1. “Stach’s Textbook of Coal Petrology”, Gebruder Borntraeger, Berlin, 1982.
2. F.T.C. Ting, in “Coal Structure”, (Ed, R.A. Meyers), Academic Press, New York, 1982, p.7.
3. R.C. Neavel, in “Chemistry of Coal Utilization” second suppl. volume, (Ed. M.A. Eliott), John Wiley-Interscience, 1981, p.91.
4. D.W. van Krevelen, “Coa1,Typology-Chemistry-Physics- Constitution”, Elsevier, Amsterdam, 1981.
5. P.H. Given, Fuel 39, 147 (1960).