1. Cook, P., and Mallard, J. R., Nature, 198, 145 (1963).
2. Stoodley, L. G., Nature, 198, 1077 (1963).
3. Varian Associates, V-4502 E.P.R. Spectrometer Systems, No. INS 1454A.
4. Hollocher, T. C., and Commoner, B., Proc. U.S. Nat. Acad. Sci., 47, 1355 (1961).
5. Wallman, H. B., MacNee, A. B., and Gaddesden, C. P., Proc. Inst. Rad. Eng., 6, 700 (1948).