Why is the ninth month called September?
late 14c., "sulphate of iron," from Old French vitriol (13c.), from Medieval Latin vitriolum "vitriol," from neuter of vitriolus, from Late Latin vitreolus "of glass," from Latin vitreus "of glass, glassy," from vitrium "glass" (see vitreous). So called from its glassy appearance in certain states. Meaning "bitter or caustic feelings" first attested 1769, in reference to the corrosive properties of vitriol (when heated it produces sulfuric acid, formerly called oil of vitriol).
vitriol vit·ri·ol (vĭt'rē-ōl', -əl)
Any of various sulfates of metals, such as ferrous sulfate, zinc sulfate, or copper sulfate.
See sulfuric acid.