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 heated in a current of chlorine, any iridium present being converted into its chloride by treatment with nitric acid and precipitated by ammonium chloride, whilst rhodium ammonium chloride goes into solution with its characteristic rose-red colour (C. E. Claus, Jour. prakt. Chem., 1843–1845). For other methods of extraction see Gibbs, ib., 1861, 84, p. 65; 1865, 94, p. 10; T. Wilm, ''Bull. soc. chim.'', 1880 (2), 34, p. 679; E. Frémy, Comptes rendus, 1854, 38, p. 1008, &c.). The metal itself is best obtained by the reduction of chlorpurpureo rhodium chloride, (Cl2Rh2·10NH3)·Cl4, in a current of hydrogen, the metal after reduction being cooled in a stream of carbon dioxide (S. M. Jorgensen, ''Zeit. anorg. Chem.'', 1903, 34, p. 82). It somewhat resembles aluminium in colour; its specific gravity varies from 11 to 12·1; and its specific heat is 0·05527 (V. H. Regnault, ''Ann. chim. phys.'', 1861, 63, p. 15). It is less fusible than platinum. It oxidizes superficially when heated, and may be distilled in the electric furnace. It is insoluble in acids, but forms a soluble sulphate when fused with potassium bisulphate (a reaction which distinguishes it from the other metals of the platinum group). It oxidizes when fused with potassium hydroxide and potassium nitrate, to the dioxide, RhO2. It absorbs hydrogen readily. Rhodium black is obtained by reducing rhodium salts with formic acid; by alcohol in the presence of alkali; or by precipitation with zinc and iron. A colloidal rhodium may be prepared by reducing the sesquichloride with hydrazine hydrate. Rhodium salts may be recognized by their characteristic reaction with freshly prepared sodium hypochlorite solution. A yellow precipitate is obtained, which on shaking for some time with acetic acid gradually dissolves to an orange-coloured solution. This solution after a short time deposits a grey precipitate, and the supernatant liquid becomes azure blue in colour (E. Demarçay, Comptes rendus, 1885, 101, p. 951).

 RHODOCHROSITE, a mineral species consisting of manganese carbonate, MnCO3, crystallizing in the rhombohedral system and isomorphous with calcite. It usually occurs as cleavable, compact or botryoidal masses, distinct crystals being somewhat rare; these often have the form of the primitive rhombohedron, parallel to the faces of which there are perfect cleavages. When pure, the mineral contains 47·7% of manganese, but this is usually partly replaced by varying amounts of iron, and sometimes by calcium, magnesium, zinc, or rarely cobalt (cobalt-manganese-spar). With these variations in chemical composition the specific gravity varies from 3·45 to 3·60; the hardness is 4. The colour is usually rose-red, but may sometimes be grey to brown. The name rhodochrosite, from the Greek  (rose-coloured), has reference to the characteristic colour of the mineral: manganese-spar and dialogite are synonyms. It is found in mineral veins with ores of silver, lead, copper, &c., or in deposits of manganese ore. Crystals have been met with in the mines at Kapnik-Bánya and Nagyág near Déva in Transylvania and at Diez in Nassau, but by far the best specimens are from Colorado. The mineral is used to a limited extent in the manufacture of spiegeleisen and ferromanganese.

 RHODODENDRON. Classical writers, such as Dioscorides and Pliny, seem, from what can be ascertained, to have called the oleander (Cerium Oleander) by this name, but in modern usage it is applied to a large genus of shrubs and trees belonging to the order of heaths (Ericaceae). No adequate distinction can be drawn between this genus and  (q.v.) the proposed marks of distinction, however applicable in particular cases, breaking down when tested more generally. The rhododendrons are trees or shrubs, never herbs, with simple, evergreen or deciduous leaves, and flowers in terminal clusters surrounded in the bud by bud-scales but not as a rule by true leaves. The flowers are remarkable for the frequent absence or reduced condition of the calyx. The funnel- or bell-shaped corolla, on the other hand, with its five or more lobes, is usually conspicuous, and in some species so much so as to render these plants greatly prized in gardens. The free stamens are usually ten, with slender filaments and anthers opening by pores at the top. The ovary is five- or many-celled, ripening into a long woody pod which splits from top to bottom by a number of valves, which break away from the central placenta and liberate a large number of small bran-like seeds provided with a membranous wing-like appendage at each end. The species are for the most part natives of the mountainous regions of the northern hemisphere, extending as far south as the Malay Archipelago and New Guinea, but not hitherto found in South America or Australia. None are natives of Britain. They vary greatly in stature, some of the alpine species being mere pygmies with minute leaves and tiny blossoms, while some of the Himalayan species are moderate-sized trees with superb flowers. Some are