Remark. Mr. Stodart has discovered the above method of covering other metals with platinum. Mr. S. found, that like gold, it may be taken from its solution by sulphuric ether.

Experiment 20. If equal parts of glacial acid of phosphorus and platinum, mixed with an eighth of charcoal, be put into a crucible, and a stratum of charcoal powder put on it; and the whole, exposed to an intense heat for one hour, a button of phosphuret of platinum of a silver white colour will be formed.

Rationale. The charcoal decomposes the phosphoric acid, carbonic acid is disengaged, and the phosphorus combines with the metal forming a phosphuret.

Remark. M. Pelletier made this preparation by exposing the phosphoric acid and platinum, mixed with charcoal, to the action of a heat equal to 329 of Wedgwood's pyrometer; or,

Experiment 21. According to Pelletier, if phosphorus be projected on red hot platinum, the metal instantly fuses, and forms with it a phosphuret of plati

num.

Experiment 22. If, into an ignited crucible a mixture of phosphuret of platinum and oxymuriate of potash be thrown, a detonation will ensue, and the platinum become separated in its metallic form.*

Rationale. The oxygen of the oxymuriatic acid unites with the phosphorous, forming the phosphoric acid, which in part combines with the potash of the decomposed oxymuriate, forming phosphate of potash, and the metal is revived; or,

Experiment 23. If nitrate of potash be used in the same manner, a detonation will also ensue.

Experiment 24. If pure potash be fused in a platinum crucible, it will be found that the vessel has lost part of its weight, which is dissolved by the potash; for,

Experiment 25. If the alkali be afterwards dissolved in water, and saturated with muriatic acid, a triple salt will be obtained by evaporation, composed of platinum, muriatic acid, and potash.

* Gren.

Experiment 26. If sulphuretted hydrogen gas be passed through a solution of platinum, or water impregnated with that gas be added, the metal will be precipitated in dark brown flakes.

Remark. Proust is of opinion, that the precipitate occasioned by sulphuretted hydrogen, is a true sulphuret of this metal. See Hydrosulphurets.

Platinum combines with most of the metals. With gold it forms an alloy of different colours, according to the quantity of the platinum. Thus, an alloy of one part of platinum with four parts of gold can scarcely be distinguished in appearance from pure platinum. The colour of the gold becomes predominant when it constitutes eight ninths of the alloy.

Various experiments have been made on this subject by Vauquelin, Hatchett, and Klaproth.*

Dr. Wollaston has discovered, that platinum is a remarkably slow conductor of caloric; for he found that when equal pieces of silver, copper, and platinum, were covered with wax, and heated at one end, the wax was melted 3 inches on the silver; 21 on the copper; and 1 inch only on the platinum. Its expansion by heat is considerably less than that of steel.

SECTION III.

OF SILVER.

Experiment 1. If sulphuretted or vitreous silver ore be roasted, and then melted with lead and borax, or some other flux to assist the fusion, metallic silver will be obtained.

* Vauquelin, Manuel de l'Essayeur p. 44, &c. Hatchett on the alloys of gold, &c. Phil. Trans. 1803. Klaproth, Journ. de Chim. iv. 29.

E

Rationale. The roasting separates the sulphur, which is altogether disengaged, if any should remain, by the subsequent operation; but, as the silver is ge-nerally alloyed with other metals, it is separated from them by cupellation, or fusion with lead or bismuth.

Remark. The extraction of silver from its ores is different in different countries, and the process is varied according to the nature of the ore. In South

America the silver is separated, after the mineral has been pounded, roasted, and washed, by trituration with mercury. The alloy thus formed is washed, then strained through leather, and exposed to heat to drive off the mercury. The silver is then melted, and cast into bars or ingots. An enumeration of the different ores of silver may be found in the classification of ores. Pure silver may be obtained in the following

manner:

Experiment 2. Dissolve the silver of commerce in nitric acid, and precipitate with a diluted solution of sulphate of iron; or precipitate with common salt, and form the precipitate into a paste with soda, and fuse it with a brisk heat. This process gives a button of pure silver.

