 For many solids composed of relatively heavy atoms (atomic number > iron), at non-cryogenic temperatures, the heat capacity at room temperature approaches 3"R" = 24.94 joules per kelvin per mole of atoms (Dulong–Petit law, "R" is the gas constant).Low temperature approximations for both gases and solids at temperatures less than their characteristic Einstein temperatures or Debye temperatures can be made by the methods of Einstein and Debye discussed below.For an ideal gas, evaluating the partial derivatives above according to the equation of state, where "R" is the gas constant, for an ideal gas

Substituting

this equation reduces simply to Mayer's relation