[1] Green vitriol, ferrous sulphate heptahydrate, a compound of iron, sulphur, and oxygen, crystallised with seven molecules of water, represented by the formula FeSO4<.>7H2O. On exposure to the air it loses water, and is gradually converted into basic ferric sulphate. For long, green vitriol was confused with blue vitriol, which generally occurs as an impurity in crude green vitriol. Blue vitriol is copper sulphate pentahydrate, CuSO4<.>5H2O.英文原著 · 《Bygone Beliefs》