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3. Courrier, R., Horeau, A., Marois, M., and Morel, F., C.R. Acad. Sci., Paris, 232, 776 (1951).
4. Blundell, M., and Rotblat, J., Nature, 167, 645 (1951).
5. This is an interesting example of an ‘autochemograph’, a possibility suggested by Boyd, G. A., and Board, F. A., Science, 110, 586 (1949).