Heat Capacity—Reflection of the Density of States

Heat capacity data, though generally given sparse consideration, have played significant roles during the past three centuries for characterizing materials. In the 18th and 19th centuries they also presented challenges to accepted theory because they defied explanation in terms of classical statistical mechanics theories of matter. The following is a brief listing of historical highlights and a summary of the fundamental quantum principles that provided a rational exposition of these data. The underlying feature of quantum, in contrast to classical mechanics, is the formalism for computing atomic/molecular states for specified models, which leads to sparse rather than closely spaced levels. This proved to be the critical factor that accounted for the observed heat capacities. When so viewed, the mystery of the propensity for the direction of change is also clarified.