Discovery of Thallium

Date: March 30, 1861 • Location: London, United Kingdom

British chemist William Crookes published his discovery of thallium using flame spectroscopy.

Sir William Crookes was an English chemist and physicist who attended the Royal College of Chemistry, now part of Imperial College London, and worked on spectroscopy. He was a pioneer of vacuum tubes, inventing the Crookes tube, which was made in 1875. Observing cathode rays generated in these tubes, Crookes posited that "radiant matter" was a unique fourth state of matter, a foundational contribution to plasma physics.