t powers of
resistance to light, air, washing, etc., the great majority are
remarkable for their fastness to those destructive agencies.

There are but two natural dye-stuffs, indigo and logwood, from which
blue tints can be dyed. With the former, a great variety of shades can
be dyed of a satisfactory character as regards fastness; with the (p. 137)
latter, only dark blues can be dyed, these are fairly fast to milling,
but only moderately so to light.

The artificial blues derived from coal tar are very numerous, and
representatives of all classes, direct, basic, acid and mordant of
dye-stuffs may be found among them. The direct blue dyes do not work
very well on wool. They are apt to dye very red, and somewhat dull
shades, which are, however, fairly fast to washing and light. The
basic blue dyes are fairly numerous, and may be used to dye from pale
sky to deep navy tints. They are apt to work somewhat unevenly on to
wool, owing to their great affinity for the fibre. They give shades
possessing some degree of resistance to light, but which are not very
fast to washing and milling, although, in this respect, there are very
great differences among them. The acid dyeing blues are fairly
numerous, but they dye a great variety of tints, usually fairly fast
to washing, milling and light. The mordant blues are pretty numerous
and of great value for dyeing wool, as they give shades which are
remarkable for their fastness to light, acids and milling, hence they
are most extensively used, especially for dyeing fabrics that are
subject to very hard wear.

#Indigo Dyeing.#--It will be most convenient to begin the description of
the methods of dyeing blues by showing how, and in what manner, indigo
is applied in wool dyeing.

The dyeing of indigo on wool is effected in two ways, either in the
usual way with acid baths, as with acid scarlets, when the so-called
indigo extract is used, or in vats, when indigo itself forms the
dye-stuff.

Indigo is, as all dyers know, or should know, a natural dye-stuff,
prepared from the leaves and twigs of the indigo plant by a species of
fermentation which produces the indigo in a soluble form from the
indigo substance in the plant, followed by oxidation which results in
the separation of the indigo from this solution.

It comes into this country in the form of lumps, which have a dark (p. 138)
blue to bronze blue colour. The dye-stuff is insoluble in water, cold
alcohol, alkalies or weak