The different species of mica have very nearly the same forms and interfacial angles, and they not infrequently occur intergrown together in parallel position.
Although bismuth is readily obtained in fine crystals by artificial means, yet natural crystals are rare and usually indistinct: they belong to the rhombohedral system and a cube-like rhombohedron with interfacial angles of 92° 20' is the predominating form.
It crystallizes in rhombohedra belonging to the hexagonal system, having interfacial angles of 87° 40'.
If T12 denote the interfacial tension, the energy corresponding to unit of area of the interface b Q FIG.
Laplace does not treat systematically the question of interfacial tension, but he gives incidentally in terms of his quantity H a relation analogous to (47).