1. van Krevelen, D. W., Coal (Elsevier, Amsterdam, 1961).
2. Lowry, H. H.(ed.), Chemistry of Coal Utilization, suppl., (Wiley, New York, 1963).
3. Bitz, M. C., and Nagy, B., Analyt. Chem., 39, 1310 (1967); Proc. natn. Acad. Sci. U.S.A., 56, 1384 (1966).
4. Yoshino, K., Fuel, 40, 457 (1961).
5. Chakrabartty, S. K., and Berkowitz, N., Fuel, 53, 240 (1974).