History of Thallium

Sir William Crookes in London, England discovered Thallium spectroscopically in 1861. When it was discovered, no one knew of its existence, therefore no one was looking for it. Crookes was trying to retrieve Selenium from the byproducts of a commercial sulphuric acid. After he had successfully taken Selenium from the byproducts, he decided to keep the waste instead of disposing of it. He thought that he would be able to find Tellurium in the residue, and it would help him in later researches. Later, Crookes went back to his residue and looked under it in a spectroscope. When he looked into the spectroscope, he did not find any blue Selenium lines, but he was not surprised since he had already removed most of the Selenium during his first experience. He had hoped to find yellow Tellurium lines, but instead he found a green spectral line instead, thus the name Thallium. After Crookes discovered Thallium, he first hesitated whether or not to contribute the new element to the periodic table, but he decided that it was simply a matter of doing the right thing. 
    The element gets its name from the Greek root “thallos” which means green twig. It was named after “thallos” because of the green spectral line that was seen when the element was first isolated.