Sir William Crookes, OM, FRS (17 June 1832 – 4 April 1919) was an English chemist and physicist. Sir William attended the Royal College of Chemistry, in London, and worked on spectroscopy.
In 1861, Crookes discovered a previously unknown element with a bright green emission line in its spectrum and named the element thallium, from the Greek thallos, a green shoot. Crookes also identified the first known sample of helium, in 1895. He was the inventor of the Crookes radiometer, which today is made and sold as a novelty item. He also developed the Crookes tubes, investigating canal rays.
In his investigations of the conduction of electricity in low pressure gases, he discovered that as the pressure was lowered, that the negative electrode appeared to emit rays (the so-called cathode rays, now known to be a stream of free electrons, and used in cathode ray display devices). As these examples indicate, he was a pioneer in the construction and use of vacuum tubes for the study of physical phenomena. He was, as a consequence, one of the first scientists to investigate what are now called plasmas. He also devised one of the first instruments for the study of nuclear radioactivity, the so-called spinthariscope.