*thallium*

n. [ NL., fr. Gr. &unr_; young or green shoot or branch, twig. So called from a characteristic bright green line in its spectrum. ] (Chem.) A rare metallic element of the aluminium group found in some minerals, as certain pyrites, and also in the lead-chamber deposit in the manufacture of sulphuric acid. It is isolated as a heavy, soft, bluish white metal, easily oxidized in moist air, but preserved by keeping under water. Symbol Tl. Atomic weight 203.7. [ 1913 Webster ]