Thallium was discovered in 1861 by William Crookes> he had been given an ore containing selenium which he extracted leaving behind tellurium. Eleven years later he tried to isolate tellurium again but instead through analysis with spectroscopy realised he had isolated a completely new element. Thalium was named after the Greek word ‘Thallos’ which means green shoot, in reference to its unique green line. Thallium is a highly toxic compound and was used in the Agatha Christie novel ‘the pale horse.’ Thallium is used in rat poison and insecticide and also used to make photovoltaic cells. It is a solid metal at room temperature with a melting point of 304°c and a boiling point of 1473°c.
Sir William Crookes