1. Merck Index, 9th ed., p. 72 (ed. M. Windholz). Rahway, New Jersey: Merck 1976
2. Barra, J.J., Krols, K., Matlak, T., in: Proc. on the Application of the M�ssbauer Effect (ed. I. Dezsi). Tihany, Hungary: Akademiai Kiado 1971
3. Rakos, M.: Czech. J. Phys.B17, 285 (1967)
4. Belloni, E.: Gazz. Chim. Ital.50, 159 (1920)
5. For a general discussion of theory, emphasizing biologically important systems, see: Swartz, H.M., Bolton, J.R., Borg, D.C., in: Biological Applications of Electron Spin Resonance, Chapt. 1. New York: Wiley 1972. The spin resonance behavior of iron specifically has been treated by Aasa, R.: J. Chem. Phys.52, 3919 (1970); Blumberg, W., in: Magnetic Resonance in Biological Systems (eds. A. Ehrenberg, B. Malmstrom, T. Vanngard). New York: Pergamon Press 1967