Sulphurous acid (sulphur dioxide) has no effect on the actual fibre,
but exercises a bleaching action on the yellow colouring matter which
the wool contains, it is therefore largely used for bleaching (p. 012)
wool, being applied either in the form of gas or in solution in water;
the method will be found described in another chapter. Wool absorbs
sulphur dioxide in large amount, and if present is liable to retard
any subsequent dyeing processes.

#Action of Other Substances.#--Chlorine and the hypochlorites have an
energetic action on wool, and although they exert a bleaching action
they cannot well be used for bleaching wool. Hot solutions bring about
a slight oxidation of the fibre, which causes it to have a greater
affinity for colouring matters; advantage is taken of this fact in the
printing of delaines and woollen fabrics, while the woollen dyer would
occasionally find the treatment of service. A paper by Mr. E. Lodge,
in the _Journal of the Society of Dyers and Colourists_, 1892 (p. 41),
may be consulted with advantage on this subject. Wool treated with
chlorine loses its felting property, and hence becomes unshrinkable, a
fact of which advantage is taken in preparing unshrinkable woollen
fabrics.

When wool is boiled with solutions of metallic salts, such as the
sulphate of iron, chrome, aluminium and copper, the chlorides of tin,
copper and iron, the acetates of the same metals, as well as with some
other salts, decomposition of the salt occurs and a deposit of the
metallic oxide on the wool is obtained with the production of an acid
salt which remains in solution. In some cases this action is
favourably influenced by the presence of some organic acid or organic
salt, as, for examples, oxalic acid and cream of tartar (potassium
tartrate), along with the metallic salt.

On this fact depends the process of mordanting wool with potassium
bichromate, alum, alumina sulphate, ferrous sulphate, copper sulphate,
etc. The exact nature of the action which occurs is not properly
understood, but there is reason for thinking that the wool fibre has
the capacity of assimilating both the acid and the basic constituents
of the salt employed.

Excessive treatment with many metallic salts tends to make the (p. 013)
wool harsh to the feel, partly owing to the scales being opened out and
partly owing to the feel naturally imparted by the absorbed metallic
salt.

The normal salts of the alkaline metals, such as so