1. R.H. Haüy, "Essai d'une Théorie sur la Structure de Crystaux", Paris (1784)
2. see C. Bunn, "Crystals", Academic Press, New York (1964), p. 7.
3. N. Max, Nature 355:115 (1992).
4. I. Stewart, New Scientist 131:29 (8 July, 1991).
5. A. Holden, “The Nature of Solids”, Columbia Univ. Press, New York (1965), Fig. 10, p. 108.