extract; work the cotton in this at 160 deg. F. for three
hours, then pass into a cold bath of 1 lb. copperas and 1/4 lb. chalk
for half an hour, then into a bath of 3 lb. bichromate of potash for
half an hour at 150 deg. F., then wash and dry.

(7) PRODUCTION OF COLOUR DIRECT UPON COTTON FIBRES.

By the action of nitrous acid upon the salts of the primary organic
amines the so-called diazo compounds are formed. An example of this
important process is that of nitrous acid on aniline hydrochloride shown
in the following equation:--

C{6}H{5}NH{2} + HCl + HNO{2} 2H{2}O + C{6}H{5}N:NCl
Hydrochloric acid Nitrous Water, Diazo-benzene
aniline, acid, chloride.

These diazo compounds are distinguished by their active properties,
especially in combining with amines in acid solutions, or with phenols
in alkaline solution to form the azo dyes, thus diazobenzene chloride
will combine with naphthol to form naphthol-azo-benzene, thus:--

C{6}H{5}N:NCl + C{10}H{7}OH + NaOH =
Diazo-benzene chloride, Naphthol, Caustic soda.

C{10}H{6}OHN:NC{6}H{5} + NaCl + H{2}O
Naphthol-azo-benzene, Salt, Water.

These azo compounds are coloured, but are perfectly insoluble in water,
alkalies, or acids; on the other hand the sulphonates of these bodies
are easily soluble and form the numerous azo dyes now so largely made
and used in wool and silk dyeing, but which on account of their being
sulphonates cannot be used for cotton dyeing.

Methods have been devised for producing the insoluble azo colours direct
upon the fibres. They are also called naphthol colours from the use of
beta-and alpha-naphthol in their production. Although these azo dyes,
when produced on the fibre, do not possess the fastness of the alizarine
dyes, yet, on account of their cheapness and relative great fastness to
soap and the action of sunlight, they are better than many of the newer
cotton dyes.

By this method (first introduced in England by Holliday) colours of
exceptional brightness and fastness can be obtained which were not
obtainable with the dyes then known. Those which are obtained from
phenols are of the first importance.

_The Diazotisation of the Amido Bases._

With most bases this must be accomplished as cold as possible below 65 deg.
F. At a higher temperature, and when allowed to stand, most diazo
compounds decompose quickly with evolution of nitrogen, which
decomposition results in the mixture l