Also called “Green Vitriol” and “Green Copperas,” is a combination
of sulphuric acid and oxide of iron, and may be made by putting iron
filings or pieces of iron into a mixture of sulphuric acid and water,
and when all action has ceased evaporating the liquid, and setting it
by till crystals form--these are of a pale green color. The sulphate
of iron of commerce is formed by exposing the iron pyrites found in
the coal districts to the air till it falls to powder; water is then
poured over this substance, and the fluid collected and evaporated.
Sulphate of iron is used chiefly to form the black dye so much in
demand to dye cloth, and also in making writing ink.