id is the main constituent of this fatty
acid.

=Colouring Matter.=--Two brown colouring matters, both containing
nitrogen, can be obtained from raw cotton. One of these is readily
soluble in alcohol, the other only sparingly so. The presence in
relatively large quantities of these bodies accounts for the brown
colour of Egyptian and some other dark-coloured varieties of cotton.

=Pectic Acid.=--This is the chief impurity found in raw cotton. It can
be obtained in the form of an amorphous substance of a light yellow
colour, not unlike gum in appearance. It 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 f