reducing intensity, the coloring of diapositives, the very important blue
printing, and various others, are daily practiced in our laboratories.

The ferrocyanide of potassium is a chemical reagent of great value,
giving rise to precipitates with the neutral or slightly acid solutions
of metals, like the beautiful brown ferrocyanide of copper, and that of
lead. When a ferrocyanide is added to a solution of a sesquioxide of
iron, Prussian blue or ferrocyanide of iron is produced. The exact
composition of this remarkable substance is not distinctly stated, as
various blue compounds may be precipitated under different circumstances.
Berzelius gives the following account: 3 eq. of ferrocyanide and 2 eq. of
sesquioxide of iron are mutually decomposed, forming 1 eq. of Prussian
blue and 6 eq. of the potassa salt, which remains in solution, or
3K_{2}Cfy + 2(Fe_{2}O_{3}3NO_{3}) = Fe_{4}Cfy_{3} + 6(KO,NO_{5}). It
forms a bulky precipitate of an intense blue, is quite insoluble in water
or weak acids, with the exception of oxalic acid, with which it gives a
deep blue liquid, occasionally used as blue ink.

Ferridcyanide of potassium, added to a salt of the sesquioxide of iron,
yields no precipitate, but merely darkens the reddish-brown solution;
with protoxide of iron it gives a blue precipitate, containing
Fe_{3}Cfdy, which is of a brighter tint than that of Prussian blue, and
is known by the name of Turnbull's blue. Hence, the ferridcyanide of
potassium is as excellent a test for protoxide of iron as the yellow
ferrocyanide is for the sesquioxide.--_E., Photo. Times_.

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