Sir William Crookes (born June 17, 1832,London, Eng.—died April 4, 1919, London) was a British chemist and physicist noted for his discovery of the element thallium and for his cathode-ray studies, fundamental in the development ofatomic physics.

After studying at the Royal College of Chemistry, London, Crookes became superintendent of the meteorological department at Radcliffe Observatory,Oxford, in 1854, and the following year gained a post at the College of Science in Chester, Cheshire. Havinginherited a large fortune from his father, he devoted himself from 1856 entirely to scientific work of various kinds at his private laboratory in London. His researches on electrical discharges through a rarefied gas led him to observe the dark space around the cathode, now called theCrookes dark space. He demonstrated that cathode rays travel in straight lines and produce phosphorescence and heat when they strike certain materials. He invented many devices to study the behaviour of cathode rays, but his theory of radiant matter, or a fourth state of matter, proved incorrect in many respects.

With the introduction of spectrum analysis by R.W. Bunsen and G.R. Kirchhoff, Crookes applied the new technique to the study ofseleniumcompounds. In 1861 he discoveredthallium in some seleniferous deposits. He continued work on that new element, isolated it, studied its properties, and in 1873 determined its atomic weight.

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During his studies of thallium, Crookes discovered the principle of theCrookes radiometer, a device that converts lightradiation into rotary motion. The principle of this radiometer has found numerous applications in the development of sensitive measuring instruments. Crookes was knighted in 1897.