s precipitated out on adding acids.

This action of alkalies has an important bearing on the scouring of
wool, for if this operation be not carried out with due care there (p. 010)
is in consequence great liability to impair the lustre and strength of
this fibre. From microscopical examination this effect of alkalies is
seen to be due to the fact that they tend to disintegrate the fibre,
loosen and open the scales, this is shown by contrasting the two
fibres A and B shown in figure 4, A being a normal wool fibre, B one
strongly treated with an alkali.

The alkaline carbonates have but little action on wool, none if used
dilute and at temperatures below 120 deg. F.

[Illustration: Fig. 4.--Showing the Effects of Scouring Agents on the
Wool Fibre. A. Unscoured Fibre. B. Badly Scoured Fibre.]

Soap has practically no action on wool, and is therefore an excellent
scouring material for wool. The carbonate of ammonia is the best and
has the least action of the alkaline carbonates, those of potash and
soda if used too strong or too hot have a tendency to turn the wool
yellow, the carbonate of potash leaves the wool softer and more
lustrous than the carbonate of soda.

The influence of scouring agents on wool will be discussed in the
chapter on cleansing wool fabrics in more detail.

Caustic or quick-lime has a similar injurious action on the wool fibre
as the caustic alkalies.

#Action of Acids.#--Acids when dilute have but little influence on (p. 011)
the wool fibre, their tendency is to cause a separation of the scales
(see fig. 5) of the wool and so make it feel harsher. Strong acids
have a disintegrating action on the wool fibre. There is a very
considerable difference between the action of acids on wool and on
cotton, and this difference of action is taken advantage of in the
woollen industry to separate cotton from wool by the process commonly
known as "carbonising," which consists in treating the fabric with a
weak solution of hydrochloric acid or some other acid, then drying it;
the cotton is disintegrated and falls away in the form of a powder,
while the wool is not affected, sulphuric acid is used very largely in
dyeing wool with the acid- and azo-colouring matters.

[Illustration: Fig. 5.--Wool Fibre Heated with Acid.]

Nitric acid affects wool in a very similar manner to the acids named
above when used in a dilute form; if strong it gives a deep yellow
colour and acts somewhat destructively on the fibre.