A soft, gray metal which readily oxidizes upon exposure to the atmosphere m.p. 303.5 ºC; 578.3 ºF b.p. 1457±10 ºC; 2655±18 ºF density 11.85 g/cc; 739.77 pound/cubic foot	memory peg
	1861 Sir William Crookes, England
	θαλλος (thallos) = green shoot (Greek)

Very soon after Bunsen and Kirchhoff announced the discovery of Rubidium (1861), Sir William Crookes (1832-1919) entered in 1848 the Royal College of Chemistry, London, where he became assistant to August Wilhelm von Hofmann (1818-1892) and began his chemical researches including those in spectroscopy. In 1851, at age 19, published his first paper, selenocyanides. For this paper he had examined some residues from a sulfuric acid plant at Tilkerode in the Harz (German), given to him by one of his teachers because they contained Selenium which could be turned to selenocyanides. Crookes saved the residues because he thought they might also contain Tellurium.

About ten year later, on examining the residues with a spectroscope he found no lines of Tellurium, and the lines of Selenium soon faded. But a beautiful green line appeared that he have never previously seen. He concluded the residues must contain a new element. This green stripe reminded Crookes, the color of the vegetation in Spring and therefore he named it Thallium, after the Greek θαλλος [thallos] = green shoot or twig (the Latin thallos is a loan word from the Greek). Crookes announced the discovery in the 30 March 1861 Chemical News which he started in 1859 and was the sole editor until 1906.

Almost simultaneously Claude-Auguste Lamy (1820-1878) examined some slime from a sulfuric acid plant at Loos which was using Belgian pyrite and observed the green spectral line. He extracted thallium sesquichloride and isolated the metal new by electrolysis. Only after his discovery, he found out that Crookes had earlier discovered and named the new element, and gave Crookes the honour. Crookes presumed that his Thallium was something of the order of Sulphur, Selenium or Tellurium but Lamy found it to be a metal. In April 1862 he reported to the French Académie des sciences (note):

• Mary Elvira Weeks, Discovery of the Elements, comp. rev. by Heny M. Leicester (Easton, Pa.: Journal of Chemical Education, 1968), pp. 607-613.
• Frank James, "The Letters of William Crookes to Charles Hanson Greville Williams 1861-2: The Detection and Isolation of Thallium." Ambix, 1981, 28: 131-157.