Early Chemical Investigations
The story of chlorine begins with Carl Wilhelm Scheele (1742-1786), a Swedish pharmacist working in his small laboratory in Köping, Sweden. On a winter day in 1774, Scheele heated pyrolusite (manganese dioxide) with muriatic acid (hydrochloric acid) and observed a pungent, greenish-yellow gas that bleached flowers and burned his lungs. He called this mysterious substance "dephlogisticated muriatic acid," believing it was muriatic acid with its phlogiston removed.
Scheele's notebook entry from December 1774 recorded: "A gas of a yellowish-green color was evolved, which had a very suffocating smell and burnt the lungs terribly." His reaction (MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂) had created the first recorded sample of pure chlorine gas.
The Elemental Controversy
Claude Louis Berthollet (1748-1822), the renowned French chemist, studied Scheele's gas extensively from 1785-1789. Initially supporting Scheele's compound theory, Berthollet discovered the gas's remarkable bleaching properties and established commercial bleaching powder production. However, he remained convinced the gas contained oxygen and called it "oxygenated muriatic acid."
The controversy intensified when Antoine Lavoisier (1743-1794) insisted all acids must contain oxygen (from Greek "oxys genes" - acid producer). This created a 30-year scientific debate about chlorine's true nature.
Humphry Davy's Definitive Proof
The mystery was solved by Sir Humphry Davy (1778-1829) at the Royal Institution in London. Using his newly invented electrolysis apparatus, Davy attempted to decompose the green gas and obtain its supposed oxygen component. Despite months of experiments in 1807-1810, he failed to extract any oxygen.
On July 12, 1810, Davy presented his groundbreaking conclusion to the Royal Society: "I am inclined to believe that the body we have been considering is not a compound, but a simple substance." He named it "chlorine" from the Greek "chloros" (pale green), finally recognizing it as a fundamental element.
Davy's colleague Michael Faraday (1791-1867) provided crucial support by successfully liquefying chlorine in 1823, demonstrating its distinct physical properties and confirming its elemental nature.
Industrial Revolution and Bleaching Powder
Charles Tennant (1768-1838), a Scottish chemist and businessman, revolutionized the textile industry by developing bleaching powder in 1799. His factory in St. Rollox, Glasgow, became the world's largest chemical works, producing calcium hypochlorite [Ca(ClO)₂] by passing chlorine gas over slaked lime.
Tennant's innovation transformed textile manufacturing - traditional bleaching with buttermilk and sunlight required 6-8 months, while chlorine bleaching took just hours. By 1815, his company produced 9,000 tons annually, employing over 1,000 workers and generating enormous profits.
The Chloralkali Industry Birth
The modern chlorine industry began with Hamilton Young Castner (1858-1899) and Karl Kellner (1851-1905), who independently developed electrolytic processes for producing chlorine and sodium hydroxide from salt water. Castner's 1892 mercury cell and Kellner's 1894 diaphragm cell competed fiercely, ultimately merging as the Castner-Kellner Company.
Herbert Henry Dow (1866-1930) revolutionized American chlorine production by founding Dow Chemical Company in 1897. His innovative electrolytic cells used Midland, Michigan's underground brine deposits, making Dow the world's largest chlorine producer by 1920.
World War I and Chemical Warfare
Chlorine's dark chapter began on April 22, 1915, at the Second Battle of Ypres when German forces released 168 tons of chlorine gas, creating the first large-scale chemical weapon attack. Fritz Haber (1868-1934), later Nobel laureate for ammonia synthesis, directed this program despite his wife Clara's opposition and eventual suicide in protest.
The attack killed 5,000 Allied soldiers and wounded 10,000 more, opening a new era of chemical warfare. Both sides subsequently developed more sophisticated chemical weapons, though chlorine's visibility and crude delivery methods limited its effectiveness.
Public Health Revolution
Chlorine's greatest legacy emerged through water disinfection. John L. Leal (1858-1914) conducted the first continuous municipal water chlorination in Jersey City, New Jersey, in 1908. Despite legal challenges from residents fearing "chemical poisoning," Leal's system eliminated typhoid fever deaths within two years.
Abel Wolman (1892-1989) and Linn Enslow refined chlorination science, establishing dosage calculations and residual monitoring that became global standards. Their work enabled safe drinking water for billions and ranks among the greatest public health achievements of the 20th century.