present in the amido compound and in the
nitrite unite together and a new compound is produced which is called a
diazo compound, and the operation is called "diazotisation".

For example when paranitroaniline is subjected to this reaction it
undergoes a change indicated in the chemical equation:--

C{6}H{4}NO{2}NH{2}, + NaNO{2}, + 2HCl = Paranitroaniline, Sodium
nitrite, Hydrochloric acid.

C{6}H{4}NO{2}N: NCl + NaCl + 2H{2}0 = Paranitro benzene Sodium
chloride, Water, diazo chloride.

The above, put into words, means that when paranitroaniline is dissolved
with hydrochloric acid and treated with nitrite of soda it forms
diazonitro benzene chloride, sodium chloride and water. Now the diazo
compounds are rather unstable bodies, but they have a great affinity for
other compounds, such as naphthol, phenylene diamine, phenol, and
combine easily with them when brought into contact with them. The new
compounds thus made form the dye-stuffs of commerce.

The azo dyes contain the characteristic group of two nitrogen atoms
shown in the formula N: N. In dealing with the production of colours
direct on the fibre this subject will be elaborated more fully.

Now many of the direct dyes, Diamine blacks, Diamine cutch, Primuline,
Diazo brown, Zambesi blues, browns, etc., contain amido groups, by
reason of having been made from such bodies as phenylene diamine, amido
naphthol, toluidine, etc., and it has been found that when dyed on the
fibre they are capable of being diazotised by passing the dyed fibre
into a bath of sodium nitrite acidified with hydrochloric acid, and if
then they are placed into a bath containing such a body as
beta-naphthol, phenylene diamine, etc., new compounds or dyes are
produced, which are characterised by being insoluble in water, and
therefore as formed on the fibre in the manner indicated are very fast
to washing, soaping and similar agencies.

Often the new or developed dye formed on the fibre differs markedly in
colour from the original dye. Perhaps in no case is this more strongly
shown than with Primuline. The original colour is a greenish yellow, but
by using various developers, as they are called, a great variety of
shade can be got, as shown in this table.

_Developer._ _Colour produced._

Beta-naphthol Bright scarlet.
Alpha-naphthol Crimson.
Phenylene diamine Brown.
Phenol