1. K. Natarajan, private communication (1981).
2. P. N. Bhargava, J. Sci. Res. BHU V(1) (1954?55), 149.
3. P. C. Das Gupta, A. Day and A. K. Mazumdar, Res. Ind. 17 (1972) 107.
4. B. K. Banerji, Ind. J. Phys. 21 (1947) 259.
5. H. P. Stout and J. A. Jenkins, Ann. Sci. Textiles Belges No. 4-12/65 (1955) 231.