle and weak.
=Composition of the Cotton Fibres.=--Of all the vegetable textile fibres
cotton is found to have the simplest chemical composition and to be, as
it were, the type substance of all such fibres, the others differing from it
in several respects. When stripped of the comparatively small quantities
of impurities, cotton is found to consist of a substance to which the
name of cellulose has been given.
[Illustration: FIG. 1.--Cotton Fibre.] [Illustration: FIG. 1A.]
Cellulose is a compound of the three elements, carbon, hydrogen and
oxygen, in the proportions shown in the following analysis:--
Carbon, 44.2 per cent., Hydrogen, 6.3 per cent., Oxygen, 49.5 per cent.,
which corresponds to the empirical formula C{6}H{10}O{5}, which
shows it to belong to the group of carbo-hydrates, that is, bodies which
contain the hydrogen and oxygen present in them in the proportion in
which they are present in water, namely H{2}O.
Cellulose may be obtained in a pure condition from cotton by treatment
with alkalies, followed by washing, and by treatment with alkaline

hypochlorites, acids, washing and, finally, drying. As thus obtained it is
a white substance having the form of the fibre from which it is procured,
showing a slight lustre, and is slightly translucent. The specific gravity
is 1.5, it being heavier than water. It is characterised by being very inert,
a property of considerable value from a technical point of view, as
enabling the fibres to stand the various operations of bleaching, dyeing,
printing, finishing, etc. Nevertheless, by suitable means, cellulose can
be made to undergo various chemical decompositions which will be
noted in some detail.
Cellulose on exposure to the air will absorb moisture or water. This is
known as hygroscopic moisture, or "water of condition". The amount in
cotton is about 8 per cent., and it has a very important bearing on the
spinning properties of the fibre, as it makes the fibre soft and elastic,
while absolutely dry cotton fibre is stiff, brittle and non-elastic; hence it
is easier to spin and weave cotton in moist climates or weather than in
dry climates or weather. Cotton cellulose is insoluble in all ordinary
solvents, such as water, ether, alcohol, chloroform, benzene, etc., and
these agents have no influence in any way on the material, but it is
soluble in some special solvents that will be noted later on.
ACTION OF ALKALIES.
The action of alkalies on cellulose or cotton is one of great importance
in view of the universal use of alkaline liquors made from soda or
caustic soda in the scouring, bleaching and dyeing of cotton, while
great interest attaches to the use of caustic soda in the "mercerising" of
cotton.
Dilute solutions of the caustic alkalies, caustic soda or