these dyes in dyeing various fabrics. This matter can only be dealt with
here in very general terms, for space is limited and the dyes are too
numerous for detailed mention. They vary very greatly in degrees of
fastness, some are absolutely fast to all influences; the blacks are
among the fastest, generally these resist washing and soaping, stand
acids well and are fast to alkalies, light however affects them more or
less, though they cannot be reckoned fugitive colours. The few direct
greens known are good colours; they stand washing, soaping and light
well, but they are affected by acids and alkalies. The blues vary very
much, generally they stand soaping and have a fair degree of fastness to
light, acids have but little action, alkalies tend to redden the shade,
while heat also affects them. The direct browns are very variable; they
are in general not fast to light; they stand washing and soaping and
resist alkalies, but are altered by acids slightly. The yellows rank
among the fastest of colours to light and washing and soaping; acids
have but little effect; they are reddened by alkalies. Among the reds
there is great variation in properties, generally they are not fast to
light, standing washing and soaping well and resisting weak alkalies;
some of them, such as the Benzo purpurines and Congo reds are very
sensitive to acids, being turned blue with very weak acids, but on
washing or soaping the original colour comes back; others, like the
Titan reds, Diamine reds and Delta purpurines are not so sensitive, but
these are affected by moderately strong acids; there are one or two reds
like Benzo fast scarlet 4 B S and Purpuramine D H, which are fast to
acids. The depth of shade which is dyed has some considerable influence
on the degree of fastness, the deeper shades of a colour are always
faster than the paler shades, particularly as regards light, a
difference of 1/2 per cent, of dye-stuff has been known to make a very
appreciable degree of difference as regards the fastness of a colour to
light.

In dyeing cotton with all the direct dyes, it is found that the whole of
the dye-stuff is not removed from the dye-bath, how much is taken up by
the cotton, and the depth of the shade which is dyed upon the cotton
chiefly depends upon three factors:--

(1) Volume of water used.
(2) Quantity of saline salts used.
(3) Degree of affinity of the dye-stuff for the fibre.

There may also be some minor factors such as tempera