The present volume is one of a number planned to extend the Physical Science Texts beyond the Advanced or Scholarship levels of the General Certificate of Education. The earlier volumes in this series were prepared as texts for class teaching or self-study in the upper forms at school, or in the first year at the university or technical college. In this next stage, the treatment necessarily assumes a greater degree of maturity in the student than did the earlier volumes, but the emphasis is still on a strongly realistic approach aimed at giving the sincere reader technical proficiency in his chosen subject. The material has been carefully selected on a broad and reasonably comprehensive basis, with the object of ensuring that the student acquires a proper grasp of the essentials before he begins to read more specialized texts. At the same time, due regard has been paid to modern developments, and each volume is designed to give the reader an integrated account of a subject up to the level of an honours degree of any British or Commonwealth university, or the graduate membership of a professional institution.