χλωρός

The Greek word chlōrós meant pale green or yellowish-green — the color of new spring growth, of unripe olives, of honey. Homer used it to describe fear turning skin pale and fresh wood still moist with sap. It was a living color word, flexible and sensory, not yet attached to any element.

Carl Wilhelm Scheele in Uppsala produced the gas in 1774 by reacting hydrochloric acid with manganese dioxide. He called it dephlogisticated muriatic acid, because phlogiston theory still dominated chemistry and nobody recognized it as an element. For thirty-six years, chemists treated it as a compound. Humphry Davy finally identified it as an element in 1810 and named it chlorine, from chlōrós, for its unmistakable pale green color.

On April 22, 1915, at the Second Battle of Ypres in Belgium, German forces released 168 tons of chlorine gas from 5,730 cylinders along a four-mile front. The yellow-green cloud drifted into French and Algerian trenches. Soldiers who did not flee suffocated as the gas dissolved in the moisture of their lungs, forming hydrochloric acid. It was the first large-scale chemical weapon attack in modern warfare. Fritz Haber, the chemist who organized the attack, called it a higher form of killing.

Within two decades, chlorine had been repurposed. Municipal water chlorination, first implemented in Jersey City, New Jersey, in 1908 and widely adopted by the 1920s, is credited with eliminating waterborne epidemics of typhoid and cholera in the industrialized world. The same element that killed soldiers in Belgium saved millions of civilians from disease. No other chemical has so cleanly divided its history between weapon and medicine.