CRYSTAL FACES AND ANGLES. On every single, or individual, crystal the dihedral or interfacial angles formed by the faces arc never reentrant. Moreover, the faces of a crystal are usually of several kinds or classes, and those which are alike, or of the same kind, are said to comprise a crystal 'form.' Faces belonging in the same form, unless perfectly smooth, have similar natural markings. The angles between faces of the same kind are identical in value. The in strument used in measuring crystal angles is called a goniometer (q.v.). Not only are the angles between the similar faces of a crystal con stant in value, but in crystals of the same sub stance, no matter where found or how produced, they are of constant value, provided only the substance is pure and the measurements are made at the same temperature. Crystal angles are, therefore, individual and characteristic for each substance, and substances may be identified by measurement of their angles. Although the
angles are characteristic and definite in size in crystals of the same substance, the size and rela tive developments of the faces themselves may vary between the widest limits, since these depend upon the accidents of growth (the feeding of crystal substances to the enlarging crystal) and not upon the crystal's characteristic struc ture—the cause is external, not internal. Crystal faces are, therefore, described in terms of their direction only, not of their absolute position in space. The groups of faces ur forms which occur upon crystals of a single substance are found to have the same kind of symmetry. though as be tween crystals of different substances the sym metry may be quite diff,,,nt. all crys tals have a centre of symmetry and one ur more planes and axes of symmetry.