The main change in this edition is the conversion to SI units. I have tried to be as consistent as possible and have been greatly aided in this task by the unflagging and meticulous care of the publishers. I have occasionally retained the °C particularly when referring to historical experiments and to the properties of water or ice near the melting point; 0 °C seems a more sensible point of reference than 273 K.

I have included some fresh material which fills out certain omissions in the first edition. I have also added some new themes which extend the coverage of the book; in particular the nature of van der Waals forces and the basic ideas of colloid stability. In some places, where it seemed relevant, I have added sections at a level rather more advanced than that required by the average readers; they may at some later stage find it useful to refer back to these items. Finally I have, as is inevitable in moving from c.g.s. to SI units, completely rewritten the introductory parts of the chapters dealing with the dielectric and magnetic properties of matter.

It remains for me to thank various colleagues and numerous correspondents who at various times have written to me proposing corrections and improvements. Though I have not been able to implement all their suggestions I am most grateful to them.