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Experiment 11. Put into a wine-glass about a scruple of the oxydized manganese and potass, prepared as above, and an equal quantity of the same compound into another glass. On one pour hot, and on the other cold water. The hot solution will exhibit a beautiful green colour; the cold one, a deep purple.

Experiment 12. If a small portion of the same compound be put into several glasses, and water at different temperatures be poured upon each, the contents of each glass will exhibit a different shade of colour. Rationale

. In the preparation the nitrate of potash is decomposed, and the black oxyd of manganese is brought to the state of a more imperfect oxyd by the ignition: it then becomes partly soluble in the potash. This solution would appear blue; but as some portion of oxyd of iron is always contained in the black oxyd of manganese, its colour is changed to green by the yellow tint of the oxyded iron. This oxyd gradually subsides, and the blue colour appears. The oxyd of manganese now attracts more oxygen from the air, and assumes a reddish brown tinge, which increases more and more, and at last becomes black. It is then precipitated, and the solution becomes limpid.

Remark. Manganese, treated in a particular manner, will unite with sulphur and phosphorus; the former combination has been effected by Bergman, and the lat. ter by Pelletier, It unites with many of the metals; these combinations are but little known. The black oxyd sold in the shops, contains from 30 to 40 per cent. of oxygen. The oxyds of this metal are used in bleaching, in purifying glass, and in glazing black earthen ware.

SECTION XXII.

OF CHROMIUM.,

Chrome is obtained from its native combinations by decomposing them by the alkaline carbonates, precipi. tating the chromic acid, and heating it strongly in a crucible.

Experiment 1. The following method is recommended by Vauquelin. Seventy-two parts of chromic acid are to be introduced into a charcoal crucible placed within another of porcelain filled with charcoal dust. The apparatus is then to be put into a furnace, and subjected to a very strong heat. Metallic chromium will then be found in the charcoal crucible. From seventytwo parts, Vauquelin obtained forty-three of metal.

Remark. This metal, which exists only in the state of a metallic oxyd, was discovered by Vauquelin. He found it in an ore called red lead ore of Siberia, or chromate of lead. The colour of this ore is red with a shade of yellow; when reduced to powder it is of a bright orange. Chromium has likewise been found in combination with iron, alumina, and silica (chromate of iron and aluminu) in the department of Var in France.

Chromate of iron has been discovered in quantities in the United States, from which the paint called chromic yellow (chromate of lead) has been prepared.

This pigment has been made in this country by several chemists.

Experiment 2. If chromate of iron be boiled in a solution of potash, the liquor then added to nitric acid, and a solution of nitrate of lead poured in, the chromic yellow, or chromate of lead, will be formed.

Experiment 3. If nitrate of mercury be used in the same manner, a chromate of mercury of the colour of vermillion will be produced.

* xperiment 4. If nitrate of silver be employed the product will be a chromate of silver of a carmine colour.

Experiment 5. If nitrate of zinc or bismuth be used, a chromate of zinc, or bismuth, of a bright yellow will be formed

Remark. In all these experiments a double decomposition ensues, by which the peculiar chromates are formed.

Chromium in its metallic state is white. Its specific gravity is 5.9. It is capable of combining with three different proportions of oxygen, and of forming three oxyds, the green, the brown, and the yellow, or chro. mic acid.

The prot oxyd is the first, obtained by exposing chromic acid to heat in close vessels; the deut oxyd is the second; and the per oxyd, or chromic acid, is the third, and contains 67 per cent. of oxygen.

SECTION XXIII.

OF URANIUM.

Experiment 1. In order to obtain uranium, the pechBlende is first freed from sulphur by heat, and cleared from the adhering impurities as carefully as possible. It is then digested in nitric acid; the metallic matter that it contains is thus completely dissolved, while part of the sulphur remains undissolved, and part of it is dissipated under the form of sulphurated hydrogen gas. The solution is then precipitated by a carbonated alkali. The precipitate has a lemon-yellow colour when it is pure. This yellow carbonate is made into a paste with oil, and exposed to a violent heat, bedded in a crucible containing and lined with charcoal.

Klaproth obtained a metallic globule 28 grains in weight, by forming a ball of 50 grains of the yellow carbonate with a little wax, and by exposing this ball in a crucible lined with charcoal to a heat equal to 1709 of Wedgwood's pyrometer.

Richter obtained in a single experiment 100 grains of this metal, which seemed to be free from all admixture.

Remark. This metal was discovered by Klaproth in the year 1789. It exists combined with sulphur and a portion of iron, lead, and silica, in the mineral termed Pechblende, or oxyd of uranium. Combined with carbonic acid, it forms the chalcolite, or green mica: and mixt with oxyd of iron, it constitutes the uranilic ochre. It is always found in the state of an oxyd with a greater or smaller portion of iron, or mineralized with sulphur and copper

The colour of uranium is iron gray; it has considerable lustre. Its specific gravity according to Klaproth is 8.1. According to Bucholz there are several oxyds of this metal. As this metal is not much known, the reader may find some further account in Thoma son's Chemistry.

SECTION XXIV.

OF MOLYBDENUM.

Molybdenum may be obtained in a state of purity in the following manner:

Experiment 1. Expose molybdena (sulphuret of molybdenum) to a moderate red heat, till the whole is reduced to the state of a fine powder, and passes through a seive. Dissolve the powder in ammonia, filter the solution, and evaporate to dryness. Heat the residuum, add a little nitric acid, and a white powder will be left, which is the oxyd of molybdenum. Mix this with oil, and expose the mixture to a violent heat. Or,

Experiment 2. Molybdic acid is to be formed into a paste with oil, dried at the fire, and then exposed to a violent heat in a crucible lined with charcoal. By this means the oxyd becomes decomposed, a black ag

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glutinated substance is obtained, very brittle umder the finger, and having a metallic brilliancy.

Remark. Molybdenum exists mineralized by sulphur in the ore called sulphuret of molybdenum. This ore, which is likewise scarce, is so similar in several of its properties to plumbago, that they were long considered as varieties of the same substance. It is of a light lead gray colour; its surface is smooth, and feels unctuous; its texture is lamellated; it soils the fingers, and marks paper bluish black, or silver gray. It may be cut with a knife. It is generally found in compact masses; seldom in particles, or crystallized. It is met with in this country, Sweden, Spain, Saxony, Siberia, and Iceland. Scheele proved that a peculiar metallic acid might be obtained from it; and later chemists have succeeded in reducing this acid to the metallic state.

The native sulphuret of molybdenum is the only ore hitherto known, which contains this metal.

Molybdenum is either in an agglutinated blackish friable mass, having little metallic brilliancy, or in black powder. The mass slightly united shows by a magnifying glass small round brilliant grains. Its weight is from 6.600 to 7.500. It is one of the most infusible of the metals. It is capable of combining with a number of metals by fusion. It forms with sulphur an artificial sulphuret of molybdenum analogous to its ore. It unites also to phosphorus. The affinity of molybdenum for oxygen, accordiog to Mr. Hatchett is very fee. ble.

For other facts respecting this metal, see Thomson, i, p. 361.

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