1. Waters, E. L., and Maki, A. H., Phys. Rev., 125, 233 (1962).
2. Kats, A., Hydrogen in Alpha Quartz, thesis, Delft, Holland (1961); see also Philips Res. Reps., 17, 133 (1962).
3. (a) Griffiths, J. H. E., Owen, J., and Ward, I. M., Nature, 173, 439 (1954). (b) See also O'Brien, M. C. M., Proc. Roy. Soc., A, 231, 404 (1955). (c) See also Defects in Crystalline Solids, Rep. Bristol Conf., 1954 (Physical Society, London, 1955): Griffiths, Owen and Ward, 81; O'Brien and Pryce, 88.
4. (a) Anderson, J. H., and Weil, J. A., J. Chem. Phys., 31, 427 (1959). (b) Anderson, J. H., and Koth, W., Bull. Amer. Phys. Soc., 4, 285 (1959). (c) Weil, J. A., Morris, A. H., and Anderson, J. H., ibid., 4, 416 (1959). (d) Weil, J. A., and Anderson, J. H., J. Chem. Phys., 35, 1410 (1961). (e) Van Wieringen, J. S., Haven, Y., and Kats, A., Proc. Eleventh Coll. ‘Ampere’ (Eindhoven, Holland, July 1962). These authors have seen the hyperfine splittings of the low-abundance isotope germanium-73 in the germanium centres.
5. Mackey, J. H., Bull. Amer. Phys. Soc., 7, 187 (1962).