Definition of thallium
: a soft poisonous metallic element that resembles lead in physical properties, occurs sparsely in a number of common ores, and is used chiefly in the form of compounds especially in photosensitive devices and formerly as a pesticide — see Chemical Elements Table
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Merriam-Webster unabridged
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Time Traveler for thallium
The first known use of thallium was in 1861
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