Page:EB1911 - Volume 27.djvu/394

Rh and North America. The Russian tundra, apart from the arctic conditions of climate and flora, may be compared with the steppes farther south.  TUNGABHADRA, a river of southern India, the chief tributary of the Kistna. It is formed by the junction of two streams, the Tunga and the Bhadra, which both rise in Mysore in the Western Ghats. The united river for nearly all its course forms the boundary between Madras and the dominions of the nizam of Hyderabad. On its right bank stood the capital of the ancient Hindu dynasty of Vijayanagar, now a wilderness of ruins. From of old its waters have been utilized for irrigation. Near its confluence with the Kistna it supplies the Kurnool-Cuddapah Canal. A project has been recently under consideration to dam the river higher up, and there construct an artificial lake that would have an area of 160 sq. m., the cost of this scheme being roughly estimated at nearly £6,000,000.  T'UNG-CHOW, a sub-prefectural city in Chih-li, the metropolitan province of China, on the banks of the Peiho in 39° 54′ N. 116° 41′ E., 12 m. E. of Peking. Its population is estimated at about 50,000.

T'ung-Chow marks the highest point at which the Peiho is navigable, and here merchandise for Peking is transferred to a canal. The city, which is faced on its eastern side by the river, and on its other three sides is surrounded by populous suburbs, is upwards of 3 m. in circumference. The walls are about 45 ft. in height and about 24 ft. wide at the top. They are being allowed to fall into decay. Two main thoroughfares connect the north and south gates and the east and west gates. The place derives its importance from the fact that it is the port of Peking. Like most Chinese cities, T'ung-Chow has appeared in history under various names. By the founder of the Han dynasty (206 ) it was called Lu-Hien; with the rise of the T'ang dynasty (618 ) its name was changed to Hūan-Chow; and at the beginning of the 12th century, with the advent of the Kin dynasty to power, Hūan-Chow became T'ung-Chow. It was at T'ung-Chow that Sir Harry Parkes, Sir Henry Loch and their escort were treacherously taken prisoners by the Chinese when they were sent forward by Lord Elgin to negotiate terms of peace after the troubles of 1860. During the Boxer outbreak in 1900 T'ung-Chow was occupied by the allied armies, and a light railway connecting the city with Peking was constructed by German military engineers.  TUNGSTEN [symbol W, atomic weight 184.0 (O = 16)], a metallic chemical element found in the minerals wolfram, an iron and manganese tungstate, scheelite, a calcium tungstate, stolzite, a lead tungstate, and in some rarer minerals. Its presence in scheelite was detected by Scheele and Bergman in 1781, and in 1783 Juan, José and d'Elhuyar showed the same substance occurred in wolfram; they also obtained the metal. Tungsten may be prepared from wolfram by heating the powdered ore with sodium carbonate, extracting the sodium carbonate with water, filtering and adding an acid to precipitate tungstic acid, H2WO4. This is washed and dried and the oxide so obtained reduced to the metal by heating with carbon to a high temperature (Hadfield, Journ. Iron and Steel Inst., 1903, ii. 38). On a small scale it is obtained by reducing the trioxide in a current of hydrogen, or the chloride by sodium vapour, or the oxide with carbon in the electric furnace; in the last case the product is porous and can be welded like iron. In the form of a powder, it is obtained by reducing the oxide with zinc and extracting with soda, or by dissolving out the manganese from its alloys with tungsten. The metal may be used uncombined, but large quantities of ferrotungsten are made in the electric furnace; other alloys are prepared by acting on a mixture of the oxides with aluminium. Tungsten has been applied in the manufacture of filament electric lamps. The metal has a crystalline structure, and melts at about 2800°. The powdered metal burns at a red heat to form the trioxide; it is very slowly attacked by moist air. It combines with fluorine with incandescence at ordinary temperatures, and with chlorine at 250–300°; carbon, silicon, and boron, when heated with it in the electric furnace, give crystals harder than the ruby. It is soluble in a mixture of nitric

and hydrofluoric acids, and the powdered metal, in aqua regia, but slowly attacked by sulphuric, hydrochloric and hydrofluoric acids separately; it is also soluble in boiling potash solution, giving a tunstate and hydrogen.

Tungsten dioxide, WO2, formed on reducing the trioxide by hydrogen at a red heat or a mixture of the trioxide, and hydrochloric acid with zinc, or by decomposing the tetrachloride with water, is a brown strongly pyrophoric powder, which must be cooled in hydrogen before being brought into contact with air. It is slightly soluble in hydrochloric and sulphuric acids, giving purple solutions. It dissolves in potash, giving potassium tungstate and hydrogen, and is readily oxidized to the trioxide.

