l may be effected in two ways, viz. by
treating the material either with sulphurous acid or with hydrogen
peroxide. Sulphurous acid may either be applied in the gaseous form or
in solution as bisulphite of soda. In working by the first method,
which is technically known as "stoving," the scoured yarn is wetted in
very weak soap containing a small amount of blue colouring matter,
wrung or hydro-extracted and then suspended in a chamber or stove.
Sulphur contained in a vessel on the floor of the chamber is now
lighted, and the door having been closed, is allowed to burn itself
out. The goods are left thus exposed to the sulphur dioxide overnight,
when they are taken out and washed in water. For piece goods a
somewhat different arrangement is employed, the pieces passing through
a slit into a chamber supplied with sulphur dioxide, then slowly up
and down over a large number of rollers and ultimately emerging again
at the same slit. Wool may also be bleached by steeping in a fairly
strong solution of bisulphite of soda and then washing well in water.
Wool bleached with sulphurous acid or bisulphite is readily affected
by alkalis, the natural yellow colour returning on washing with soap
or soda. A more permanent bleach is obtained by steeping the wool in
hydrogen peroxide (of 12 volumes strength), let down with about three
times its bulk of water and rendered slightly alkaline with ammonia or
silicate of soda. Black or brown wools cannot be bleached white, but
when treated with peroxide they assume a golden colour, a change which
is frequently desired in human hair.

_Bleaching of Silk._

In raw silk, the fibre proper is uniformly coated with a proteid
substance known as _silk-gum, silk-glue_ or _sericine_ which amounts to
19-25% of the weight of the material, and it is only after the removal
of this coating that the characteristic properties of the fibre become
apparent. This is effected by the process of "discharging" or
"boiling-off," which consists in suspending the hanks of raw silk over
poles or sticks in a vat containing a strong hot soap solution (30% of
soap on the weight of the silk). The liquor is kept just below boiling
point for two or three hours, the hanks being turned from time to time.
During the process, the sericine at first swells up considerably, the
fibres becoming slippery, but as the operation proceeds it passes into
solution. It is important that only