1. Beketow: Comptes Rendus, 1859, vol. 48, p. 442.
2. Schaufelberger and Roy: Transactions of the Institution of Mining and Metallurgy, London, 1954–1955, vol. 64, p. 375. AIME Chicago Meeting, February 1956. U. S. Patent 2,694,005.
3. Joannis: Comptes Rendus, 1897, vol. 125, p. 948; 1903, vol. 136, p. 615.
4. Klumpp, U. S. Patent 2,227,783.
5. Wilcox: U. S. Re. 12,815.