Tungsten trioxide, WO3, occurs in nature as wolframine, a yellow mineral found in Cumberland, Limoges, Connecticut and in North Carolina. It is prepared as shown above, or by other methods. It is a canary-yellow powder, which becomes a dark orange on heating; the original colour is regained on cooling. On exposure to light it assumes a greenish tinge. A crystalline form was obtained by Debray as olive-green prisms by igniting a mixture of sodium tungstate and carbonate in a current of hydrochloric acid gas, and by Nordenskjold by heating hydrated tungstic acid with borax. Partial reduction of tungsten trioxide gives blue or purple-red products which are intermediate in composition between the dioxide and trioxide. Tungsten trioxide forms two acids, tungstic acid, H2WO4, and metatungstic acid, H2W4O13; it also gives origin to several series of salts, to which the acids corresponding are unknown. Thus we have salts of the following types M2O(WO3)n, where n=1, 2, 3, 4, 5, 6, 7, 8, and also (M2O)m(WO3)n, where m, n=2, 5; 3, 7; 4, 3; 5, 12; M standing for a monovalent metal. The (M2O)5(WO3)12 or M10W12O41 salts are called paratungstates. Tungstic acid, H2WO4, is obtained as H2WO4·H2O by precipitating a tungstate with cold acid; this substance has a bitter taste and its aqueous solution reddens litmus. By using hot acid the yellow anhydrous tungstic acid is precipitated, which is insoluble in water and in all acids except hydrofluoric. It may be obtained in a flocculent form by exposing the hexachloride to moist air. Metatungstic acid, H2W4O13·7H2O, is obtained by decomposing the barium salt with sulphuric acid or the lead salt with hydrochloric acid. It forms yellow octahedral, which become anhydrous at 100°, and are converted into the trioxide on ignition. It is readily soluble in water, and on boiling the aqueous solution a white hydrate is first deposited which after a time is converted into the trioxide. Graham obtained a colloidal tungstic acid by dialysing a dilute solution of sodium tungstate and its equivalent of hydrochloric acid; on concentrating in a vacuum a gummy product is obtained, which still remains soluble after heating to 200°, but it is converted into the trioxide on heating to redness. When moistened it becomes adhesive. The solution has a bitter taste and does not gelatinize, even under the influence of boiling acids.

Of the salts, the normal tungstates are insoluble in water with the exception of the alkaline tungstates; they are usually amorphous, but some can be obtained in the crystalline form. The metatungstates of the alkalis are obtained by boiling normal tungstates with tungstic acid until the addition of hydrochloric acid to the filtrate gives no precipitate. The most important tungstate is the so-called tungstate of soda, which is sodium paratungstate, Na10W12O41·28H2O. This salt is obtained by roasting Wolfram with sodium carbonate, lixiviating, neutralizing the boiling filtrate with hydrochloric acid and crystallizing at ordinary temperatures. The salt forms large monoclinic prisms; molecules containing 25 and 21 H2O separate from solutions crystallized at higher temperatures. The salt is used as a mordant in dyeing and calico printing, and also for making textiles non-inflammable. Several other sodium tungstates are known, as well as potassium and ammonium tungstates. Many salts also occur in the mineral kingdom: for example, scheelite is CaWO4, stolzite is PbWO4, farberite is FeWO4, Wolfram is (Fe,Mn)WO4, whilst hübnerite is MnWO4.

By partial reduction of the tungstates under certain conditions products are obtained which are insoluble in acids and alkalis and present a bronze-like appearance which earned for them the name of tungsten bronzes. The sodium compound was first obtained by Wöhler on reducing sodium tungstate with hydrogen; coal-gas, zinc, iron or tin also effect the reduction. It forms golden cubes which are unattacked by alkalis or by any acid except hydrofluoric. It appears to be a mixture of which the components vary with the materials and methods used in its production (Philipp, Ber., 1882, 15, p. 499). A blue bronze, Na2W5O15, forming dark blue cubes with a red reflex, is obtained by electrolysing fused sodium paratungstate; a purple-red variety, Na2W3O9, and a reddish yellow form result when sodium carbonate and sodium tungstate are heated respectively with tungsten trioxide and tinfoil. Similar potassium tungsten bronzes are known.

Tungstic acid closely resembles molybdic acid in combining with phosphoric, arsenious, arsenic, boric, vanadic and silicic acids to form highly complex acids of which a great many salts exist. Of the phosphotungstic acids the most important is phosphoduodecitungstic acid, H3PW12O40·nH2O, obtained in quadratic pyramids by crystallizing mixed solutions of orthophosphoric and