Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/809

Rh merce usually contains both gold and silver, often in con siderable quantity, which circumstance has probably given rise to the story current about its transmutation into these metals.

{{11fine|Bismuth ma} be readily obtained in crystals by pouring it when melted into a heated iron ladle, and cooling it until a crust is formed on the surface, which must then be pierced by a red-hot iron rod, and the liquid metal poured off. The solidified portion adhering to the ladle is found to be covered with hopper-shaped crystals, which are usually beautifully irised, owing to the formation of a thin film of oxide on the surface, showing the colours of thin plates. This colouring is only obtained when the metal is quite free from arsenic. It may be purified by melting with about 10 per cent, of nitre, and keeping it constantly stirred at a temperature not much above its melting point, whereby the more oxidizable metals are removed, and form a slag at the surface. Another method of purifying it from arsenic is by fusing it with from 3 to 5 per cent, of zinc, covering the surface with charcoal to prevent oxidation of the zinc, which takes off the whole of the arsenic, and is subsequently removed by treatment with hydrochloric acid, the purified bismuth remaining insoluble. When prepared by any of these processes, Bismuth is a hard, brittle metal, and the fracture is highly crystal line and white, with a perceptible red tinge l&amp;gt;y reflected light. The crystalline form is rhombohedral, the angle of the primary rhom- bohedron being 87 40, or very close to a cube. The specific gravity is 9 S3, but when subjected to great pressure the density is reduced to 9-6. The melting point is 264 C. (507 Fahr.) (Paulberg), or 268 - 3 (515) (Riemsdijk). Like water it may be cooled 6 or 7 C. below its freezing point ; but when solidification sets in the tempera ture rises to 480 Fahr., and continues until the mass is completely solidified. Like ice it expands about -^ of its volume in solidifi cation, a property which is communicated to its alloys, rendering them valuable for taking casts of incised or relief surfaces for reproduction, as printing-blocks by electrotype or other processes. It may be distilled by heating to a higher temperature in hydrogen. Despretz volatilized it by subjecting it to the current from 600 Bunsen elements. The spectrum of the vapour in the voltaic arc shows numerous brilliant green lines, one strong and one fainter line on the red, and a faint line on the orange field (JIasson). The coefficient of expansion by heat is 001341, calorific conducti- bility 61, silver being 1000 (Calvert and Johnson), and specific heat 0-0305 (Kopp). The electric conductivity is T19 at 14 C., silver being 100 at (Matthiesen). According to Matteuci the conductivity varies in the crystals according to the direction of the cleavages. It is the most strongly diamagnetic of all metals.}|undefined}

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