Suppose its colour to be orange-brown. We find first that it imparts no
colour to boiling water; that protochloride of tin takes out its colour;
that plunged into a solution of ferroprussiate of potash it becomes
blue; and that a piece of it being burned, leaves a residuum of peroxide
of iron; we may thence conclude that the dyeing matter is peroxide of
iron.
Suppose we have a blue stuff which may have been dyed either with indigo
or with Prussian blue, and we wish to know what it will become in use.
We inquire first into the nature of the blue. Hot water slightly
alkaline will be coloured blue by it, if it has been dyed with sulphate
of indigo; it will not be coloured if it was dyed in the indigo vat, but
it will become yellow by nitric acid. Boiling water, without becoming
coloured itself, will destroy the Prussian blue dye; an alkaline water
will convert its colour into an iron rust tint; nitric acid, which makes
the indigo dye yellow, makes that of Prussian blue green. The liquor
resulting from boiling alkaline water on the Prussian blue cloth, will
convert sulphate of iron into Prussian blue.