Chlorine

Chlorine was first prepared in 1774 by Carl Wilhelm Scheele, who took it for a compound; Humphry Davy established in 1810 that it was an element and gave it its name. That name comes from the Greek chloros, pale or yellow-green, describing the colour of the dense, choking gas. Chlorine is a member of the halogens, intensely reactive and never found free in nature, occurring instead as chloride, above all in common salt and in seawater. It is manufactured on a vast scale by the electrolysis of brine, and its compounds range from table salt and hydrochloric acid to the plastic PVC and the chlorine-based bleaches and disinfectants that keep drinking water and swimming pools safe.

Systematic chlorination of drinking water began in the early twentieth century, first at Middelkerke, Belgium in 1902 and then, decisively for the United States, at Jersey City, New Jersey in 1908, and it is credited with one of the largest documented declines in waterborne disease in history, virtually eliminating typhoid and cholera from treated municipal supplies. That achievement sits uneasily beside chlorine's use as a chemical weapon at the Second Battle of Ypres in 1915, so that within a single decade the same element became simultaneously one of public health's greatest triumphs and one of warfare's most feared new weapons.

Davy's choice of chloros fixed the element's identity to a colour, dissolving Scheele's older label of dephlogisticated marine acid air and Lavoisier's mistaken oxymuriatic acid. Chlorine has no ancient or alchemical persona, being recognised only in the industrial age, and its cultural weight is modern and double-edged. As a disinfectant and bleach it transformed public health, making drinking water safe and cloth reliably white; as a war gas first released at Ypres in 1915 it opened the era of chemical warfare. The element's name, meaning simply pale green, sits oddly against both the lives its chemistry has saved and those it has taken.

The horror of Ypres helped drive the 1925 Geneva Protocol, the international treaty banning the use of chemical and biological weapons in war, though the treaty notably did not ban their production or stockpiling. Chlorine's grim role did not end in 1915: improvised chlorine barrel bombs were documented in Syria's civil war in the 2010s, prompting investigations by the Organisation for the Prohibition of Chemical Weapons, a reminder that this common industrial and sanitary gas has never fully shed the weaponised identity Davy's chloros first fixed on it as merely a colour.