1. Pfeil, E., and Ruckert, H., Ann., 641, 121 (1961) and references therein.
2. Mayer, R., and Jaschke, L., Ann., 635, 145 (1960).
3. Hayck, B., Konetschny, H., and Schnell, E., Angew. Chem. Intern. Edit., 5, 669 (1966).
4. Gabel, N., and Ponnamperuma, C., Nature, 216, 453 (1967).
5. Reid, C., Orgel, L. E., and Ponnamperuma, C., Nature (in the press).