The author deals with the question whether a sample of goods is dyed
with indigo alone or with a mixture of indigo and other blue coloring
matters. His method may be summarized as follows: Threads of the
material in question should give up no coloring matter to boiling
water. Alcohol at 50 and at 95 per cent. (by volume) ought to extract
no color, even if gently warmed (not boiled). Solution of oxalic acid
saturated in the cold, solution of borax, solution of alum at 10 per
cent., and solution of ammonium molybdate at 33-1/3 per cent. ought
not to extract any coloring matter at a boiling heat. The borax
extract, if subsequently treated with hydrochloric acid, should not
turn red, nor become blue on the further addition of ferric chloride.
Solutions of stannous chloride and ferric chloride with the aid of
heat ought entirely to destroy the blue coloring matter. Glacial
acetic acid on repeated boiling should entirely dissolve the coloring
matter. If the acetic extracts are mixed with two volumes of ether and
water is added, so as to separate out the ether, the water should
appear as a slightly blue solution, the main bulk of the indigo
remaining in suspension at the surface of contact of the ethereal and
watery stratum. This acid watery stratum should be colorless, and
should not assume any color if a little strong hydrochloric acid is
allowed to fall into it through the ether. No sulphureted hydrogen
should be evolved on boiling the yarn or cloth in strong hydrochloric
acid. On prolonged boiling, supersaturation with strong potassa in
excess, heating and adding a few drops of chloroform, no isonitrile
should be formed.--_W. Lenz_.
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