Crookes, Sir William (1832-1919) English physicist and chemist. In 1861, as a result of his spectroscopic work, he discovered the element thallium. Around 1875 he began investigating the activity of cathode-rays in high-vacuum tubes of his design, establishing that the rays were streams of charged particles. This work resulted in his invention of the Crookes Tube and led directly to the discovery of the electron by J J Thomson in 1897 and the development of atomic physics. He was also the inventor of the Crookes Radiometer and the spinthariscope which measured alpha radiation. He was knighted in 1897 and elected Fellow of The Royal Society in 1863.