Barium, a metal which is only found in nature in a combined state, most commonly as sulphate in Barytes, or heavy spar, and as carbonate in Witherite. Its compounds are characterised by high density, whence its name (Gk. barys, heavy). The metal itself is very difficult to prepare, and was first isolated by Sir H. Davy in 1808, though he probably only obtained an amalgam. It has a specific gravity 4.0, atomic weight 137, burns in air if heated, and decomposes water rapidly. It forms an oxide, BaO, closely resembling lime and known as Baryta. It also forms an oxide, which has been the starting point of many attempts for the manufacture of oxygen gas. Baryta is used in sugar refining; and certain salts, as the nitrate and chloride, are largely used in pyrotechny - for green fires - and in chemical analysis.