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256 OXIDES, EAETHS, AND ACIDS. As, arsenic 75 74, oxygen 24'28. _ In crystals, crystalline crusts, and columnar masses, stalac- titic, earthy, investing other minerals. It is probably produced by the decomposition of ores con taining arsenic, in veins with metallic arsenic, realgar, proustite galena, Ac. It is found at Joachimsthal in Bohemia, Kapnik m Transylvania, Bieber in Hanau, Markirch in Alsace An- dreasberg m the Harz, Gistain in the Pyrenees. Sublimed at a high temperature in close vessels arsenious acid is amorphous, glassy and perfectly transparent, o = 37386. Exposed tg the air, it gradually becomes crystalline and opaque A solution of glassy arsenious acid in warm hydrochloric acid on cooling, deposits crystals which, as they form, were observed by H. Hose to become luminous. In the process of roasting cobalt ores containing arsenic, it is sometimes obtained in thin pearly, flexible, prismatic crystals isomorphous with valentinite’ These when sublimed yield octahedrons. A boiling solution of potash saturated with arsenious acid and left to cool usually deposits prismatic crystals of arsenious acid. ’ 132. ICE.—Eis; Hausmann. Bhombohedral. 0 ill, a oil. FIG. 281. ao ad 60 0 Combination, oa. Twins. Twin-face a. Groups of twin-crystals occur frequently in snow, producing star-shaped figures. Cleav age. 0. Eracture conchoidal. Pellucid in a high degree. Lustre vitreous. The indices of refraction for rays of different colours are as follows:— Bed Orange Yellow 1'3070 1-3086 1-3095 Green Blue Violet , 1-3115 1-3160 1-3170 Double refraction very feeble. The surface of ice formed on still water is the face 0, the centre of the coloured rings sur rounding the optic axis being seen, when placed in a polarizing apparatus, in a direction perpendicular to the surface. Colour less, in large masses greenish or blueish. Sectile, rather brittle. 11 = i'6. o = 0'918 at o° c.