Remembered for discovery of thallium and researches on cathode rays.
In 1848 Crookes entered the Royal College of Chemistry, London, where he became assistant to A. W. Hofmann and began his chemical researches including those in spectroscopy; His first paper was published in 1851. In 1854 he became Superintendent of the Meteorological Department at the Radcliffe Observatory, Oxford, and in the following year was appointed lecturer in chemistry at the Chester College of Science. He returned to London in 1856, and in 1859 became the editor of the newly-founded Chemical News, a position he occupied till 1906.
Crookes's observation of a striking green line in the spectrum of selenium led to his announcement in 1861 of the existence of a new element, thallium. For his work on thallium, Crookes was elected F.R.S. in 1863. Crookes published in 1873 his final and remarkably accurate, determination of the atomic weight of thallium.
In his Bakerian Lecture of 1878 and his British Association Lecture of 1879, he announced various striking properties of "molecular rays" including the casting of shadows, the warming of obstacles and the deflection by a magnet. The title "Radiant Matter" employed by Crookes in his British Association Lecture of 1879 referred to ordinary matter in a new state in which the mean free path was so large that collisions between molecules could be ignored. Other important work by Crookes included a paper of 1881 containing an experimental confirmation of the prediction by J. Clerk Maxwell that the viscosity of a gas would be independent of pressure.