Experiment 27. If a piece of silk, moistened with phosphorized ether, be immersed in the nitro-muriatic solution, its surface is immediately gilt with a fine coat of gold.

Experiment 28. If sulphuretted hydrogen be substituted for hydrogen gas, in the reduction of gold, the same effect will take place.

Experiment 29. If a piece of white silk be dipped in an aqueous solution of nitro-muriate of gold, and exposed while wet to sulphurous acid gas, the whole piece will in a few seconds be covered with a coat of reduced gold, which remains permanent.

Experiment 30. If a piece of white silk be immersed in an aqueous solution of muriate of gold thoroughly dried in the dark, and then exposed to sulphurous acid vapours, it will suffer no change; nor, if it be wetted with alcohol and then replaced in the vapour, will any sign of reduction appear: but if it be wetted with pure water, and then exposed to the vapour, metallic gold will immediately be seen on its surface.*

Remark. Various experiments of a similar kind may be found in an Essay on Combustion, by Mrs. Fulhame, and in count Rumford's paper, in the Phil. Trans. 1798, p. 449. Mrs. Fulhame has proved, that moisture is absolutely necessary for the reduction of gold. She observes, that in the experiment with ether, no reduction takes place unless water be present. She accounts very ingeniously for these phenomenæ, by supposing the water to be decomposed. The combustible combines

* A glass funnel is a convenient apparatus for these experiments. The silk may be suspended by a thread passed through it, and made fast to the funnel with a cork. The funnel is then to be placed on a table, and by moving it a little over the edge of the table, a lighted match may be readily introduced, and when the glass is full of vapour the match may be withdrawn The vapour is confined by sliding the funnel back upon the table; and thus the phenomena of the experiment may be easily observed. See Parke's Chemical Catechism.

with its oxygen, whilst its hydrogen, by uniting with the oxygen of the gold, reproduces water.*

Experiment 31. If a solution of gold in nitro-muriatic acid be applied to bone or ivory, it will be stained of a purplish red colour.

Experiment 32. If the same solution be applied to feathers, they will be changed to the same colour as in the last experiment.

* Experiment 33. If a diluted solution of gold be applied to silk by means of a brush, the places thus affected will undergo the same change.

Remark. A solution of gold, has, therefore, the property of staining animal and vegetable substances of a purplish red. After a time these stains become of a beautiful brown, and then black. As a strong affinity subsists between the oxyd of gold and the substance to which it is applied, the stain cannot be effaced.

Experiment 34. If to a saturated solution of gold in nitro-muriatic acid, diluted with three times its bulk of distilled water, liquid ammonia be gradually added, a yellow precipitate will be obtained. When washed and dried on a chalk stone, it forms the fulminating gold.. Rationale. The solution of gold, in tal exists in the state of muriate, is the ammonia; one part of the alkali acid, forming muriate of ammonia, whilst another part combines with the gold, thus precipitated, in the form, as Dr. Thomson expresses it, of aurate of ammonia. See Ammonia.

which that medecomposed by unites with the

Experiment 35. If muriate of ammonia be added to a solution of gold in nitro-muriatic acid, and a fixed alkali added, the gold will be precipitated in the state of fulminating gold.†

* Professor Coxe, of the University of Pennsylvania, has lately published an Essay on Acidification, in which he has adduced several of these experiments, which are apparently anomalies, in order to modify the anti phlogistic theory.

↑ Gren. ii. 202.

Rationale. When muriate of ammonia is added to a solution of gold, and the mixture decomposed by potash, or soda, the fixed alkali unites with the muriatic acid, as well from the solution, as from a part of the muriate of ammonia; and the oxyd and ammonià thus disengaged unite, and form fulminating gold.

Experiment 36. If oxyd of gold be obtained, by adding lime water to a solution of gold, and afterwards treated with liquid ammonia, fulminating gold will be produced.

Rationale. Analogous to experiment 34.

Experiment 37. If a few grains of fulminating gold be heated on the point of a knife, a violent detonation will be produced.

Rationale. The theory of the detonation of fulminating gold is imperfectly understood. If it contain only ammonia and oxyd of gold, the following changes would ensue. When the temperature is raised, the ammonia as well as the oxyd of gold suffer decomposition. The hydrogen of the former unites with the oxygen of the oxyd of gold, and reduces the latter to a metallic state. The water thus formed, together with the azote, are liberated in a highly expanded state. The detonation is produced by the violent impulse of these aeriform bodies on the surrounding atmosphere. Mr. Accum supposes, however, that fuiminating gold is a triple compound, namely, an ammoniacal oxymuriate of gold, and that, in combustion, the products are metallic gold, nitrogen gas, muriatic acid and water.

