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A chemical element, in the periodic table thallium has the symbol Tl and atom number 81. Discovered by Sir William Crookes in 1861 in England. The name is derived from the Greek word "thallos" meaning "green twig" or green shoot. The name comes from Thallium's bright green spectral emission lines.

The metal is very soft and malleable, able to be cut with a knife. Its melting point is 577 K and its boiling point is 1746 K. A heavy layer of oxide builds up on thallium if left in air, and in the presence of water the hydroxide is formed.

The element and its compounds are toxic and should be handled carefully. The toxicity has led to its use (now discontinued) as a rat poison. Amongst the distinctive effects of thallium poisoning are loss of hair, and damage to peripheral nerves. The hair loss was the cause of thallium being used in the treatment of ringworm, but that use has long been abandoned. There is a persistent story that the CIA, in an attempt to discredit Fidel Castro had actually planned to poison him with thallium so that his beard would fall out, but apparently sanity prevailed and the attempt was never made.

The detective fiction writer, Agatha Christie, used thallium as the agent of murder in her novel The Pale Horse - the first clue to the murder method coming from the hair loss of the victims.