1. Spencer, “The Metals of the Rare Earths,” London, 1919;
2. Mellor, “A Comprehensive Treatise on Inorganic and Theroretical Chemistry,” Vol. VII;
3. Hopkins, “Chapters in the Chemistry of the Rare Earths,” Illinois, 1939
4. Inform. Circ. 7233, 1943, United States Bur. Mines; Minerals Year Book, United States Bur. Mines (Annually); “The Mineral Industry”by Roush (N.Y.) (Annually); Also Refs. 1(a), 1(b), 1(c)
5. B.P. 510, 198 1939