Barium was first isolated by Humphry Davy in 1808, by the electrolysis that yielded him so many of the reactive metals; its heavy ores had been known earlier as baryta. The name comes from the Greek barys, heavy, reflecting the unusual density of its minerals, above all barite, or heavy spar. Barium is a soft, silvery alkaline-earth metal, too reactive to occur free, and known in daily life through its compounds. Its dense, X-ray-opaque sulfate is swallowed as the contrast medium that reveals the digestive tract on a radiograph and is used in the heavy drilling muds of the oil industry, while other barium compounds give the green of fireworks and feature in specialty glasses.
Barium's medical safety depends entirely on its chemical form: the sulfate used in imaging is so insoluble that it passes through the body virtually inert, whereas soluble barium salts such as the carbonate are acutely toxic and were historically used as rat poison and, in a documented mass poisoning of British soldiers in 1945, as an accidental contaminant of flour. The same heavy, opaque sulfate that makes the barium meal work in radiography also makes barite valuable as a weighting agent in drilling mud, where its density helps control pressure deep in oil and gas wells. Davy isolated barium in the same short run of electrolysis experiments that also yielded calcium, strontium and magnesium, a single method opening up nearly the whole alkaline-earth column of the table within a few months.