they will
probably come to be called, differ from those produced on the fibre by
the original dye-stuff, thus the Diamine jet blacks and some of the
Diazo blacks give, with paranitroaniline, browns of various shades.

In this section also may be considered the method of dyeing cotton by
using the direct colours in the ordinary way, and then "topping," as it
is called, with a basic dye in a fresh bath.

Practically in the "coupling process" of dyeing only diazotised
paranitroaniline is used as the coupler, although other amido bases of a
similar nature are available.

When paranitroaniline is used as the source for the coupling bath it is
well to prepare a stock bath of diazotised paranitroaniline, which may
be done in the following manner:--

=Preparation of diazotised paranitroaniline.=--Take 1 lb.
paranitroaniline, mix with 1 gallon boiling water and 1 quart
hydrochloric acid, stir well, when the paranitroaniline will dissolve
the solution may if necessary be assisted by a little heat. Now add
1-1/2 gallons of cold water, and set aside to cool, when the
hydrochloride of paranitroaniline will separate out in the form of fine
crystals; when the mixture is quite cold (it cannot be too cold) there
is added 1/2 lb. sodium nitrite dissolved in 1/2 gallon cold water, stir
well for fifteen to twenty minutes, by the end of which time the
paranitroaniline will have become fully diazotised, cold water is added
to bring up the volume of the mixture to 10 gallons. This stock bath
well prepared and kept in a cool, dark place will keep good for three to
four weeks. This bath contains 1 lb. of paranitroaniline in 10 gallons,
and it is a good rule to allow 1/2 lb., or 5 gallons of this stock bath
to each pound of dye-stuff used in dyeing the ground colour to be
developed up.

To prepare the coupling bath there is taken 5 gallons of the stock bath,
1 lb. sodium acetate with sufficient water for each 1 lb. of dye that
has been used.

This bath is used cold, and the cotton is worked in it for half an hour,
then it is taken out, washed well and dried.

Nitrazol C is a ready prepared diazotised paranitroaniline in a powder
form which keeps well if stored in a dry place. The method of using is
to take 8 lb. Nitrazol C, stir into a paste with water and then add this
paste to the coupling bath, together with 2 lb. soda and 3/4 lb. acetate
of soda. This bath is used cold and the dyed cotton is immersed in it
for half an hour, then ta