At 25 oC, bromine is a dense, freely flowing, corrosive, dark red liquid that is easily vaporized into a brownish-red vapor. It was discovered in 1826 and it occurs mainly as the bromide ion, Br-, in salts such as NaBr, KBr, MgBr2, and CaBr2 in sea water, underground salt brines, and salt beds. Bromine is prepared by bubbling chlorine through seawater, which contains dissolved NaBr. Bromine vapor is blown out of the reaction vessel by a current of air and collected. It takes about 2500 L of seawater to produce 160 g of bromine. Bromine is used as a disinfectent. Uses of bromine-containing compounds include: silver bromide in light-sensitive photographic film, sodium bromide as a mild nerve sedative, methyl bromide as a soil fumigant, and 1,2-dibromoethane, a petrol anti-knock compound which reacts with lead in automobile engines to form volatile PbBr2. The PbBr2 escapes into the atmosphere and prevents buildup of nonvolatile lead compounds inside engines. Although this ultimately makes car engines run smoothly, it poses a serious health hazard in the environment.