William Crookes

Sir William Crookes (/krʊks/; 17 June 1832 – 4 April 1919) 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.

He is credited with discovering the element thallium, announced in 1861, with the help of spectroscopy. He was also the first to describe the spectrum of terrestrial helium, in 1865. Crookes was the inventor of the Crookes radiometer, but did not discern the true explanation of the phenomenon he detected. Crookes also invented a 100% ultraviolet blocking sunglass lens. For a time, he was interested in spiritualism and became president of the Society for Psychical Research.

At the end of his life, Crookes was appraised as "superb experimentalist" known for the originality of his inquiry into physics and chemistry. He was praised for his industriousness and intellectual qualities. His interests, ranging over pure and applied science, economic and practical problems, and psychic research, made him a well-known personality and earned him a substantial income. He received many public and academic honours.