Gray-white metal m.p. 1545±15 ºC; 2813±27 ºF b.p. 1947 ºC; 3537 ºF density 9.321 g/cc (25 ºC); 581.891 pound/cubic foot (77 ºF)	memory peg
	1879 Per Theodor Cleve, Sweden
	Thule, for the ancient Greeks the northernmost habitable region of the world (Scandinavia)

The story of discovery and naming of this element began with Carl Gustav Mosander splitting old yttria into three new elements, yttria proper, erbia, and terbia (cf. Yttrium). In 1860 the Swedish chemist Nils Johan Berlin (1812-1891) denied the existence of Mosander’s erbia, and gave this name to his terbia.

In 1878, Jean Charles Galissard de Marignac, professor of Chemistry at the University of Geneva, separated Berlin's erbia into two new earths, erbia and ytterbia (note). Marignac's erbia was the following year split by Per Theodor Cleve (1804-1905) into erbia proper and two new elements, which he named Thulium and Holmium (note). Analysis of Holmium showed later that it also contained Dysprosium.

The chemical symbol for Thulium was first Tu, but it was altered into Tm by the International Commission on Atomic Weights, since the symbol Tu was also in use for Tungsten (Wolfram, present day symbol W). The editors of Gmelins Handbuch regret this decision: the "m" does not belongs to the root of Thulium and is therefore not characteristic. John and Gordon Marks suggested in 1994 the symbol Θu (note).

See also: Chronological list of discovery of the rare earths, their names in different languages etc. on the Yttrium page

Thule

To the ancient Greeks, Thule (also Tile or Ultima Thule) was the northernmost habitable region of the world. One of the first recorded sea journeys in the Atlantic was by Pytheas of Massalia, who sailed to England in about 325 B.C. from his Greek colony in what is now Marseilles, France (although he probably traveled over land to the port of Corbilo and sailed from there.) He wrote about his voyage in a book called About the Ocean of which no copies exist, but he is quoted in other works.

Pytheas visited Britan, where tin was traded, and possibly Ireland, the Hebrides, and the Orkneys. Pytheas heared of an island six days sailing to the north of Britain, called Thule, and visited it. We do not know where Thule was, but probably it was (part of) the Norwegian coast, although Iceland, the Shetlands and Faeroer have also been identified as such by historians. Pytheas says that Thule was an agricultural country, and that it produced honey. He said he was shown the place where the sun went to sleep, and he noted that the night in Thule was only two to three hours. One day further north the congealed sea began, he claimed. As Strabo says (as quoted in Chevallier 1984): "Pytheas also speaks of the waters around Thule and of those places where land properly speaking no longer exists, nor sea nor air, but a mixture of these things, like a 'marine lung', in which it is said that earth and water and all things are in suspension as if this something was a link between all these elements, on which one can neither walk nor sail."

On maps, Thule usually appears north or northwest of England and Ireland or in the northernmost parts of Asia. It has been associated with early reports of Iceland, Norway, or the Shetland Islands. Pytheas's Thule was probably Trondheim, in Norway.
(From: André Engels, Discoverers Web, and Dave's Mythical Creatures and Places)

Cleve himself, and after him most chemical sources, write that Thule is an old name for Scandinavia, which is not the case.

Further reading:

• Mary Elvira Weeks, Discovery of the Elements, comp. rev. by Heny M. Leicester (Easton, Pa.: Journal of Chemical Education, 1968), pp. 667-699.
• Seltene Erden. Gmelins Handbuch der anorganische Chemie, 8. Aufl.; System-Nummer 39 (1938).