If the calorically perfect gas approximation is used, then the ideal gas law may also be expressed as follows p = ρ ρ ( γ γ − − 1 ) e where ρ ρ is the number density of the gas (number of atoms/molecules per unit volume), γ γ = C p / C v is the (constant) adiabatic index (ratio of specific heats), e = C v T is the internal energy per unit mass (the "specific internal energy"), C v is the specific heat capacity at constant volume, and C p is the specific heat capacity at constant pressure.