ulphonation, an operation of the greatest (p. 067)
importance and value in the preparation of dye-stuffs. The preparation
of indigo extract or indigo carmine from indigo is also a case of
sulphonation. The sulpho-acids of the azo colours, of the basic dyes,
and of indigo are usually insoluble in water, although there are great
differences in their properties in this respect. They will combine
with bases such as soda, calcium and potash to form salts which are
soluble in water, and it is usually in the form of sodium salts that
these azo and acid dye-stuffs are sold to the dyer and calico printer.
It is this power of combination with bases that makes them of value in
wool dyeing. As Knecht and other authorities have pointed out, the
wool fibre contains a basic principle capable of combining with acid
bodies, and in wool dyeing with the colouring matters under
discussion, this combination occurs between the sulpho-acid of the
dye-stuff and the basic principle of the wool fibre.

This points to the fact that the dye-stuffs of this class do not
combine with the wool in the form in which they are supplied to the
dyer as sodium salts, which is shown by a property that many if not
all of them possess, of not dyeing the wool fibre in a neutral bath.
If a piece of wool be immersed in a solution of, say, a scarlet or
indigo extract, which is neutral it is not dyed. The dye-liquor may
penetrate thoroughly throughout the fabric, but if the piece of wool
be lifted out, and allowed to drain, nearly all the liquor will drain
away, and leave the wool nearly if not quite white, showing that the
dye-stuff in the form in which it is sold has no affinity for the wool
fibre. If now a few drops of sulphuric acid be added to the dye-liquor
the wool will become dyed. The sulphuric acid liberates the free
sulpho-acid of the dye-stuff, and this is now in a form to combine
with the wool fibre, which it does. This is the fundamental principle
underlying the acid method for dyeing wool with the acid group of
colouring matters.

The practical application of the principle laid down above is a (p. 068)
matter of simplicity compared with the other methods of dyeing. The
composition of the bath is given above. It is best to enter the wool
at from 150 deg. to 160 deg. F. and then to raise the temperature slowly
to the boil. This method of proceeding gives time for the free colour
acid of the dye to be liberated from the dye-stuff on the one