1. A. G. Gregory and T. J. Veasey, J. Mater. Sci. 8 (1973) 324.
2. Idem, ibid 7 (1972) 1327.
3. R. J. P. Lyon, NASA Tech. Note D-1871 (Washington, 1963).
4. M. E. Tyrrell, G. V. Gibbs, and H. R. Shell, US Bur. Mines no. 549 (1961).
5. N. A. Toropov and N. A. Sirazhiddinov, in ?Structure of Glass?Volume 5, Structural Transformations in Glass at High Temperatures?, Ed. E. A. Porai-Koshits (Consultants Bureau, New York, 1965).