The following pages are derived in part from papers published in Reviews of Modern Physics and the Journal of Physical Chemistry and in part from lectures given for several years at The Johns Hopkins University. I am obliged to the editors of the two journals for leave to incorporate parts of the papers in the present work and to those who attended the lectures for their interest. I am indebted especially to Dr. Lloyd W. Walter and Dr. John S. Thomsen, from whose doctoral dissertations I have drawn the treatment of several subjects, as indicated in the notes. My colleague, Dr. T. H. Berlin, has helped me from time to time and encouraged me always. Mrs. Mary B. Rowe has been very patient in typing and retyping the manuscript.