XVIII EXTERNAL FORM. These characters are comprehended in the following list. § 6. Only a small proportion of the specimens admitted into our collections can be said to possess precise external forms, since they mostly exhibit on one side or the other, and are some times entirely bounded by, surfaces produced by fracture; tbcre are comparatively few minerals which are found in masses abso lutely isolated. § 7. Nevertheless there are many minerals to which particular external forms belong; some few are found in single or separate crystals, and the surfaces of others are coated by them. A crystal may be defined as a more or less symmetrical, geo metrical solid, commonly bounded by plane surfaces, which in mineralogical language are termed planes or faces. An edge is formed by the meeting of two planes. A solid angle is a point formed by the meeting of three or more planes. A prism is rarely found having only three sides, very com monly four, six, eight, or more sides ; the sides, or lateral planes, surround its axis, which is an imaginary line passing down the middle of the prism, from the centre of the upper terminal plane to the centre of the lower; the terminal planes are also called the bases. But prisms are found both very long and very short; when long, and the crystals slender and curved, they are termed capillary; when straight, acicular ; when the prism is short, the crystal is said to be tabular. A pyramid is formed by the meeting of three or more planes at a point, which is termed the apex, each plane being bounded by edges i considered separately, a pyramid is supposed to have a base, which is the case in regard to the tetrahedron ; but in respect of most other forms, it is only imaginary, as in the in stance of the octahedron, which often is termed a double four sided pyramid ; and also the dodecahedron with triangular faces, which is frequently denominated a double six-sided pyramid. External form. Structure. Fracture. Frangibilily. Hardness. Transparency. Lustre. Colour. Flexibility. Double refraction. Touch, Taste, and Odour. Streak. Powder. Adhesion to the tongue. Magnetism. Electricity. Phosphorescence. Specific gravity. External Form.