1. Littlewood, K., and Monaghan, M. T., Nature, 192, 128 (1961).
2. Van Krevelen, D. W., and Schuyer, J., Coal Science, 336 (Elsevier Pub. Co., 1957).
3. Dryden, I. G. C., Residential Conf. Sci. in the Use of Coal, Sheffield, 1958, A-13 (Institute of Fuel, London).
4. Given, P. H., in Coal, by Francis, W., 717 (Edward Arnold. Ltd., London, 1961).
5. Brown, J. K., Fuel, 38, 55 (1959).