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An important germicide agent of drinking water and swimming pools since Victorian times. It was discovered by Scheele in 1774, but Humphrey Davy first proved it to be an element (atomic number 17) in 1810.
It has a suffocating smell, and was used as a chemical weapon during the First World War, as it attacks the membranes of the nose, throat and lungs.
There are current worries over its continued use in water, as it produces by-products - such as "Trihalomethanes" - thought to induce cancers. See 'Water; Drinking'