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Cb,formerly the symbol for the element columbium, now called niobiumniobium
, metallic chemical element; symbol Nb; at. no. 41; at. wt. 92.90638; m.p. about 2,468°C;; b.p. 4,742°C;; sp. gr. 8.57 at 20°C;; valence +2, +3, +4, or +5.
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A reservoir, esp. for catching and retaining surface drainage over a large area, in which sediment may settle.