In his investigations of the conduction of electricity in low pressure gases, he discovered that as the pressure was lowered, the negative electrode (cathode) 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 spinthariscope.
From 1850 to 1854 he filled the position of assistant in the college, and soon embarked upon original work, not in organic chemistry where the inspiration of his distinguished teacher, August Wilhelm von Hofmann, might have been expected to lead him, but on certain new compounds of the element selenium. These formed the subject of his first published papers in 1851.
Leaving the Royal College, he became superintendent of the meteorological department at the Radcliffe Observatory in Oxford in 1854, and in 1855 was appointed lecturer in chemistry at the Chester training college. In 1856 he married Ellen, daughter of William Humphrey, of Darlington, by whom he fathered three sons and a daughter.
From this time his life was passed in London, devoted mainly to independent work. After 1850, he lived at 7 Kensington Park Gardens, where in his private laboratory all his later work was carried out. Crookes's life was one of unbroken scientific activity. He was never one of those who gain influence by popular exposition. The breadth of his interests, ranging over pure and applied science, economic and practical problems, and psychical research, made him a well-known personality, and he received many public and academic honours. In 1859 he founded the Chemical News a science magazine, which he edited for many years and conducted on much less formal lines than is usual with journals of scientific societies.
Crookes was knighted in 1897, and in 1910 received the order of merit. He died in London on 4 April 1919, two years after his wife, to whom he had been much devoted. Crookes is buried in London's Brompton Cemetery.
Crookes' attention had been attracted to the vacuum balance in the course of the thallium researches. He soon discovered the phenomenon upon which depends the action of the well-known little instrument, the Crookes radiometer, in which a system of vanes, each blackened on one side and polished on the other, is set in rotation when exposed to radiant energy. Crookes did not, however, provide the true explanation of this apparent "attraction and repulsion resulting from radiation".
Crookes published numerous papers on spectroscopy, a subject which always had a great fascination for him, and he conducted research on a large variety of minor subjects. In addition to various technical books, he wrote a standard treatise on Select Methods in Chemical Analysis in 1871, and a small book on Diamonds in 1909.
In 1903, Crookes turned his attention to the newly discovered phenomena of radioactivity, achieving the separation from uranium of its active transformation product, uranium-X (later established to be protactinium). He observed the gradual decay of the separated transformation product, and the simultaneous reproduction of a fresh supply in the original uranium. At about the same time as this important discovery, he observed that when "p-particles", ejected from radio-active substances, impinge upon zinc sulphide, each impact is accompanied by a minute scintillation, an observation which forms the basis of one of the most useful methods in the technique of radioactivity.
Crookes' report on this research, in 1874, concluded that these phenomena could not be explained as conjuring, and that further research would indeed be useful. Crookes was not alone in his views. Fellow scientists who came to believe in Spiritualism included Alfred Russel Wallace, Oliver Joseph Lodge, Lord Rayleigh, and William James (Doyle 1926: volume 1, 62). Nevertheless, most scientists were convinced that Spiritualism was fraudulent, and Crookes' final report so outraged the scientific establishment "that there was talk of depriving him of his Fellowship of the Royal Society." Crookes then became much more cautious and didn't discuss his views publicly until 1898, when he felt his position was secure. From that time until his death in 1919, letters and interviews show that Crookes was a believer in Spiritualism (Doyle 1926: volume 1, 169 – 170, 249 – 251).