1. van Krevelen, D. W., Ann. Mines Belg., 53, 750 (1954).
2. Schuyer, J., Brennstoff.-Chem., 37, 74 (1956).
3. Akamatu, H., and Magamazu, K., J. Colloid Sci., 2, 593 (1947).
4. Friedel, R. A., Nature, 179, 1237 (1957).
5. Friedel, R. A., and Queiser, J. A., Fuel, 38, 369 (1959).