It is our everyday experience that materials in the solid, liquid, and gas phases are equally present in the world around Nevertheless it is the physics of solids that has acquired a privileged status in the vast field of research into the physical properties of macroscopic bodies. The reason for this is that while investigating liquids and real gases, let alone materials in so-called mesomorphic phases can yield a lot of new and interesting results, it is the processes taking place in solids that have, in the first place, led to a great many fundamentally new concepts. Moreover, solids have a much larger scope of application. .
