Portrait of English physicist Sir William Crookes (1832-1919), discoverer of the element Thallium. Crookes is probably best known for his invention of the radiometer, a device consisting of a set of pivoted vanes in a vacuum. One side of each vane is blackened, the other shiny, & in the presence of sunlight the vanes turn steadily. Although never more than a toy, Crookes' radiometer did provide evidence for the kinetic theory of gases. Crookes developed methods for producing better vacuums & studied cathode rays, showing that they were deflected by a magnet. He was the first to realise that they were beams of charged particles. Crookes also did important work on radioactivity.