right|thumb|The element thallium, discovered by Crookes thumb|right|The mineral crookesite, a selenide of copper, thallium and silver (), named for Crookes His first important discovery was that of the element thallium, made with the help of flame spectroscopy. Crookes discovered a previously unknown element with a bright green emission line in its spectrum. He named the element thallium, from Greek , , meaning "a green shoot or twig". Crookes's findings were published on 30 March 1861.Crookes, William (30 March 1861) "On the existence of a new element, probably of the sulphur group," Chemical News, vol. 3, pp. 193–194; reprinted in: Crookes, William (18 May 1861) "Further remarks on the supposed new metalloid," Chemical News, vol. 3, p. 303.Crookes, William (19 June 1862) "Preliminary researches on thallium," Proceedings of the Royal Society of London, vol. 12, pages 150–159.
right|thumb|The element thallium discovered by Crookes thumb|right|The mineral crookesite a selenide of copper thallium and silver () named for Crookes His first important discovery was that of the element thallium made with the help of flame spectroscopy Crookes discovered a previously unknown element with a bright green emission line in its spectrum He named the element thallium from Greek   meaning "a green shoot or twig" Crookes's findings were published on 30 March 1861Crookes William (30 March 1861) "On the existence of a new element probably of the sulphur group" Chemical News vol 3 pp 193–194; reprinted in Crookes William (18 May 1861) "Further remarks on the supposed new metalloid" Chemical News vol 3 p 303Crookes William (19 June 1862) "Preliminary researches on thallium" Proceedings of the Royal Society of London vol 12 pages 150–159