1. N. V. Sidgwick, The Chemical Elements and Their Compounds, Vol. II, London: Oxford Univ. Press, 1950 (1962 reprint), p. 1345.
2. J. G. Pereira, Anales soc. espan. fis. quim.13, 144 (1915).
3. G. S. Johar, U. Agarwala, and H. S. Sodhi, Talanta18, 1052 (1971).
4. E. de Somer, Arch. intern. pharmacodynam.24, 1 (1914).
5. P. J. Durrant and B. Durrant, Introduction to Advanced Inorganic Chemistry, 2nd ed., London: Longman. 1970. p. 977.