Figs. 443 and 444.—Secondary forms of first system.
Fig. 445.—Pentagonal dodecahedron.
The second crystalline form is the Hexagonal, and in this system three of the four axes are equal and in the same plane, inclined at an angle of 60°, with a principal axis at right angles to the others. In crystals of this system are found quartz and calc-spar.
The third system is termed the Quadratic or the diametric. This form has three axes, all at right angles, two being equal and the other longer or shorter than the former two. In this system crystallize sulphate of nickel, zircon, oxide of tin, etc.
Fig. 446.—Galena, or sulphide of lead.
Fig. 447.—Oxide of tin.
The fourth, or Rhombic system, or the trimetric. Here we have three rectangular axes, all unequal and intersecting at right angles. The sulphate and nitrate of potassium crystallize in this system.