Chlorine

Chlorine announced itself unmistakably: a yellow-green gas with a suffocating odor that bleached everything it touched. Carl Wilhelm Scheele produced it in his Uppsala pharmacy laboratory in 1774 by dripping hydrochloric acid onto manganese dioxide. He called it dephlogisticated marine acid air—a name that reflected the pre-Lavoisier understanding of chemistry as the manipulation of phlogiston.

Scheele recognized the gas's extraordinary reactivity. It bleached flowers, killed insects, and attacked metals. But within the phlogiston framework, he could not understand what he had made. The adjacent possible included the substance but not yet the concept of elements.

Humphry Davy resolved the confusion in 1810. Using electrochemistry—itself a new tool—he demonstrated that Scheele's gas contained no oxygen and was itself an element. He named it chlorine from the Greek chloros, yellow-green. The first halogen had been identified.

The discovery cascaded into industry. Claude Berthollet recognized chlorine's bleaching power in 1785 and developed eau de Javel, the first chlorine bleach. The textile industry adopted it immediately—chemical bleaching replaced months of sun-bleaching linen in fields. Charles Tennant patented bleaching powder (calcium hypochlorite) in 1799, making chlorine bleaching portable and storable.

Water purification followed. After John Snow traced London's 1854 cholera outbreak to contaminated water, chlorination gradually became standard. By 1908, Jersey City was chlorinating its entire water supply. The practice spread globally, likely saving more lives than any other public health intervention.

But chlorine's reactivity cut both ways. German forces released chlorine gas at Ypres in April 1915, inaugurating modern chemical warfare. The same property that killed waterborne pathogens could kill soldiers. The gas mask emerged as desperate countermeasure.

Chlorine enabled polyvinyl chloride (PVC), chlorofluorocarbons, countless pharmaceuticals, and the entire chlor-alkali industry. The yellow-green gas from Scheele's pharmacy became foundational to industrial chemistry—a substance too reactive to exist free in nature, too useful to do without.