st and
importance in the dyeing of wool, as an acid condition of the bath is
necessary for dyeing by far the great majority of colouring matters on
this fibre. Alkaline salts, such as Glauber's salt and common salt,
exert little or no influence on this felting property, and can be
added to dye-baths with impunity, and in many cases with good effect,
so far as the quality of dyeing is concerned.

So far as the properties of the wool are concerned, it is seen (p. 061)
that an acid condition of the dye-bath will work better than an
alkaline condition, and wherever it is possible to use acids such
should be added.

What has been said in regard to wool is equally true of all fibres
derived from animals in the same way as wool is, such as horse-hair,
fur of rabbits, hares and other animals, although, of course, there
are some minor differences between different furs in their resistance
to the action of acids and alkalies.

The next feature that influences the methods of dyeing wool is the
varying properties of the dye-stuffs, or colouring matters. It is
obvious that those which, like Magenta or Saffranine, have a strong
affinity for the wool fibre must be dyed differently from those which,
like Alizarine and Gambine, have no direct affinity for the wool
fibre, and, further, which require the aid of mordants before they can
be dyed, and on the character of which mordants the colour that is
fixed on the fibre depends.

The dye-stuffs, independently of the question whether they be derived
from natural sources or be of artificial origin, may be roughly
divided into five groups, some of which may also be subdivided again
as will be shown later on. These groups may be named the (1) Neutral,
(2) Basic, (3) Acid, (4) Mordant, and (5) Indigo dye-stuffs. The first
two classes are practically dyed in the same way; but as there is a
great difference in the chemical composition of the colouring matters
comprised in them, it will be best to consider them separately.

_First Method_.--This method is used in applying the now large and
increasing group of azo dye-stuffs, which are characterised by being
able to dye unmordanted cotton from a simple boiling bath. The
dye-stuffs that are applied by the method now to be described include
such as Benzopurpurine, Chrysamine, Chrysophenine, Titan red, Titan
yellow, Benzo brown, Diamine red, Diamine brown, Diamine blue, (p. 062)
Congo blue, Congo red, etc. The dyeing is done in a bath a