The English chemist and physicist Sir William Crookes (1832-1919) discovered the element thallium and invented the radiometer, the spinthariscope, and the Crookes tube.

William Crookes was born in London on June 17, 1832. His education was limited, and despite his father's wish that he become an architect, he chose industrial chemistry as a career. He entered the Royal College of Chemistry in London, where he began his researches in chemistry. In 1859 he founded the Chemical News, which made him widely known, and remained its editor and owner all his life.

Most notable among Crookes's chemical studies is that one which led to his 1861 discovery of thallium. Using spectrographic methods, he had observed a green line in the spectrum of selenium, and he was thus led to announce the existence of a new element, thallium. While determining the atomic weight of thallium, using a delicate vacuum balance, he noticed several irregularities in weighing, which he attributed to the method. His investigation of this phenomenon led to the construction in 1875 of an instrument that he named the radiometer.

In 1869 J. W. Hittorf first studied the phenomena associated with electrical discharges in vacuum tubes. Not knowing of this, Crookes, 10 years later, made a parallel but more extensive investigation. In his 1878 report he pointed out the significant properties of electrons in a vacuum, including the fact that a magnetic field causes a deflection of the emission. He suggested that the tube was filled with matter in what he called the "fourth state;" that is, the mean free path of the molecules is so large that collisions between them can be ignored. Tubes such as this are still called "Crookes tubes," and his work was honored by naming the space near the cathode in low pressure "Crookes dark space."

Crookes also made useful contributions to the study of radioactivity in 1903 by developing the spinthariscope, a device for studying alpha particles. He foresaw the urgent need for nitrogenous fertilizers, which would be used to cultivate crops to meet the demands of a rapidly expanding population. Crookes did much to popularize phenol (carbolic acid) as an antiseptic; in fact, he became an expert on sanitation. Mention should also be made of the serious and active interest he took in psychic phenomena, to which he devoted most of 4 years.

Crookes was knighted in 1897. His marriage lasted from 1856 until the death of his wife in 1917; they had 10 children. He died in London on April 4, 1919.

Further Reading

• A biography of Crookes is Edmund E. Fournier d'Albe, The Life of Sir Wm. Crookes (1923). For background information see Alexander Findlay, A Hundred Years of Chemistry (1937; 3d ed. 1965), and Eduard Farber, The Evolution of Chemistry: A History of Its Ideas, Methods and Materials (1952; 2d ed. 1969).
• Hall, Trevor H., The medium and the scientist: the story of Florence Cook and William Crookes, Buffalo, N.Y.: Prometheus Books, 1984.