1. V. M. Goldschmidt, Geochemistry, Oxford University Press, London (1954).
2. E. A. Smith, The Sampling and Assay of the Precious Metals, Charles Griffin & Company Ltd., London (1947).
3. E. Wichers, J. Amer. Chem. Soc. 43 (1921).
4. W. F. Meggers, C. C. Kiess, and F. J. Stimson, U.S. Bur. Std. Sci. Papers 18, 235 (1922).
5. W. Gerlach and E. Schweitzer, Z. Anorg. Allgem. Chem. 173, 92 (1928).