1. Certain material in Section 4.1 is adapted (by kind permission of the Clarendon Press) from J. Shorter, Correlation Analysis in Organic Chemistry: an Introduction to Linear Free-Energy Relationships, Chap. 2 (Oxford Chemistry Series, 1973).
2. L. P. Hammett, J. Amer. Chem. Soc., 59, 96 (1937).
3. L. P. Hammett, Physical Organic Chemistry, Chap. 11 (McGraw-Hill, New York, 2nd edn., 1970).
4. O. Exner, in Advances in Linear Free Energy Relationships, Chap. 1, N. B. Chapman and J. Shorter, eds. (Plenum, London, 1972). This is a review of the Hammett equation.
5. H. H. Jaffé, Chem. Rev., 53, 191 (1953).