Archaic term for sulphuric acid.The alchemists called sulfuric acid oil of vitriol, a reference to its corrosive nature....
In the late 18th and early 19th centuries, extra acid in the form of vinegar or tartaric acid (or in one recipe oil of vitriol), were added to boiled sugar, and modern drops evolved.
Mix one fourth pound of indigo with half an ounce of oil of vitriol & a little water; shake the bottle every day till the whole is dissolved.