osing its power of producing
colour, or at the most gives unsatisfactory results. For this reason it
is therefore always necessary to work as cold and as quickly as
possible.

The amido-azo bodies, whose compounds with the phenols are also
distinguished by their great fastness, are in this respect an exception.
They can be diazotised at the ordinary temperature, and their diazo
compounds are much stabler than those, for example, of alpha-and
beta-naphthylamine or of aniline, which must always be used as quickly
as possible.

From anisidine, phenetidine and amido-diphenylamine, still more stable
diazo compounds can be obtained, but the prices of these bases are
rather high, and the colours produced with them are not fast to light.

The cheapest and most convenient method of obtaining nitrous acid for
diazotising is by the action of a mineral acid, preferably hydrochloric
acid, upon nitrite of soda.

For diazotising one molecule of base requires one molecule of
hydrochloric acid to form a salt of the base, a molecule of nitrite of
soda, and another molecule of hydrochloric acid to decompose the
nitrite. The diazotisation is better carried out and the diazo solution
rendered more stable if another molecule of hydrochloric acid and an
excess of nitrite of soda are used. The presence of an excess of nitrite
can be determined by testing the diazo solution with potassium iodide
starch paper, which in the presence of excess of nitrite gives the blue
iodine starch reaction.

In carrying out the diazotisation, the base is first dissolved in the
whole amount of hydrochloric acid which has to be used, and the solution
is filtered. The diazotisation takes place in the manner shown in the
equation:--

C{6}H{5}NH{2} + HCl + HCl + NaNO{2} =
Aniline hydrochloride, Hydrochloric acid, Sodium nitrite,

NaCl + C{6}H{5}N:NCl + H{2}0
Salt, Diazo-benzene chloride, Water.

The bases which form salts soluble with difficulty, such as nitroaniline
and the amido-azo bodies, offer special difficulties in diazotising.

It has been found that the operation with these is best carried out if
the chemically pure bases in paste form are mixed with the requisite
amount of nitrite, and the diluted paste then poured into the
hydrochloric acid.

It has been found by experience that the colour is developed much
brighter upon the fibre when the diazo solutio