Sir William Crookes discovered Thallium in 1861 in London. In 1850, Crookes received a deposit from a sulphuric acid factory in Tilkerode, Germany which contained selenium. He extracted the selenium, but he was left with residues that seemed to contain tellurium in it. Eleven years later, when Crookes needed some tellurium, he tried to do the experiment again.
However, this time, he was unable to isolate any tellurium. Wondering what happened, he decided to analyze the residues using a spectroscope and noticed a bright green line in the spectrum, which did not match with any known element. After many experiments, Crookes concluded that the line was due to a new element. He announced his discovery in March 1861.
Claude-Auguste Lamy had independently discovered and isolated pure thallium in 1862 in France.
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