hand,
and for its combination with the wool fibre on the other. In dyeing
pale tints with acid dye-stuffs it is a good plan not to add the acid
until after the goods have been entered into the bath and worked for a
short time to enable them to become impregnated with the dye-liquor;
the acid may be then added, and the dyeing may be finished as usual.

By this plan of working more even dyeings can be obtained than by
simply entering the goods direct into an acidified dye-liquor.

Any kind of acid may be employed, but generally sulphuric acid is
used, partly because it is cheap, and partly because it is the
commonest acid known. Acetic acid is also used in many cases.

_Fourth Method_.--We now come to the fourth method of dyeing wool.
Strictly, perhaps, it is not a single method, but a group of methods,
which are used to supply a certain class of dye-stuffs to the wool
fibre; but as the governing principle depends upon the peculiar
property of the dye-stuffs now to be noticed, which underlies all the
variations of the process of dyeing, it has been thought better to
speak of the fourth method rather than to subdivide further, in which
case the fundamental principle might be lost sight of.

The class of dye-stuffs included in the fourth group was named by
Bancroft the "adjective" group, because they require the aid of a
second body, named the mordant, to properly develop and to fix the
colour of the dye-stuff on the wool. It is sometimes known as the
"mordant dye-stuff" class, and this is perhaps its best name. This (p. 069)
group of colouring matters comprises dye-stuffs of both natural and
artificial origin, the latter of which are getting very numerous and
valuable, and bid fair to displace the natural members of the group.
With but few exceptions the adjective dye-stuffs are not colouring
matters of themselves, _i.e._, they will not dye wool or other fibres
by themselves. Some are coloured bodies, such as fustic, logwood,
Persian berries, Anthracene yellow, etc., but many are not so, and
some possess but little colour, which, moreover, gives no clue to the
colours that can be developed therefrom.

All the colouring matters of this class possess either a distinctively
acid character, or belong to the class of phenols, which, while not
being true acids, still possess weak acid functions that enable them
to combine with bases like acids. These bodies have the property of
combining with bases and metallic oxides, such