1. H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL U.K., Department of Chemical Engineering, UMIST, P.O. Box 88, Manchester, M60 1QD U.K., and Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, Merseyside, L63 3JW U.K.