6. Scratches glass easily. Yields with difficulty to the knife. 7. Does not yield to the knife. Yields to the edge of a file, though with difficulty. 8, 9, 10. Harder than flint. Specific gravity cannot well be determined without the aid of a balance, and hence its value here is not great. As in the preceding chapter, alphabetic arrangement will be employed. ORES OF ANTIMONY ~Stibnite~, Sb_{2}S_{3}, Sb . 71, S . 29.--[A]H = 2, G = 4.52-4.62. Of lead-gray color and metallic lustre. Consists of a large number of needle-shaped crystals. Brittle. Fuses in candle flame. In an ignition tube yields a sublimate of sulphur. On Ch. before the Bp. it is volatilized, giving antimony coating and tinges the flame pale blue. [Footnote A: H = Hardness, G = Specific Gravity.] ORES OF ARSENIC ~Native Arsenic, As.~--This contains traces of Sb, Ag, Fe, Co, and Ni. H = 3.5, G = 5.7-5.8. Dark gray in color. Fracture tin-white, tarnishing rapidly. Volatilizes before the Bp. on Ch. without melting, giving white coating of arsenious acid and characteristic garlic odor. In ignition tube it sublimes, giving arsenical ring. ~Realgar~, AsS, As . 70, S . 30.--H = 1.5-2, G = 3.56. Bright red to orange-red color and resinous lustre. In an ignition tube it fuses and finally sublimes. The sublimate when cool is red and transparent. Fuses readily before the Bp. on Ch. and burns with pale yellowish flame, emitting gray-white fumes having garlic odor. ~Orpiment~, As_{2}S_{3}, As . 61, S . 39.-- H = 1.5-2.0, G = 3.4-3.5. Lemon-yellow in color and resinous or pearly lustre. Sectile. Before the Bp. on Ch. behaves like realgar, but in an ignition tube it gives a dark yellow sublimate which is transparent. ORES OF BISMUTH ~Native Bismuth, Bi.~--This contains traces of As, Te, and S. H = 2.0-2.5, G = 9.7-9.83. Color, silver-white, slightly tinged with red. Metallic lustre. Brittle when cold, but may be laminated when hot. Before the Bp. on Ch. behaves like pure Bi. ~Bismuthite~, Bi_{2}O_{3} . 90, CO_{2} . 7, H_{2}O . 3,-- H = 4.0-4.5, G = 6.9-7.8. Usually of a white or light greenish color and vitreous lustre, in acicular crystallizations. In an ignition tube decrepitates, yielding water and turning gray. Before the Bp. on Ch. it fuses easily and is reduced to metallic globule, coating the Ch. with Bi_{2}O_{3}. With Na_{2}CO_{3} it occasionally gives the sulphur reaction. ORES OF CHROMIUM ~Chromic Iron Ore~, FeO . 32, Cr_{2}O_{3} . 68.--Al_{2}O_{3}, Fe_{2}O_{3}, MnO, and MgO are commonly present. H = 5.5, G = 4.32-4.57. Occurs usually massive. Color, iron-black to brownish black. In many varieties strongly magnetic. Lustre, shining and somewhat metallic. Heated in an ignition tube, remains unchanged. Infusible before the Bp. on Ch. Before the Bp. on Ch. with Na_{2}CO_{3} and KCN yields metallic iron. In borax bead it slowly dissolves to a clear transparent glass, which is a beautiful green when cool. ORES OF COBALT