1. CSIRO Molecular Science, P.O. Box 184, North Ryde New South Wales 1670, Australia, School of Chemistry F11, University of Sydney, New South Wales 2006, Australia, Department of the History of Science, Harvard University, Science Center 371, Cambridge, Massachusetts 02138, Barwon Water, Post Office Box 659, Geelong Victoria 3220, Australia, and Philips Research Laboratories, Professor Holstlaan 4 (WY72) 5656 AA Eindhoven, The Netherlands