1. Contribution from the Additives Analytical Research Department and Additives Research Department, Ciba Specialty Chemicals Corporation, 540 White Plains Road, P.O. Box 2005, Tarrytown, New York 10591, Physics, Novartis Services AG, Postfach, CH-4002, Basel, Switzerland, and Additives Research Department, Ciba Specialty Chemicals Corporation, P.O. Box 64, CH-1723 Marly 1, Switzerland