decomposed so that the base may combine with the essential constituent
of the wool fibre, while it is obvious that this decomposition and
then the union of the colour base with the wool must take time, and as
it is effected more easily and completely at the boiling point, it is
advisable to work the goods in the bath so as to fully insure that
they are given the necessary time for the chemical change to take
place.

The dye-bath is generally completely exhausted of colour, but if
fairly clean it need not be thrown away, but used for another batch of
wool by simply adding more Glauber's salt and dye-stuff. After a (p. 066)
time the bath gets too dirty to used, when it may be thrown away, and a
new dye-liquor made up.

In dyeing for pale shades it is best to add the dye-stuff in small
quantities at intervals during the process of dyeing, and to run the
goods quickly through the bath, so as not to give the dye-stuff too
much opportunity to become absorbed by a portion of the goods only.

Working according to the hints given above, the dyeing of wool with
the basic coal-tar colours may be carried out in a very satisfactory
manner.

_Third Method_.--This method consists in dyeing the wool in a bath
containing the dye-stuff, a little acid (usually sulphuric) with the
addition of Glauber's salt, or some other alkaline salt, the essential
feature or principle being that the bath is an acid one. This method
is applicable to the large group of azo dye-stuffs derived from coal
tar, and also to the acid dyes prepared from the basic coal-tar
colours by the process of sulphonation.

It is also used to apply indigo carmine to wool, probably the only
good example of a natural dye-stuff applied by this process. Most of
the natural colouring matters, such as logwood and fustic, belong to
another group of dye-stuffs.

The simple azo dyes are combinations of two or more organic bases,
united together by a peculiar and characteristic group of nitrogen
atoms. Such azo colours are, however, insoluble in water, and
therefore they cannot be used in dyeing and textile colouring,
although the firm of Messrs. Read Holliday & Sons years ago patented a
process whereby these insoluble azo colours could be developed on the
cotton fibre direct, and thus fabrics made from that fibre could be
dyed in fast colours. When these insoluble azo colours are treated
with sulphuric acid they are converted into sulpho acids, undergoing
what is called s