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++}, 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.


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 corresponding to the ferrous salts are the most common, the ones corresponding to the ferric salts being difficult to obtain. Thus we have the chlorides NiCl{2}·6H{2}O and CoCl{2}·6H{2}O; the sulphates NiSO{4}·7H{2}O and CoSO{4}·7H{2}O; the nitrates Ni(NO{3}){2}·6H{2}O and Co(NO{3}){2}·6H{2}O.

Nickel is largely used as an alloy with other metals. Alloyed with copper it forms coin metal from which five-cent pieces are made, with copper and zinc it forms German silver, and when added to steel in small quantities nickel steel is formed which is much superior to common steel for certain purposes. When deposited by electrolysis upon the surface of other metals such as iron, it forms a covering which will take a high polish and protects the metal from rust, nickel not being acted upon by moist air. Salts of nickel are usually green.

Compounds of cobalt fused with glass give it an intensely blue color. In powdered form such glass is sometimes used as a pigment called smalt. Cobalt salts, which contain water of crystallization, are usually cherry red in color; when dehydrated they become blue.