Barium.

ETYM New Lat., from Greek barys heavy.
White metallic element. barium meal, drink containing a compound of barium which is opaque to X-rays, taken before or during X-ray examination of digestive tract.
Soft, silver-white, metallic element, symbol Ba, atomic number 56, atomic weight 137.33. It is one of the alkaline-earth metals, found in nature as barium carbonate and barium sulfate. As the sulfate it is used in medicine: taken as a suspension (a “barium meal”), its movement along the gut is followed using X-rays. The barium sulfate, which is opaque to X-rays, shows the shape of the gut, revealing any abnormalities of the alimentary canal. Barium is also used in alloys, pigments, and safety matches and, with strontium, forms the emissive surface in cathode-ray tubes. It was first discovered in barytes or heavy spar.
A soft silvery metallic element of the alkali earth group; found in barite; SYN. Ba, atomic number 56.

A crystalline insoluble compound BaSO4 that is used especially as a pigment and extender, as a filler (as in fluids used in gas and oil drilling), and as a substance opaque to X rays in medical photography of the alimentary canal.