William Crookes announces his discovery of thallium

Background. Flame spectroscopy became an approved method for determining mineral composition following Bunsen and Kirchhoff's improvements in 1859-1860. This technique relies on the unique light spectrum emitted or absorbed by each element. Crookes, an English chemist who attended the Royal College of Chemistry, utilized this method to analyze residues from sulfuric acid production, leading to the identification of thallium.

Methodology. Crookes employed flame spectroscopy, a technique where elements are put into a gaseous state to examine their spectra. The process involves diffracting light to observe consistent frequencies unique to each atomic element. This allowed Crookes to detect the presence of thallium in chemical residues, distinguishing it from other elements through its specific spectral signature.

Publication. Crookes published his findings on March 30, 1861, establishing priority over Claude-Auguste Lamy, who discovered thallium independently in the same year. Both chemists worked on residues from sulfuric acid production. The announcement was made in London, where Crookes was active, and his work was disseminated through scientific channels, including his own magazine, Chemical News.