Rationale. The nitric acid dissolves the silver, forming a nitrate of silver, and, in the process with common salt, the muriatic acid precipitates it in the form of a muriate of silver, whilst the nitric acid passes to the soda, forming nitrate of soda. When the precipitate, or muriate of silver, is exposed with soda to the action of heat, the muriatic acid quits the silver, and unites with the soda.

Experiment 3. If to a silver ore soluble in nitric acid, that acid be added, and the solution decomposed by muriate of soda, every 100 parts of the precipitate will indicate 75 of silver; but,

Experiment 4. If the ore should be insoluble in nitric acid, which is the case with the corneous silver ore, it may be decomposed, and the silver obtained from it, by treating it with carbonate of soda in a crucible, as stated in Experiment 2.

Rationale. The muriated or corneous silver ore is decomposed by the carbonate of soda, muriate of soda is formed, and the silver is separated.

Experiment 5. If the solution of silver should contain lead, as that metal is also precipitated by muriate. of soda, the silver may be separated by immersing into it a strip of copper, which precipitates it in a

metallic state.*

Remark. Silver, like gold, has been known from the earlier ages. It is a white metal. When melted its specific gravity is 10.474; when hammered 10.510. It is very malleable. It may be beat into leaves of 10 inch thick. It is so ductile, that it may be

1

drawn out into a wire much finer than a human hair. A single grain may be extended about 400 feet in length. A wire of silver 0.078` inch in diameter is eapable of supporting a weight of 187.13lbs. avoirdupois, without breaking.

When completely red hot, silver melts, and increases in splendour. The temperature necessary for the fusion of silver is 229 of Wedgwood. When exposed for a long time to heat, it gradually loses in weight.

Experiment 6. If silver be melted, and its surface gradually cooled; and if the liquid part of the metal be poured out as soon as the surface congeals, crystals of silver may be obtained.

Remark. By this method Tillet and Mongez, junior, procured silver in four sided pyramids, both insulated and in groups.

Experiment 7. If silver be dissolved in nitric acid and the solution decomposed by adding lime water, an oxyd of silver will be precipitated.

Remark. Silver unites with oxygen by precipitation, although it is asserted, that if silver be melted and long exposed to the air it will absorb oxygen, but the proportion is small. Klaproth found the oxyd pro

* For the different methods of analysing silver ores, see Klaproth's Essays, i. p. 554, &c,

duced by precipitation to contain 89 parts of silver, and 11 parts of oxygen. Thomson considers it the per-oxyd. The prot-oxyd, according to Proust, may be formed in the following manner:

Experiment 8. Dissolve silver in nitric acid, and then heat the solution in contact with a portion of the metal in the state of powder.*

Experiment 9. If the precipitate obtained by adding lime water to a solution of silver in nitric acid, be mixed with liquid ammonia, and stirred till it assumes a black colour, and the product then dried in the air, the fulminating silver of Berthollet will be formed.

Rationale. The oxyd of silver unites with a part of the ammonia, forming an ammoniated oxyd of silver. Remark. This powder is so extremely dangerous, that when once prepared it cannot be touched without danger. It expiodes by the mere touch of any substance. A single grain is sufficient to shew its effect.

In the fulmination of this powder, by the slightest friction, or the application of heat, the oxygen of the oxyd unites with the hydrogen of the ammonia forming water, whilst the silver is reduced, and azotic gas emitted. The aqueous vapour, as well as the gas being considerably augmented in volume by the caloric which is disengaged, accounts for the extreme power of the powder. It is said that the oxyd of silver, combined with the ammonia, contains more oxygen than the oxyd of gold in the preparation of fulminating gold, to which some attribute its superior

power.

Experiment 10. If the liquor which is separated from the precipitate, be slowly evaporated, crystals are formed, which explode more violently than the powder.

Experiment 11. If oxymuriate of alumina, formed by passing oxymuriatic acid gas through water containing alumina, be digested on phosphate of silver,

* Nicholson's Journal, xv. 376.