This monograph is one of a connected series of three by the same author: Stationary states. The nature of atoms, and Bonds between atoms. Like its fellows, it was written for the Conference on New Instructional Materials in Physics, sponsored by the Commission on College Physics with the support of the National Science Foundation, and held at the University of Washington in the summer of 1965.The purpose of the conference was to create effective ways of presenting physics to college students who are not preparing to become professional physicists. Such an audience might include prospective secondary-school physics teachers, prospective practitioners of other sciences, and those who wish to learn physics as one component of a liberal education.The form of these, and of other monographs originating at that conference, implements a principle of experimental pedagogy promulgated there. Each monograph is 'multi-level': each consists of several sections arranged in order of increasing sophistication. Their authors hope that such papers can be fragmented for use in existing formal courses, or can be associated with other papers to provide the basis for new kinds of courses.