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cesium
noun
a soft, silver-white, ductile, metallic chemical element, one of the alkali metals and the most electropositive of all the elements: it ignites in air, reacts vigorously with water, and is used in photoelectric cells: symbol, Cs; at. no., 55: a radioactive isotope () with a half-life of 30.17 years is a fission product and is used in cancer research, radiation therapy, etc.
See cesium in American Heritage Dictionary 4
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