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William Crookes (June 17, 1832 - April 4, 1919) was an English chemist, physicist and spectroscopist. In 1861, he discovered a previously unknown element with a bright green emission line in its spectrum and named the element thallium. In 1895, he identified the first known sample of helium. He developed the Crookes tubes (vacuum tube), investigating cathode rays. He was a pioneer in the use of vacuum tubes for the study of physical phenomena. He was one of the first scientists to investigate what are now called plasmas and identified it as the fourth state of matter. He also devised one of the first instruments for the study of nuclear radioactivity, the spinthariscope.