The ratio of specific heat at constant pressure to specific heat at constant volume $(\gamma)$ for a gas is $\left(1+\frac{2}{\mathrm{f}}\right)$ where $\mathrm{f}$ is the number of degrees of freedom of a molecule of a gas. The ratio of ' $\gamma_{d}$ ' for rigid diatomic to ' $\gamma_{\mathrm{m}}$ ' for monoatomic is