dyes are applied to cotton, linen,
wool and other fibres we can divide them into five groups, thus:--

Group 1. Direct dyes.
" 2. Basic dyes.
" 3. Acid dyes.
" 4. Mordant dyes.
" 5. Miscellaneous dyes.

First group, direct dyes, are now very numerous; they dye cotton, linen
and other vegetable fibres from a plain bath, and do not require those
fibres to be prepared in any way. Hence the reason of their being named
direct, or by some the substantive colours. They will also dye wool and
silk.

The second group, basic dyes, comprise some of the oldest of the
coal-tar dyes; they dye wool and silk direct from plain baths, but
require cotton, linen and other vegetable fibres to be previously
prepared in baths of tannic acid, sumach or other tanning material.

The third group, acid dyes, are very numerous, and from both their
chemical composition and mode of dyeing can be divided into several
sub-groups. Their principal feature is that they dye wool and silk from
baths containing Glauber's salt and some acid, hence their name of "acid
dyes". They do not dye cotton or linen well, some not at all, others are
absorbed to a slight extent by the cotton, but only pale tints are
produced, while others may be used along with metallic mordants to dye
bright but pale and fugitive shades. The acid dyes comprise such as Acid
green, Formyl violet, Acid magenta, Azo scarlet, Orange. Thiocarmine R.,
Patent blues, Wool greens, indigo extract, Eosines, etc.

The fourth group, mordant dyes, includes the alizarine series of
coal-tar colours, logwood, Brazil wood and most natural colours, and
some others. The principal feature of these dyes is that they require
the cotton to be prepared with some metallic oxide, like those of
chrome, alumina and iron, before dyeing, and the colour which is got
depends partly upon the particular dye-stuff used and partly upon the
oxide with which the cotton has been prepared.

The fifth group includes a few dyes like indigo, which are dyed on to
cotton by various and special processes.

The processes of cotton dyeing employed to-day may be comprised under
eight heads, namely:--

(1) Direct dyeing.
(2) Direct dyeing followed by fixation with metallic salts.
(3) Direct dyeing followed by fixation with developers.
(4) Direct dyeing followed by fixation with couplers.
(5) Dyeing on tannic mordant.
(6) Dyeing on metallic mordant.
(7) Developing the colour direct o