4) Mulcahy, M. F. R., Morley, W. J., Smith, I. W., "The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization," ed. by Durie, R. A., Butterworth-Heinemann, London (1991), p.386.
5) Kimura, H., Fujii, S., "Sekitan: Kagaku to Kogyo," Sankyo-syuppan, Tokyo (1984), p.254.