1. Butler, G., and Thirsk, H. R., J. Electrochem. Soc., 100, No. 7 (1953). Maxwell, K. H., Butler, G., and Thirsk, H. R., J. Chem. Soc., (810), 4210 (1952). Butler, G., and Thirsk, H. R., Acta Cryst., 5, 288 (1952). Boult, E. H., and Thirsk, H. R., J. Phot. Science, 3, No. 5 (1955).