errocyanide, or yellow
prussiate of potash (K_{4}FeC_{6}N_{6}), and potassium ferricyanide, or
red prussiate of potash (K_{3}FeC_{6}N_{6}). These compounds are the
potassium salts of the complex acids of the formulas H_{4}FeC_{6}N_{6}
and H_{3}FeC_{6}N_{6}.

~Oxidation of ferrous salts.~ It has just been seen that when a ferrous
salt is treated with an oxidizing agent in the presence of a free acid a
ferric salt is formed:

2FeSO_{4} + H_{2}SO_{4} + O = Fe_{2}(SO_{4})_{3} + H_{2}O.

In this reaction oxygen is used up, and the valence of the iron is
changed from 2 to 3. The same equation may be written

2Fe^{++}, 2SO_{4}^{--} + 2H^{+}, SO_{4}^{--} + O
= 2Fe^{+++}, 3SO_{4}^{--} + H_{2}O.

Hydrogen ions have been oxidized to water, while the charge of each iron
ion has been increased from 2 to 3.

In a similar way the conversion of ferrous chloride into ferric chloride
may be written

Fe^{++}, 2Cl^{-} + Cl = Fe^{+++}, + 3Cl^{-}.

Here again the valence of the iron and the charge on the iron ion has
been increased from 2 to 3, though no oxygen has entered into the
reaction. As a rule, however, changes of this kind are brought about by
the use of an oxidizing agent, and are called oxidations.

The term "oxidation" is applied to all reactions in which the valence of
the metal of a compound is increased, or, in other words, to all
reactions in which the charge of a cation is increased.

~Reduction of ferric salts.~ The changes which take place when a ferric
salt is converted into a ferrous salt are the reverse of the ones just
described. This is seen in the equation

FeCl_{3} + H = FeCl_{2} + HCl

In this reaction the valence of the iron has been changed from 3 to 2.
The same equation may be written

Fe^{+++}, 3Cl_{-} + H = Fe^{++}, + H^{+} + 3Cl_{-}

It will be seen that the charge of the iron ions has been diminished
from 3 to 2. Since these changes are the reverse of the oxidation
changes just considered, they are called reduction reactions. The term
"reduction" is applied to all processes in which the valence of the
metal of a compound is diminished, or, in other words, to all processes
in which the charge on the cations is diminished.

NICKEL AND COBALT

These elements occur sparingly in nature, usually combined with arsenic
or with arsenic and sulphur. Both elements have been found in the free
state in meteorites. Like iron they form two series of compounds, but
the salts