a. Chem. [f. L. ferr-um iron + -OUS.] A term applied to compounds in which iron combines as a divalent, e.g., ferrous oxide, FeO, also called iron protoxide.
c. 1865. G. Gore, in Circ. Sc., I. 190/2. Ferrous sulphate (protosulphate of iron).
1873. J. P. Cooke, New Chem., 173. Ferrous and ferric sulphates … correspond to ferrous and ferric oxides.
1876. Harley, Mat. Med., 204. Iron forms with chlorine two compounds, protochloride or ferrous chloride … and the sesquichloride, perchloride, or ferric chloride.
