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Painting with gold upon porcelain, or glass,

Is done with the powder of gold which remains behind after driving off the muriatic acid from a solution of that metal, or by precipitating this solution by immersing plates of copper in it. It is laid on with super-saturated borate of soda and gum-water, or oil, burned in and polished.

The gilding of glass

Is commonly effected by covering the part with a solution of super-saturated borate of soda, and apply. ing gold-leaf upon it, which is afterwards fixed by burning

The edges of tea cups, &C.

Are frequently gilt in a less durable manner by appłying a very thin coat of amber varnish,

upon

which gold-leaf is to be fixed, and when the varnish is dry, the gold is burnished.

The gilders of wood, and other compositions designed to supply the place of carved work, make use of gold-leaf, which is either laid on with size, or boiled oil, and afterwards burnished.

For information on hot gilding, Grecian gilding, cold gilding, and wet gilding, see Gren's Chemistry, vol. ii. p. 282. For the muriate of gold, see Salts of Gold.

SECTION II.

OF PLATINUM.

Experiment 1. If equal parts of crude platinum and super-tartrate of potash be mixed, and introduced into a crucible, and exposed for two hours to a violent heat, the platinum fuses, and if it be now exposed to a very strong heat under a muffle, the platinum will remain in a malleable state ; or,

Experiment 2. If common platinum be dissolved in nitro-muriatic acid, and the solution decomposed by muriate of ammonia ; and the precipitate thus obtained be exposed to a violent heat, stamping it when of a white heat into a mass, pure platinum will be produced; or,

Experiment 3. According to Richter, dissolve crude platinum in nitro-muriatic acid, and add potash until a precipitate begins to appear, and then sulphate of potash, till the whole is precipitated. Wash the precipitate, dry it, and mix it with 1.5 times its weight of soda, freed from its water of crystallization; introduce the mixture into a crucible, apply heat gradually, and pure platinum will be obtained ; or,

Experiment 4. According to Jannetty, triturate common platinum with water to wash off every contami. nating matter that water can carry away. Mix the platina with about one-fifth part of arsenious acid and one fifteenth part of potash; putting the whole in a proper crucible, in the following manner: having well heated the crucible and the furnace receiving it, put in one-third of the mixture, apply to this a strong heat, and add one-third more : after a renewed application of heat, throw in the last portion. After a thorough fusion of the whole, cool and break the mass. Then fuse it a second time, and, if necessary, even a third time, till it ceases to be magnetic. Break it into small pieces, and melt those pieces in separate

crucibles, and in portions of a pound and a half of the platinum to each crucible, with an equal quantity of arsenious acid and half a pound of potash. After cooling the contents of the different crucibles in a horizontal position, in order to have them throughout of equal thickness, heat them under a muffle to volatilize the arsenious acid, and maintain them in this state without increase of heat, for the space of six hours. Heat them, next, in common oil, till the oil shall liave evaporated to dryness. Then immerse them in nitric acid, boil them in water, heat them to redness in a crucible, and hammer them into a dense mass. They are now fit to be heated in a naked fire, and hammered into bars for the purposes of commerce.

The rationale of these different processes will be given in the course of the experiments on this metal.

Remark. Platinum, which from its colour has been called white gold, has been found in small grains, combined with palladium, osmium, rhodium, iridium, iron, copper, &c. It was unknown, however, as a distinct metal, before the year 1749. Many experiments were instituted upon this new metal at different periods since its discovery. Dr. Wollaston and Mr. Tennant have thoroughly investigated the subject.

Platinum, when pure, is of a white colour. Its hardness is 8. Its specific gravity, after being hammered is 23.000 ; it is therefore the heaviest body known. It is ductile and malleable. It possesses con-siderable tenacity; a wire of platinum 0.078 inch in diameter is capable of supporting a weight of 274.31lbs. avoirdupois, without breaking. It is one of the most infusible of all metals. It may be fused by the heat of a burning glass, the assistance of oxygen gas, and by the galvanic spark. When pure, its parts may

be welded together, but it first must be heated to whiteness. It may be oxydized in the following manner :

Experiment 5. Dissolve platinum in nitro-muriatic acid, and add lime water until the precipitation ceases'; a yellowish brown powder is thus obtained. Dissolve this powder in nitric acid, and apply heat to drive off the acid. The product is now the per oxyd of platinum.

Rationale. The nitro-muriatic solution is decomposed by the lime water, and oxyd of platinum is precipitated. The metal is still further oxydized by the assistance of nitric acid, which is in part decomposed, and the metal is finally oxydized to the maximum.

Remark. The per oxyd of platinum heated to redness parts with its oxygen. It contains 87 parts of platinum and 13 of oxygen in the hundred.

Experiment 6. If the per oxyd be gradually heated, it parts with a portion of its oxygen, assumes a green colour, and, according to Chenevix, is changed into the protoxyd of platinum.

Remark. The prot-oxyd is composed of 93 platinum, and 7 oxygen in the hundred.

Experiment 7. If to ten parts of nitro-muriatic acid, one part of platinum be added, a violent action will ensue, and the metal will become dissolved ; and,

Experiment 8. If this solution be evaporated, very small crystals of muriate of platinum will be deposited.

Rationale. The nitro-muriatic acid first oxydizes the metal, and then dissolves the oxyd ; the evaporation of which, in the last experiment, produces the muriate of platinum, a proof that the platinum is combined with the muriatic acid. The platinum may be precipitated from this menstruum by various bodies; as,

Experiment 9. If to a solution of platinum in nitromuriatic acid, either soda, potash, or lime water be added, the metal will be precipitated in the form of an oxyd; but,

Experiment 10. When potash is added, it first throws down a triple compound, consisting of the oxyd of platinum, alkali, and acid. If more alkali be used, the precipitate is pure oxyd of platinum.

Experiment 11. Soda used in the same manner produces the same effect.

Experiment 12. Ammonia also, at first, precipitates. a triple compound, an ammoniacal muriate of platinum, and towards the end a pure calciform powder; or oxyd of platinum.

Experiment 13. If to a solution of muriate of platinum, another of inuriate of ammonia be added, a precipitate will be formed.

Rationale. As this precipitate is a sub-muriate of platinum and ammonia, a triple salt, it is obvious, that the inuriate of ammonia not only separates the metal from its solution, but combines with it into an ammoniacal muriate of platinum, from which the platinum may be separated by exposing the precipitate to a strong heat, the muriate of ammonia being evolved; but,

Experiment 14. If muriate of ammonia be added to a solution of gold, no precipitate will appear. Hence by this test platinum may always be known from gold.

Experiment 15. Prepare a very dilute and colourless solution of platina by dropping a small quantity of the nitro-muriate of that metal into a glass of water. If a single drop of the solution of muriate of tin be added to this, a bright red precipitate will be instantly produced.

Remark. Hence the muriate of tin may be used as A test for platinum.

Experiment 16. If prussiate of potash be mixed with a solution of platinum, no effect will ensue; or,

Experimeni 17. If sulphate of iron be used in the same manner, no precipitate will be formed; but,

Experiment 18. If tincture of galls be added, a precipitate of a dark green colour will appear, which becomes gradually paler by standing.

Remark. Should a precipitate be formed in either of the other cases, the presence of other metals may be inferred. In the latter experiment, the gallic acid in the tincture is the precipitant.

Experiment 19. "If nitro-muriate of platina be mixed with a fourth part of its bulk of ether, and the mixture suffered to setde, the ethereal solution of platina will float, and may then be poured off. If polished brass and some other metals be immersed in this so. lution, the surface will be instantly covered with a coat of platina. This process may be applied to many useul purposes.

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