LI. OSMIUM.

When crude platinum has been dissolved in chlorohydric acid, a part remains undissolved in the state of a black powder. This powder is an alloy, from which the osmium is separated by means of fusion with soda or nitrate of potassa. It is obtained as an oxide, which is easily reduced to the metallic state. Thus obtained, it is a black powder, which by pressure is rendered compact, when it acquires a white colour and metallic lustre, but less brilliant than that of platinum. It is brittle, and easily pulverized. When in the state of powder, as it is obtained from the oxide, it takes fire when heated in the open air, burning without residue, and diffusing the characteristic odour of osmic acid; and it is readily oxidated by nitric acid. But when rendered dense by the application of a red heat, it is not attacked by acids, and may be heated without undergoing oxidation. When placed in the flame of a spiritlamp, it communicates to the flame the appearance of that produced by olefiant gas, H2 C2, isomeric with nitrogen. Its specific gravity when most dense is 10. Its combining weight has been estimated at nearly 96.

Osmium combines with oxygen in the following proportions, on the assumption that its combining weight is nearly 96.

The equioxide is a dark greenish-black powder, produc

ing deep-green solutions, is slowly soluble in acids, is reduced by hydrogen at ordinary temperatures, and detonates when heated with combustible substances. The sesquioxide is a dark powder, which likewise detonates with combustible matters. The binoxide is of a brown colour nearly black, and, like the equioxide, is reduced by hydrogen, and detonates when heated with combustible bodies. The teroxide, or osmous acid, exists only in combination, and performs the functions of an acid. The quadroxide, termed osmic acid, is altogether peculiar, and distinct from any known oxide of platinum or the allied metals. It is formed by the combustion of osmium, and may be prepared by several processes. It is obtained in the form of brilliant white crystals, is volatile, and has a pungent offensive odour, resembling that of certain chlorides; and its vapour, like that of chlorine itself, is acrid and irritating, producing cough and painful feelings. Besides these oxides, Berzelius conceives that there is formed a blue oxide, apparently intermediate between the equioxide and sesquioxide.

Osmium likewise combines with sulphur in various proportions. When it is heated in the vapour of sulphur, combination takes place with evolution of light; and sulphuretted hydrogen precipitates all the known chlorides.

of osmium.

Osmium combines with chlorine, and probably in as many proportions as with oxygen. The equichloride is of a greenish colour, volatile, and capable of crystallizing. It dissolves in a certain quantity of water, but is decomposed when the quantity is large. It combines with chlorides of the alkaline metals. The sesquichloride has as yet only been obtained in combination. The bichloride, produced by burning osmium in a current of chlorine, is obtained as a deep-red powder, which dissolves in water, producing a

yellow solution, and is decomposed by an excess of water. It combines with chloro-bases in the manner of the corresponding chloride of platinum.

This substance is always found native combined or associated with iridium, and, according to M. Clauss, with another metal which he has termed Ruthenium. Though osmium differs, with respect to some of its compounds, from platinum, there is a sufficient analogy between the two substances to shew that they must be placed in the same class of chemical products.

LII. IRIDIUM.

Iridium, so named from the variety of colour which its solutions present, is obtained from its alloy with osmium by a series of processes. It is an exceedingly brittle metal. It is said, when fused, to have the aspect of platinum, but it is of all known metals the most infusible, having only yet yielded to the most powerful galvanic action. If, when in a fine state of division, it be exposed to a red heat in the open air, it becomes oxidated. It resists the action of all the acids, and even aqua regia acts upon it feebly, and only when in large quantity.

Iridium combines with oxygen in various proportions. The equioxide is a heavy black powder, soluble in alkaline solutions which it colours purple. The sesquioxide is a bluish-black powder, insoluble in acid solutions, decomposable with detonation when heated with a combustible body, and reducible by hydrogen without the aid of heat; which last result would seem to be due to this, as is justly observed by Berzelius, that like platinum powder it condenses the gas in its interstices, and so produces combination with the oxygen of the oxide at a low temperature. The binoxide is known only in combination with alkaline bases. The teroxide is obtained as a hydrate, so similar to the hydrate of oxide of rhodium that the two substances cannot be distinguished by the eye. This acid detonates when heated.

Iridium combines with sulphur in various proportions. It does so directly when heated in the vapour of sulphur; and each of the four known chlorides is precipitated by sulphuretted hydrogen.

Iridium combines with chlorine, forming compounds corresponding with the oxides. The equichloride is a light powder of an olive-green colour, formed when iridium is brought in contact with chlorine at an incipient red heat. It combines with chlorides of the alkaline bases in the same manner as chloride of platinum. The sesquichloride is volatile, and of a light-green colour, and its aqueous solution is of a deep-red. It combines with metallic chloro-bases. The bichloride is found chiefly in combination with the chlorides of the alkaline metals; and the terchloride is only known in such a combination, forming Ir Cl3 + K Cl.

The combining weight of iridium has been supposed to be the same as that of osmium; and the two substances closely resemble one another in their characters and reactions, and are always found associated or combined in the native state.