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Thallium, chemical element, symbol Tl; for physical constants see Periodic Table. Thallium was discovered spectroscopically by William Crookes in 1861. It occurs in crookesite, loandrite, orbaite, and other minerals. It is obtained commercially from flue dusts remaining from pyrite calcination. Thallium is a tin-white, soft, reactive metal. It is soft enough to be scratched with the finger nail. The metal oxidizes in air and should be kept covered by an inert liquid. Thallium is a suspected carcinogen. The element and it compounds are poisonous, and contact with the skin should be avoided. The sulphate has been used as an insecticide and rodenticide, but its use in the United States is now prohibited. Thallium and its compounds are used in photo cells, infrared detectors, low-temperature mercury switches, and special glasses.

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