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Rhodium, like palladium, iridium, and osmium, is found associated with platinum. It is obtained in the state of a black powder, but being fused, for which an intense temperature is required, it has the colour and lustre of platinum. It is brittle, hard, and capable, when fused, of resisting the action of all acids, but when alloyed with certain metals, it dissolves along with these metals in hydrochloric acid. Its solutions have a fine rose-colour, whence its name godov, a rose. Its specific gravity is about 11, and its combining weight is estimated at 52.2. Its combinations, so far as they have been examined, are similar to those of the other bodies of the same class.
Palladium is obtained from an acid solution of the crude ore of platinum. It is precipitated from the solution in the form of a powder, which is rendered malleable by a process of purification and alternate heating. The metal resembles platinum in its external characters, being, like it, of a white colour, and in lustre inferior to silver. It is malleable like platinum, but more hard; it is more fusible than platinum, and, when intensely heated by the oxyhydrogen blowpipe, is dissipated in bright sparks. It is not acted upon by exposure to the air, but when strongly heated in oxygen gas, its surface acquires a fine blue colour manifesting the commencement of oxidation. It is acted upon by nitric, and even by sulphuric acid, in which respect it differs from platinum, which is not acted upon by these acids ; but its proper solvent, as in the case of gold and platinum, is nitrohydrochloric acid. Its density is about 11.8, being little more than half that of platinum, and its combining weight is computed to be 53.36. Its combinations, in so far as they have been examined, are similar to those of the bodies of the same class.
These five bodies, then, platinum, osmium, iridium, rhodium, and palladium, are all connected together, and possess the same general physical and chemical characters. The existence of this single group of bodies should of itself demonstrate, that the theory of the existence of 55 distinct elements cannot be maintained. Even if it could be conceived that platinum was a distinct chemical element, how could it be conceived that iridium likewise was so, found only in connexion with the other, possessing all its essential properties, and not differing more from it than some other bodies do from their normal characters after being subjected to a red heat and before ? We treat with ridicule the opinion of the alchymists, that the elements were three,salt, sulphur, and mercury ; but two of the three we still retain as elements, and, although we now know that the other is not an element, the discovery does not date far back in the history of chemical science. In place of this elementary salt, we have now more than 50 elements, so that if we have corrected the opinion of our precursors in this one point, we have erred at least in an equal degree in another direction. In seeking to reduce all bodies to simple elements, the early cultivators of chemistry followed out, though with imperfect lights, a natural train of thought. They had even improved upon the earlier conception, that the elements were four,-earth, air, fire, and water. But even this latter idea was a natural one. For earth, air, and water are still in one sense the root of many bodies, though the earlier philosophers did not know that there were many earths and airs, and that water was derived from elements more simple than itself; and with respect to their element of fire, we can hardly say that the idea was extravagant, when we know, that almost until the age in which we live, the same doctrine, modified into the theory of phlogiston, retained its place among the admitted truths of science.
Platinum and the allied bodies are found in the oldest rock formations, or in their detritus washed into crevices, hollows, and plains. In this respect platinum is found under the same conditions as gold, tin, and other perfect metals, which are generally found in veins, sometimes in threads as fine as a hair, passing through the mass of rock with which they are associated. Many theories have been proposed to account for the production of metals under these circumstances. One of the latest ascribes their formation to electrical agency. We are the less entitled to reject this hypothesis, now that we know the close relation, if not the absolute identity, of chemical and electrical forces, so that when we ascribe an effect to electricity, we may be said to ascribe it to a chemical action. But the hypothesis infers that the matter on which the agent acted was already formed. Now, what was the matter? Was it a set of molecules, proper to each metal to be formed, and the sole element of each metal, which in this case only could be called a simple body. Or was each metal formed by new combinations of pre-existing elements ? We have a knowledge of the one kind of change in every chemical combination ; but of the other kind of action we have no knowledge whatever. The solid crust of the earth consists essentially of silica, alumina, lime, magnesia, the oxides of iron, soda, and potassa. Each and all of these may be supposed to possess the materials for forming new products, and amongst these products, the class of crystalline bodies which we term metals; and we should no more hesitate to believe that gold, platinum, and silver have been produced by chemical actions, in this vast magazine of matter, than that any of the rarer minerals have been so produced.
This beautiful substance has, like gold, been known from the earliest records of mankind. It exists in various ores and mineral species. It is found native, or in combination with gold, mercury, copper, lead, sulphur, arsenic, antimony, and most abundantly as a sulphuret, either alone or mixed with other sulphurets.
Silver is of a yellowish-white colour, and, when burnished, the most brilliant of all the metals, except polished steel. It is soft when pure, and may be cut with a knife. It is malleable and ductile in a high degree. In the former property, that of malleability, it is superior to all the metals except gold, and may be beaten into leaves of
Tooroo part of an inch in thickness. It is so ductile, that it may be drawn into threads finer than the human hair. In tenacity, it is superior to gold, a wire of .078 of an inch in diameter being capable of sustaining a weight of 178.13 lb. avoirdupois. It fuses at a temperature of about 1873° F., and, on cooling slowly, may be crystallized, its crystalline form being the cube or octahedron. When exposed in the state of leaf, or fine wire, to a powerful voltaic action, or when acted upon by the oxyhydrogen blow-pipe, it burns brilliantly with a light-green flame, emitting bright sparks. It is not oxidated by simple exposure to the air, nor by being kept under water; but if it be exposed long in a state of fusion, it partially combines with the oxygen of the air; or if it be heated to redness in contact with glass or porcelain, it attracts oxygen, and forms an oxide which combines with the earthy matter of the glass or porcelain, forming an enamel. Silver, besides its chemical union with oxygen, absorbs, when exposed to heat, in the state of fusion, a