1. R. F. Guller andE. N. Bunthing,J. Res. Nat. Bur. Stand. 23 (1939) 275.
2. A. Bishay andM. Maklad, ?Proceedings of the Cairo Solid State Conference?, edited by A. Bishay (Plenum, New York, 1967) p. 213.
3. A. Abou el Azm andA. L. Hussein,Glastech. Ber. 32K(8) (1959) 66.
4. P. J. Bray, M. Leventhal andH. O. Hooper,Phys. Chem. Glasses 4 (1963) 47.
5. M. Leventhal andP. J. Bray,ibid. 6 (1965) 11.