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 teacher, August Wilhelm von Hofmann, might have been expected to lead him, but on new compounds of selenium. These formed the subject of his first published papers in 1851. He worked at the department at the Radcliffe Observatory in Oxford in 1854, and in 1855 was appointed lecturer in chemistry at the Chester Diocesan Training College. In 1856 he married Ellen, daughter of William Humphrey, of Darlington, by whom he fathered three sons and a daughter. Married and living in London, he was devoted mainly to independent work. In 1859, he founded the Chemical News, a science magazine which he edited for many years and conducted on much less formal lines than his usual with journals of scientific societies.
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 wrote a standard treatise on Select Methods in Chemical Analysis in 1871. Crookes was effective in experimentation. The method of spectral analysis, introduced by Bunsen and Kirchhoff, was received by Crookes with great enthusiasm and to great effect. His first important discovery was that of the element thallium, announced in 1861, and made with the help of spectroscopy. By this work his reputation became firmly established, and he was elected a fellow of the Royal Society in 1863.
He developed the Crookes tubes, investigating cathode rays. He published numerous papers on spectroscopy and conducted research on a variety of minor subjects. 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 and identified it as the fourth state of matter in 1879. He also devised one of the first instruments for the study of nuclear radioactivity, the spinthariscope.
Crookes investigated the properties of cathode rays, showing that they travel in straight lines, cause fluorescence in objects upon which they impinge, and by their impact produce great heat. He believed that he had discovered a fourth state of matter, which he called "radiant matter", but his theoretical views on the nature of "radiant matter" were to be superseded. He believed the rays to consist of streams of particles of ordinary molecular magnitude. It remained for Sir J. J. Thomson to expound on the subatomic nature of cathode rays (consisting of streams of negative electrons). Nevertheless, Crookes's experimental work in this field was the foundation of discoveries which eventually changed the whole of chemistry and physics.
Crookes' attention had been attracted to the vacuum balance in the course of thallium research. He soon discovered the phenomenon upon which depends the action of 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 became interested in spiritualism in the late 1860s. In this he was possibly influenced by the untimely death of his younger brother Philip in 1867 at age 21 from yellow fever contracted while on an expedition to lay a telegraph cable from Cuba to Florida. In 1867, Crookes from the influence of Cromwell Fleetwood Varley attended a séance to try and get in touch with his brother.
Between 1871 and 1874, Crookes studied the mediums Kate Fox, Florence Cook, and Daniel Dunglas Home. After his investigation he believed that the mediums could produce genuine paranormal phenomena and communicate with spirits. Psychologists Leonard Zusne and Warren H. Jones have described Crookes as gullible as he endorsed fraudulent mediums as genuine.
Edward Clodd claimed Crookes had a poor eyesight that may of explained his belief in spiritualist phenomena and quoted William Ramsay as saying Crookes is "so shortsighted that, despite his unquestioned honesty, he cannot be trusted in what he tells you he has seen." Biographer William Hodson Brock noted that Crookes was "was evidently short-sighted, but did not wear spectacles until the 1890s. Until then he may have used a monocle or pocket magnifying glass when necessary. What limitations this imposed upon his psychic investigations we can only imagine."
After studying the reports of Florence Cook the science historian Sherrie Lynne Lyons wrote that the alleged spirit "Katie King" was Cook herself and at other times an accomplice. Regarding Crookes, Lyons wrote "Here was a man with a flawless scientific reputation, who discovered a new element, but could not detect a real live maiden who was masquerading as a ghost." Cook was repeatedly exposed as a fraudulent medium but she had been "trained in the arts of the séance" which managed to trick Crookes. Some researchers such as Trevor H. Hall suspected that Crookes had an affair with Cook.
In a series of experiments in London at the house of Crookes in February 1875, the medium Anna Eva Fay managed to fool Crookes into believing she had genuine psychic powers. Fay later confessed to her fraud and revealed the tricks she had used. Regarding Crookes and his experiments with mediums, the magician Harry Houdini suggested that Crookes had been deceived. The physicist Victor Stenger wrote that the experiments were poorly controlled and "his desire to believe blinded him to the chicanery of his psychic subjects."
In 1906, William Hope tricked Crookes with a fake spirit photograph of his wife. Oliver Lodge revealed there had been obvious signs of double exposure, the picture of Lady Crookes had been copied from a wedding anniversary photograph, however, Crookes was a convinced spiritualist and claimed it was genuine evidence for spirit photography.
Crookes joined the Society for Psychical Research, becoming its president in the 1890s: he also joined the Theosophical Society and the Ghost Club, of which he was president from 1907 to 1912.
(Sources: wikipedia, www.aecea.info)