« PreviousContinue »
Molybdenum is derived from its ores, of which the most common, termed molybdena, is a bisulphuret, usually associated with ores of tin. From this substance is derived molybdic acid, Mo 03, and from this, again, the metal molybdenum. The acid, which is a light porous substance, is exposed, along with charcoal, to a strong heat, by which means the oxygen is made to quit the acid. The metal may likewise be obtained, and more easily, by placing the acid in a tube of porcelain, strongly heated, and passing over it a current of hydrogen, which, combining with the oxygen of the acid, forms water.
Molybdenum is so infusible, that it has as yet only been obtained in grains, or in pieces imperfectly agglutinated. Its colour is a silver-white, with a shade of yellow. Its specific gravity is about 8.6, and it is exceedingly brittle. When heated in open vessels, it absorbs oxygen, and is converted into molybdic acid. But its affinity for oxygen is not very great, and hence it is not subject to alteration. when exposed to the atmosphere, or kept under water. Its combining weight has been reckoned 47.96. It combines with oxygen, forming-
It combines in a similar manner with chlorine and sul
This metal was first distinguished as a peculiar substance by Klaproth in 1789. He derived it from the mineral pitchblende, in which it exists in the state of an oxide, along with silica, peroxide of iron, and other substances. It is found likewise in a few other minerals, but in comparatively small quantity. It is obtained by different processes. One is founded upon the principle of bringing the metal to the state of an oxide, and then reducing the oxide to the metallic state, by passing over it a current of hydrogen, or by mixing it with charcoal, and exposing it to the heat of a blast furnace. It is obtained in the state of powder, which no heat has yet been found sufficient to melt into a mass. The powder has a reddish brown colour and metallic lustre. Its specific gravity, according to Klaproth, is 8.100, according to Bucholz, 9. It undergoes no change, when exposed to the air at common temperatures, but when heated to redness it takes fire, and is converted into an oxide. Its combining weight has been estimated at 217.26; but this is certainly an error, and M. Regnault, founding on Dulong's law of specific heat, reduces it to one-fourth of this estimate, or 54.31. It seems to be allied to chromium, communicating beautiful colours to its compounds.
XIX. TUNGSTEN or WOLFRANIUM.
This metal exists in a few minerals, of which one is tungstate of lime, an opaque white substance, found in Sweden, of great weight, and thence termed tungsten or heavy stone. It was first obtained from this mineral, and afterwards from another, called by the Germans wolfram, which is sometimes found in the mines, and is a tungstate of manganese and iron. From the latter, the metal is termed by the Germans wolfranium. Both these minerals yield tungstic acid W 03, and from this acid is obtained the metallic base, by exposing the acid to the action of charcoal, or of hydrogen, at a red heat, as in the case of molybdenum.
Tungsten thus obtained is a powder of a dark-gray colour, possessing considerable brilliancy. It is brittle, and exceedingly hard. Its specific gravity is about 17.4, so that, next to gold, platinum, and iridium, it is the heaviest of the metals. It is fusible with the greatest difficulty, and when fused has the appearance of steel, and is not changed by exposure to the air. When heated to redness, in its state of powder in the open air, it takes fire, and is converted into tungstic acid. Its combining weight has been estimated at from 94.7 to 97.7. It combines with oxygen, forming
It combines in the same manner with sulphur and chlorine. It is exceedingly similar to molybdenum, and both are allied to iron and manganese
This metal, discovered in 1801, was derived from a black mineral, with golden streaks, found in the British Museum, and supposed to have been brought from Massachusetts, in North America, on which account, the name columbium was applied to the metal. It was soon afterwards obtained from two other rare minerals, which were named respectively tantalite, and yttro-tantalite, in the first of which it existed along with oxides of iron and manganese, in the second, along with the earth yttria, and it has been found in a few minerals of the rarest species.
From these minerals tantalic acid, Ta O3, is obtained, and from this acid the metal. It is in the form of a black powder, which, under the burnisher, assumes the metallic lustre, with an iron-gray colour. It is fusible only at a high temperature, and is not altered by exposure to the atmosphere. When heated in the open air, it takes fire below a red heat, and glows with a vivid light, but without flame, being converted into tantalic acid. Its combining weight has been estimated, doubtless erroneously, at 184.9. It combines with oxygen, chlorine, and sulphur. With oxygen it forms
1. Tantalic oxide,
This metal was obtained by Seftstroem in 1830, in the iron derived from the iron-mine of Taberg, in Sweden, and afterwards procured in larger quantity from the slag of the same ore; and soon afterwards, the same metal was discovered in a mineral in Scotland, in which it exists as vanadiate of lead. It is obtained in the form of a powder, which has not yet been fused. It has a white colour, somewhat like silver, and a strong metallic lustre. It has been obtained by another process, in the form of a dark pulverulent substance, which has little of the external characters of a metal, though, by pressure, it assumes a certain lustre. In this state it takes fire when heated in the open air, and is converted into the protoxide.
This metal is so exceedingly brittle, that it cannot be moved from the glass-tube in which it is prepared, without falling into powder. It is but little changed by exposure to the air, but, after a time, its lustre begins to be impaired, and it acquires a reddish tint.
Its combining weight has been estimated at 68.66. It combines with oxygen, sulphur, chlorine, and other substances. Its combinations with oxygen are—
2. Binoxide, which has also the properties of an acid, Va O
This latter acid is fusible, but retains its oxygen at a strong heat. It is very sparingly soluble in water. Like chromium, it has the property of communicating colour to its salts. Its neutral salts are white and yellow, its acid salts fine orange red.