An electrolytic cell used industrially for the production of sodium hydroxide. The usually iron cell is filled with brine (sodium chloride solution) and employs liquid mercury as the cathode. The sodium liberated there forms an amalgam with the mercury, which is run off and reacted with water to give sodium hydroxide (and hydrogen); the mercury is then re-used. Chlorine gas produced at the anode is another valuable by-product. The Castner–Kellner cell was invented independently by Hamilton Castner (1858–99) and Carl Kellner (1851–1905) in the 1890s.