ither be
effected by dyeing or tinting with a colouring matter like Victoria
blue 4R or acid violet, or by treatment in wash stocks with a
suspension of ultramarine in weak soap until the colour is uniformly
distributed throughout the material. The yarn is now straightened out
and dried.

The bleaching of cotton yarn is a very straightforward process, and it
is very seldom that either complications or faults arise, providing
that reasonable care and supervision are exercised.

The _raison d'etre_ of the various operations is comparatively simple.
The effect of boiling with alkali is to remove the pectic acid, the
fatty acids, part of the cotton wax and the bulk of the colouring
matter, while the albuminoids are destroyed and the motes swelled up.
If soap be used along with the alkali, the whole of the wax is removed
by emulsification. In the operation of bleaching proper, the calcium
hypochlorite of the chloride of lime through coming into contact with
the carbonic acid of the atmosphere suffers decomposition according to
the equation, Ca(OCl)2 + CO2 + H2O -> CaCO3 + 2HOCl, and the
hypochlorous acid thus liberated destroys the colouring matter still
remaining from the first operation, by oxidation. At the same time the
motes which were swelled up by the alkali are broken up into small
fragments and are thus removed. In the operation of souring, the lime
which has been deposited on the fibres during the treatment with
bleaching powder is dissolved, while at the same time any other
metallic oxides (iron, copper, &c.) are removed.

_Cotton Pieces._--By far the largest bulk of cotton is bleached in the
piece, as it can be more conveniently and more economically dealt with
in this form than in any other. Though similar in principle to yarn
bleaching, the process of piece bleaching is somewhat more complex
because the pieces contain in addition to the natural impurities of
the cotton a considerable amount of foreign matter in the form of size
which has been incorporated with the warp before weaving, with the
object of strengthening it. This size consists essentially of starch
(farina), with additions of tallow, zinc chloride, and occasionally
other substances such as paraffin wax, magnesium chloride, soap, &c.,
all of which must be removed if a perfect bleach is to result.
Besides, mineral oil stains from the machinery of the weaving-shed are
of comm