[f. Gr. θαλλ-ός a green shoot (θάλλειν to bloom), from the brilliant green line distinguishing its spectrum + -IUM.] A rare metal, bluish white in color with leaden luster, extremely soft and almost devoid of tenacity or elasticity; occurring in small quantities in iron and copper pyrites. Atomic weight 204; symbol Tl.
1861. Crookes, in Chem. News, 16 March, III. 193. On the Existence of a New Element. Ibid., 18 May, 303. I have thought … to propose for it the provisional name of Thallium, from the Greek θαλλός, or Latin thallus, a budding twig … which I have chosen as the green line which it communicates to the spectrum recals with peculiar vividness the fresh colour of vegetation at the present time.
1871. Roscoe, Elem. Chem., 262. Thallium was discovered in 1861 by Crookes, by means of spectrum analysis, in the deposit in the flue of a pyrites burner.
1874. trans. Lommel’s Light, 114. The splendid green light of Thallium is more strongly refracted than the yellow light of Sodium.
b. attrib. and Comb., as thallium alloy, spectrum; thallium glass, a variety of glass of great density and refracting power, in the manufacture of which thallium is used instead of lead or potassium; thallium green, the color of the thallium line, the vivid green line of the thallium spectrum.
1868. Watts, Dict. Chem., V. 745. The length of the wave of the green thallium-line is 0.0005348 millimetre. Ibid., Thallium-salts are highly poisonous. Ibid., 758. Thallium-glass.