Remark. Bergman observes, that fulminating gold is composed of five parts of yellow oxyd of gold, and one part of animonia. Basil Valentine describes the preparation of this powder. Many philosophical inquiries were instituted respecting it. Whether struck violently, triturated in a mortar, or heated to the temperature of about 250 degrees, the powder fulminates. -Its force, however, is inferior to that of gun powder.

Experiment 38. If to a solution of gold in nitro-muriatic acid, a solution of sulphate of iron be added, the gold will be precipitated in a metallic form; but,

Experiment 39. If red sulphate of iron be added in the same manner, no effect will take place.

Rationale. Gold, it must be observed, is held in solution in consequence of the metal being oxydized, which ensues in the act of solution, and that when the oxygen is abstracted, it not only becomes insoluble, but it also takes the metallic form. This abstraction of oxygen is effected by the sulphate of iron. When this is added, it seizes the oxygen of the oxyd, and the gold is precipitated in a metallic state; the sulphate changing in part to the red or oxy-sulphate. That this is the modus operandi, is obvious from the last experiment; for on the addition of the red sulphate, no precipitation ensues.

Experiment 40. If a piece of thread be passed through a small piece of phosphorus previously freed from moisture by immersing it in alcohol, and then suspended in an aqueous solution of nitro-muriate of gold, in a few minutes the phosphorus will become covered with pure gold. See Phosphorus.

Experiment 41. If hydrosulphuret of potash be dropt into a solution of gold, a black powder will be precipitated. See Hydrosulphurets.

Rationale. The acid of the solution unites with the alkali of the hydrosulphuret, and the gold together with the sulphur is precipitated. This precipitate, according to Proust, consists of gold and sulphur merely in the state of mixture.

Experiment 42. If some leaf gold be digested with heat, in a solution of sulphuret of potash, the metal will be dissolved; or,

Experiment 43. If three parts of sulphur, and three of potash be melted together, and one part of gold added, the same effect will take place.

Remark. Hence it is, that gold is soluble both in the dry and humid way in alkaline sulphurets. The solution is of a yellow colour, and is decomposed by acids. When an acid is dropt in, the gold is precipitated along with sulphur. This precipitate, from the experiments of Bucholz, is composed of 82 parts gold and 18 sulphur. The opinion of some chemists,

that Moses dissolved the golden calf of the Israelites, as related in Exodus xxxii, by the agency of sulphur and potash, appears somewhat probable.

Experiment 44. If half an ounce of gold and an ounce of glacial acid of phosphorus be introduced into a crucible with charcoal, and exposed to an intense heat, a phosphuret of gold, according to Pelletier, will be formed.

Rationale. The charcoal unites with the oxygen of the phosphoric acid, forming carbonic acid, which is disengaged, and the phosphorus unites with the gold into a phosphuret of gold.

Remark. The phosphuret of gold is composed of twenty-three parts of gold and one of phosphorus.

Experiment 45. If an alloy, say of gold and copper, be introduced into a crucible with two parts of sulphuret of antimony, and brought to the state of fusion, the gold will be separated from the copper, or other metal, and found united to the antimony at the bottom of the crucible.

Rationale. The sulphur of the sulphuret is partly volatilized, and partly combines with the copper, if it be used, whilst the antimony unites with the gold.

Experiment 46. If the alloy of gold and antimony, the product of the last experiment, be exposed to a white heat, the gold will remain in the crucible in a

separate state.

Rationale. When a white heat is applied, it volatilizes the antimony, and the gold is left in the crucible.

Experiment 47. .If gold be combined with mercury, in the proportion of one forty-eighth part, and the compound exposed in a mattrass, furnished with a capillary tube, to the action of heat in a sand bath, the metals will become oxydized.

Remark.

Gold, therefore, when united to mercury is easily oxydized.

Cupellation. It is necessary, as gold is frequently alloyed with copper, lead, &c. that some process should be employed to effect the separation of the gold. Gold is usually purified by a process called cupellation. A vessel of a porous nature, usually made

D