1. Orrock CM., Ph.D. thesis, University of London (1986).
2. White J.H. and Wahl C.V., American Patent No 1.862.559 (1932).
3. Pitt C.D. and Rawlings R.D.,Met Sci., 17, p. 261, (1983).
4. Persiano A.I.C., Ph.D. thesis, University of London (1986).
5. Kawahara K.,J. Mat. Sci., 18, p. 1709 (1983).