Experiment 31. If the amalgam thus found be heated, the mercury will be volatilized.

Experiment 32. If a solid amalgam of silver be melted, and suffered to cool undisturbed, it crystallizes in octahedral crystals, or else in four sided columns, with tetrahedral terminations.

Experiment 33. If copper be presented to mercury, a union will ensue, as in a former experiment. Experiment 34. If to a solution of gold in nitromuriatic acid, mercury be added, the gold will be precipitated in the form of a brown powder.

Rationale. The mercury takes the oxygen from the oxyd of gold held in solution, and, while the gold is precipitated, it unites with the muriatic acid, forming sub-muriate of mercury, which, being insoluble, also precipitates.

Remark. It is on account of the considerable affinity of mercury for oxygen, that the Arbor Diana is produced by mercury and silver. See Silver. Since mercury precipitates silver, but not copper from its solution in nitric acid, it furnishes an easy method to part the silver from its alloyed copper. Mercury, if assisted by ammonia, is likewise a means of reviving the silver from its muriate. For this purpose, the muriate is triturated with carbonate of ammonia; mercury is then added, and the trituration continued. An amalgam of silver is produced, from which the mercury is separated by heat. As siver has a greater affinity for sulphur than mercury, if the sulphuret of mercury be distilled with the filings of silver, the sulphur will unite with the silver, and the mercury be reduced.

For the combinations of mercury with the different acids, see Salts of Mercury.

SECTION V.

OF PALLADIUM.

Experiment 1. If crude platinum be dissolved in nitro-muriatic acid, and the solution, previously freed from any excess of acid, mixed with prussiate of mercury, the solution will become turbid, and a pale yellowish white matter will fall. If this be collected, washed, dried, and exposed to a strong heat, a white matter will be left, which is palladium.

Experiment 2. To the product of the last Experiment add sulphur and borax, and fuse the mixture in a crucible; a metallic button of palladium will be obtained.*

Rationale. The solution of platinum is decomposed by prussiate of mercury, which according to Dr. Wollaston, precipitates palladium, exclusively of all other metals. This property, which the mercurial prussiate possesses, of precipitating palladium only, is accounted for on the great affinity of mercury for the prussic acid, which prevents the precipitation of iron or copper. In the opinion of Dr. Wollaston, the muriate of palladium in the solution is not decomposed solely by the superior affinity of mercury for muriatic acid, but is assisted also by the greater affinity of prussic acid for palladium.

Remark. This metal, which has received the name of palladium, or new silver, was discovered by Dr. Wollaston, who found it in the ore of platinum. It was called palladium from the planet that had been discovered, about the same period, by Dr. Olbers. This metal has given birth to a very extraordinary controversy, and several sets of experiments have

*See Wollaston on Palladium. Phil. Trans. 1805.

been made on it. Mr. Cloud, of the mint of the Uni ted States, has improved the process for obtaining palladium from crude platinum.

Palladium is a white metal, rather harder than wrought iron, as malleable as platinum, and about 11.8 specific gravity. It requires a violent heat for fusion, and is not altered by exposure to the air. When heated it acquires a blue colour, which is the commencement of oxydizement.

Sulphuric acid boiled upon palladium, dissolves a portion, and acquires a red colour. Nitric acid acts more powerfully. Muriatic acid, assisted by heat, also dissolves it; but the nitro-muriatic acid dissolves with facility. The solution is decomposed by potash or lime water, which precipitates the metal of a fine orange colour, not however pure, but in combination with a portion of the acid and precipitant. With sulphur, it readily unites into a sulphuret. With phosphorus it also combines, forming a phosphuret. Liquid ammonia dissolves a portion of it, and assumes a bluish hue.

Muriate of tin precipitates palladium of a brown colour, and is considered a very delicate test for this metal; but as prussiate of mercury separates palladium from all its solutions, it is therefore the best test for the discovery of it.

Green sulphate of iron, it is said, will precipitate palladium in a metallic state. The prussiate of palladium, separated by a neutral solution of prussiate of mercury, has the property, when heated to about 500° of Fahrenheit, of detonating, with a noise similar to that occasioned by firing an equal quantity of gun powder. Hydrosulphurets, and water impregnated with sulphuretted hydrogen gas occasion a dark brown sediment from solutions of palladium. See Hydrosulphurets.

Palladium unites with various metals, an account of which may be seen in the papers of Chenevix, and Wollaston in the Philosophical Transaction of 1803, 4, and 5. For the combination of this metal with acids, see Salts of Palladium..

SECTION VI.

OF RHODIUM.

Experiment 1. Into a solution of the ore of platinum, which has been previously precipitated by the admixture of muriate of ammonia, and from which the ammoniacal muriate of platinum has been separated, suspend a cylinder, or thin slips of zinc, or iron. All the metals which are left in the solution are thus thrown down in the form of a black powder. This being done, transfer the precipitate, without drying it, into a flask containing very dilute nitric acid, and assist the action by a gentle heat; by this means the copper and lead become dissolved. Then transfer the residue into dilute nitro-muriatic acid, and when the solution is effected, add to it a portion of muriate of soda, equal in weight to about th of the ore of platinum employed for obtaining the first solution: evaporate the whole by a gentle heat. The dry mass obtained contains the muriate of soda and platinum, palladium, and rhodium. On washing this precipitate repeatedly in small quantities of alcohol, the muriate of platinum and soda, as well as the palladium, become separated, and the salt of rhodium remains behind in a state of purity; from this the rhodium may be obtained, by merely dissolving it in water, and precipitating the solution by suspending in it a cylinder of zinc. The black powder which is obtained is rhodium. The rhodium thus prepared acquires a white metallic lustre with borax, but it appears infusible by any degree of heat. It may be rendered fusible by arsenic, and also by sulphur, both of which may again be separated by a continued heat. The metallic button thus obtained is not malleable.*

* See Phil. Trans, 1804. p. 428,

Remark. Rhodium is of a white colour. Its specific gravity is more than 11. It is extremely infusible. It is said that no degree of heat hitherto applied is capable of melting it. It is not oxydized by exposure to heat and air, neither is it much acted upon by acids.

Rhodium readily combines with sulphur, and, like palladium, is rendered fusible by it. With arsenic also it unites, but the arsenic or sulphur may be expelled by means of heat. It forms alloys with the greater number of the metals. It does not, like platinum and palladium, destroy the colour of gold when alloyed. with it.*

It may be made to unite with oxygen; the only oxyd known of a yellow colour. This oxyd may be obtained in the following manner:

Experiment 2. If the salt of rhodium mentioned in Experiment 1, be dissolved, and a solution of potash added, the rhodium will be precipitated in the form of a yellow powder, being the oxyd of this metal.

Rationale. The potash unites with the acid, and the metal is separated in the form of a yellow oxyd.

Experiment 3. If to a solution of rhodium, prussiate of potash be added, no decomposition will ensue. Experiment 4. If to a portion of the same solution, muriate of ammonia be poured in, no precipitation will

ensue.

Experiment 5. If hydrosulphuret of ammonia be used in the same manner, no effect will take place. Experiment 6. If the carborated alkalies be added, no change will ensue; but,

Experiment 7. If the pure alkalies be employed, a precipitate of the yellow oxyd will be formed.

Remark. Hence it is, that this metal differs from others in its chemical properties.

Experiment 8. If an alloy of silver or gold with: rhodium, be digested in nitric or nitro-muriatic acid, the rhodium will remain untouched; but,

Experiment 9. If it be alloyed with three times its weight of bismuth, copper or lead, each of these alloys

See Dr. Wollaston's paper in the Phil. Trans..