There are two divisions of the hexagonal system; the hexagonal system properly so-called, and its rhombohedral division. All forms are referred to four axes, three equal axes inclined to each other at angles 60 degrees in a common horizontal plane, and a fourth vertical axis at right angles, and longer or shorter. The rhombohedral division comprises crystals having but three planes of symmetry, intersecting at angles of 120 degrees in the vertical axis. They are regarded as half forms of the corresponding hexagonal crystals. In the orthorhombic system there are three unequal axes at right angles to each other. In the monoclinic system there are three unequal axes, of which one, the lateral axis, is inclined to the vertical, while the angles between the others are right angles. In the triclinic system there are three unequal axes and these intersections are all oblique. The student who wishes to pursue this subject further should consult Dana's System of Mineralogy. Physical Mineralogy. Cleavage is the line of easiest separation in a mineral. It may be perfect, imperfect, interrupted, etc. Fracture, referring to any surface except that of a cleavage fall, may be uneven, conchoidal (shell-like), hackly (rough), etc. Tenacity refers to such qualities as brittle, sectile, malleable, flexible, or elastic. Hardness is represented by the difficulty with which a smooth surface is scratched. The scale in general ore was devised by Mohs. It is: 1. Talc. Scratched by the finger nail. 2. Gypsum. Ditto, but with more difficulty. Will not scratch a copper coin. 3. Calcite. Scratched by a copper coin. 4. Fluorite. Is not scratched by a copper coin and does not scratch glass. 5. Apatite. Scratches glass, but with difficulty. Is readily scratched by a knife. 6. Feldspar. Scratches glass with ease. Is difficult to scratch by knife. 7. Quartz. Cannot be scratched by a knife and readily scratches glass. 8. Topaz. Harder. 9. Corundum. Harder. 10. Diamond. Scratches any other substance. Hardness may be intermediate. For instance, any mineral that scratched quartz and is soft enough to be scratched by topaz, in turn would be rated at 7.5. Specific Gravity. This is the density of mineral and other substances compared with that of water. It is particularly valuable in determining heavy metals. To find the specific gravity of any solid body divide its weight in air by the loss of weight in water, at a temperature as near 60 degrees F. as possible, and the quotient will equal the specific gravity. In the case of gases, such as nitrogen, oxygen, etc., hydrogen is taken as the unit. Luster. There are seven kinds of luster, viz: Metallic, the luster of metals; adamantine, that of the diamond; vitreous, of broken glass; resinous, of the yellow resins; greasy; pearly; silky. There are five degrees of intensity of luster recognized, viz: Splendent; shining; glistening; glimmering; dull.