Chlorine

Scheele made chlorine in 1774 and thought it was a compound; Davy proved it an element in 1810 and named it for its colour. Its public-health record is extraordinary. In 1908 John Leal chlorinated the Jersey City water supply without permission and without telling anyone, was hauled into court, and won — typhoid deaths in American cities collapsed over the following two decades, and chlorination is routinely ranked among the great public-health achievements of the century. The same reactivity was turned the other way at Ypres in April 1915, when Germany released 168 tonnes of chlorine gas in the first large-scale chemical attack of the war. Chemically chlorine is the classroom example of a fractional atomic weight: 76% ³⁵Cl and 24% ³⁷Cl averaging to 35.45, a ratio that shows up as a telltale doublet in mass spectra.