Whose discovery first identified thallium by its bright green spectral line?

William Crookes discovered thallium in 1861 by studying the spectrum of residues from sulfuric acid production. The new element stood out because of a bright green spectral line, which inspired its name from the Greek word thallos, meaning green shoot. Thallium was one of the early successes of spectroscopy in chemistry. It showed that light could reveal the presence of elements that were hard to isolate chemically. Crookes later became a major figure in physical chemistry and spectroscopy.