Sir William Crookes (1832-1919), English chemist and physicist who discovered the element thallium. Crooke’s interest in the photographic process in the 1850s led him to the new science of spectroscopy. Using spectroscopic techniques, Crookes discovered in 1861 a previously unknown element with a bright green emission line in its spectrum; he named the element thallium, from the Greek word `thallos'—a green shoot. Crookes invented the radiometer in 1875 and investigated electrical discharges through highly evacuated "Crookes tubes." These studies laid the foundation for J. J. Thomson's subsequent research in discharge-tube phenomena. Crookes maintained an interest in agriculture and warned in 1898 that the world's population would face starvation unless new fertilizer sources were discovered.