# Encyclopædia Britannica, Ninth Edition/Thallium

See also Thales of Miletus on Wikipedia; Thallium in the 11th edition; and the disclaimer.
THALLIUM, one of the rarer elements of chemistry. Its discovery is one of the outcomes of Bunsen and Kirchhof's method of spectrum analysis. When Crookes, in 1861, applied this method to the flue-dust produced in the roasting of a certain kind of pyrites he observed in its spectrum a green line foreign to all then known spectra, and concluded that his substance must contain a new element, to which he gave the name of thallium, from θαλλός, a green twig. Crookes presumed that his thallium was something of the order of sulphur, selenium, or tellurium; but Lamy, who anticipated him in isolating the new element, found it to be a metal. Our present knowledge of the chemistry of thallium is based chiefly upon the labours of Crookes.
The chemical character of thallium presents striking peculiarities. Dumas once called it the "ornithorhynchus paradoxus of metals." As an elementary substance, it is very similar in its mechanical and physical properties to lead; like lead it forms an almost insoluble chloride and an insoluble iodide. But the hydroxide of thallium, in most of its properties, comes very close to those of the alkali metals; it is strongly basilous, forms an insoluble chloroplatinate, and an alum strikingly similar to the corresponding potassium compounds. Yet, unlike potassium or lead, it forms a feebly basic sesquioxide similar to manganic oxide, Mn$_2$O$_3$.
Traces of thallium exist in many kinds of pyrites, as used for vitriol-making. The only known mineral of which it forms an essential component is the "crookesite" of Skrikerum, Smaland, Sweden, which, according to Nordenskiold, contains 33.3 of selenium, 45.8 of copper, 3.7 of silver, and 17.2 of thallium in 100 parts. Crookesite, however, is scarce. The best raw materials for the preparation of thallium are the flue-dusts produced industrially in the roasting of thalliferous pyrites and the "chamber muds" accumulating in vitriol-chambers wrought with such pyrites; in both it is frequently associated with Selenium (q.v.). The flue-dust from the pyrites of Theux, near Spa (Belgium), according to Böttcher, contains 0.5 to 0.75 per cent, of thallium; that of the pyrites of Meggen, according to Carstanjen, as much as 3.5 per cent.; while that of the pyrites of Ruhrort yielded 1 per cent, of the pure chloride to Gunning.
For the extraction of the metal from chamber mud, the latter is boiled with water, which extracts the thallium as Tl$_2$SO$_4$. From the filtered solution the thallium is precipitated by addition of hydrochloric acid, as TlCl, along, in general, with more or less of chloride of lead. The mixed chlorides are boiled down to dryness with oil of vitriol to convert them into sulphates, which are then separated by boiling water, which dissolves only the thallium salt From the filtered solution the thallium is recovered, as such, by means of pure metallic zinc, or by electrolysis. The (approximately pure) metallic sponge obtained is washed, made compact by compression, fused in a porcelain crucible in an atmosphere of hydrogen, and cast into sticks. Methods for the final purification of the metal will easily be deduced from what follows.
The metal is bluish white; it is extremely soft but almost devoid of tenacity and elasticity. Its specific gravity is 11.86. It fuses at 290°C.; at a white heat it boils and can be distilled in hydrogen gas. When heated in air it is readily oxidized, with formation of a reddish or violet vapour. When exposed to the air it readily draws a film of oxide; the tarnished metal when plunged into water reassumes its metallic lustre, the oxide film being quickly dissolved. When kept in contact with water and air it is gradually converted into hydroxide, Tl$_2$OH$_2$O or TlOH. This hydrate, TlOH, most conveniently prepared by decomposing the solution of the sulphate with baryta water, crystallizes from its ​solution in long yellow needles, TlOH or TlOH + H$_2$O, which dissolve readily in water, forming an intensely alkaline solution, which acts as a caustic, like, for instance, potash-ley, and like it greedily absorbs carbonic acid from the atmosphere. But, unlike the alkalies, it readily loses its water at 100°C. and even at the ordinary temperature, to pass into the state of anhydrous Tl$_2$O, which is black or black-violet.
The chloride, TlCl, is readily obtained from the solution of any thallous salt (e.g., the sulphate), by addition of hydrochloric acid, as a white precipitate similar in appearance to chloride of silver, like which it turns violet in the light and fuses below redness into a (yellow) liquid which freezes into a horn-like flexible mass. The specific gravity of this "horn" thallium, as one might call it, is 7·02. One part of the precipitated chloride dissolves at 0°C. in 500 parts of water, and in 70 parts at 100°C. It is less soluble in dilute hydrochloric acid. Carbonate of soda solution dissolves it pretty freely.
The iodide, TlI, is a yellow precipitate, which requires 16,000 parts of cold water and still more of solution of iodide of potassium for its solution.
The chloroplatinate, PtCl 6 Tl 2, readily obtainable from thalloussalt solutions by addition of chloride of platinum (PtCl 6 H 2 ), is a yellow precipitate soluble in no less than 15,600 parts of cold water.
The carbonate, T1 2 C0 3, comes closer to the lithium compound than to any other ordinary carbonate. It forms resplendent monoclinic prisms, soluble at 18°C. in 19 1 and at 100 in 4 46 parts of water. A stable bicarbonate, T1HC0 3, does exist.
The sulphate, TUS0 4, forms rhombic prisms isomorphous with K 2 S0 4 . It dissolves at 18 C. in 20 8 and at 101 in 5 2 parts of water. It unites with vitriol into an acid salt, T1HS0 4 + 3H 2 0, and with sulphate of alumina into an "alum," A1 2 (S0 4 ) 3 T1 2 S0 4 +
Thallic salts are related to thallous pretty much as manganic are to manganous. The chloride, TIClj, is obtained as a solution by passing chlorine into a suspension of thallous chloride in water. The solution, when evaporated in vacua, deposits colourless crystals, T1C1 3 + H 2 0. For the oxide, if chlorine be passed into a solution of thallous chloride in carbonate of soda a brown precipitate is produced, which, after drying, has the composition T1 2 3 + H 2 0. When heated with strong hydrochloric acid it evolves chlorine and yields T1C1; when heated with oil of vitriol it yields oxygen gas and thallous sulphate. Thallic sulphate, however, does exist, in crystals, T1 2 (S0 4 ) 3 + 7H 2 0, soluble in dilute sulphuric acid, but decomposed by water, with precipitation of hydrated TI 3 .
Analysis.—All thallium compounds volatile or liable "to dissocia tion at the temperature of the flame of a Bunsen lamp impart to such flame an intense green colour. The spectrum consists of only one line, which, of course, has a definite position in the spectrum, and consequently is easily identified,—a most delicate test.
From solutions containing it as thallous salt the metal is easily precipitated as chloride, iodide, or chloroplatinate by the corre sponding reagents (see supra). Sulphuretted hydrogen, in the presence of free mineral acid, gives no precipitate; sulphide of ammonium, from neutral solutions, precipitates T1 2 S as a dark brown or black precipitate, insoluble in excess of reagent. Thallic salts are easily reduced to thallous by means of solution of sulphur ous acid, and thus rendered amenable to the above reactions. The atomic weight of thallium was determined very carefully by Crookes. He found it Tl = 204 2, being 16.(W. D.)