NO. NAME. LOCALITY. SP. GR. 1. Sulphur, Girgenti, 2.070 2. Hyalite, Waltsch, 2.160 3. Opal, Scheiba, 2.212 4. Natrolite, Brevig, 2.246 5. Pitchstone, Meissen, 2.284 6. Obsidian, Lipari, 2.362 7. Pearlite, Hungary, 2.397 8. Leucite, Vesuvius, 2.465 9. Adularia, St. Gotthard, 2.570 10. Elæolite, Brevig, 2.617 11. Quartz, Middleville, 2.650 12. Labradorite, Labrador, 2.689 13. Calcite, Rabenstein, 2.715 14. Dolomite, Muhrwinkel, 2.733 15. Dolomite, Rauris, 2.868 16. Prehnite, Kilpatrick, 2.916 17. Aragonite, Bilin, 2.933 18. Actinolite, Zillerthal, 3.020 19. Andalusite, Bodenmais, 3.125 20. Apatite, Ehrenfriedersdorf, 3.180 Among the heavy solutions employed may be mentioned: Thoulet’s solution of potassium-mercuric iodide (KI: HgI_{2} = 1: 1.24), maximum specific gravity 3.196. Klein’s solution of cadmium borotungstate (2H_{2}O, 2CdO, B_{2}O_{3}, 9WoO_{3} + 16H_{2}O), maximum specific gravity 3.6. These two solutions can be mixed with water in any proportion without being decomposed. Solution of barium mercuric iodide, maximum specific gravity, 3.588, cannot be diluted with water. Methylene iodide (CH_{2}I_{2}), specific gravity 3.3243 at 16° C. varying with the temperature, can be diluted with benzole but not with water. Nitrates of silver and thallium[145] (AgNO_{3}: TlNO_{3} = 1.1) fuse at about 75° C. to a clear mobile liquid with specific gravity over 4.5. Can be mixed while melted with water in all proportions; but cannot be used for separation of sulphides, as these nitrates are attacked by them. Possible chemical action between the minerals and heavy solutions must not be overlooked in this method of separation. The funnel-shaped apparatus for these separations must be so arranged, with stop-cocks, etc., that the heavier material collected at the bottom can be easily drawn off or removed at any stage of dilution. These separations, for various reasons, are not always complete, but the best results are obtained when the processes are repeated several times. _Electro-magnetic Separation._ All iron-bearing minerals may be separated from those free from iron by an electro-magnet. The factors influencing the attraction of a mineral by an electro-magnet are not definitely known, and do not seem to depend only on the percentage of iron.