Barium
Barium is a chemical element, number 56 on the periodic table. It is a metallic element that only occurs naturally as an oxide or in mineral form.

As noted by The Doctor while working to separate Tuvok and Neelix following their transporter accident, barium (as barium sulfate) absorbs X-rays and makes a good contrast agent. ()

See also 
 Algobarium solution

External link
 

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Elements