Chlorine is a chemical element; its symbol is Cl. As a free element it is a greenish-yellow gas with a strong, sharp smell, and it is poisonous if you breathe much of it. Cities put a little chlorine into tap water and swimming pools because it kills germs so well. Stronger chlorine chemicals sit in household bleach. Humphry Davy named it in 1810 from a Greek word for pale green, because of the colour of the gas. Though actually, the heavy smell people notice at a busy pool is often not pure chlorine. It is probably other chemicals made when chlorine meets sweat and urine — so a well-run pool can smell less, not more. And bleach in a bottle is not chlorine gas; it is usually a chlorine compound in water, and mixing it with acid or ammonia is dangerous. Chlorine the element, a chloride (chlorine joined to something else), and chloroform are related words, not the same stuff.