Sulphuric acid. (a) Sulphur trioxide (see under Sulphur); -- formerly so called on the dualistic theory of salts. [Obs.] (b) A heavy, corrosive, oily liquid, H₂SO₄, colorless when pure, but usually yellowish or brownish, produced by the combined action of sulphur dioxide, oxygen (from the air), steam, and nitric fumes. It attacks and dissolves many metals and other intractable substances, sets free most acids from their salts, and is used in the manufacture of hydrochloric and nitric acids, of soda, of bleaching powders, etc. It is also powerful dehydrating agent, having a strong affinity for water, and eating and corroding paper, wood, clothing, etc. It is thus used in the manufacture of ether, of imitation parchment, and of nitroglycerin. It is also used in etching iron, in removing iron scale from forgings, in petroleum refining, etc., and in general its manufacture is the most important and fundamental of all the chemical industries. Formerly called vitriolic acid, and now popularly vitriol, and oil of vitriol. -- Fuming sulphuric acid, or Nordhausen sulphuric acid. See Disulphuric acid, under Disulphuric. -- Sulphuric anhydride, sulphur trioxide. See under Sulphur. -- Sulphuric ether, common anæsthetic ether; -- so called because made by the catalytic action of sulphuric acid on alcohol. See Ether, 3 (a).