This solution is allowed to stand for some time (in order that any calcium sulphate and basic ferric sulphate may separate), and is then evaporated until ferrous sulphate crystallizes on cooling; it is then drawn off and evaporated until it attains a specific gravity of 1.40.
Another kind of alteration which pyrites may suffer has been termed "vitriolization," since the products are ferrous sulphate, with free sulphuric acid and sometimes a basic ferric sulphate.
This consists in stacking the broken ore in heaps and adding a mixture of sodium sulphate and ferric chloride in the proportions necessary for the entire conversion of the iron into ferric sulphate.
Fuming or Nordhausen Oil of Vitriol, a mixture or chemical com pound of H 2 SO 4, with more or less S03, has been made for centuries by exposing pyritic schist to the influence of atmospheric agents, collecting the solution of ferrous and ferric sulphate thus formed, boiling it down into a hard mass ("vitriolstein") and heating this to a low red heat in small earthenware retorts.
Ferri hydroxidum (U.S.P.), the hydrated oxide of iron, made by precipitating ferric sulphate with ammonia, is used solely as an antidote in arsenical poisoning.
