Greenish yellow poisonous gas with a pungent odor. A member of the halogen family of elements, it is one of the most reactive substances and is found in nature only in combined-form such as table salt (sodium chloride). Manufactured from the electrolysis of brine, it is an important bleaching agent (used in paper and textile industry) and disinfectant (used in water purification). Its presence in upper atmosphere (mainly as a component of chlorinated organic compounds) causes depletion of the ozone layer. Discovered in 1774 by the Swedish chemist Carl Wilhelm Scheele (1742-86).