Simple approximate equations expressing $C_V$ and $C_p$ of gases as functions of $p$ and $T$ are given. They are derived by neglecting all powers of $\frac{1}{V}$ higher than the first and replacing $\frac{1}{V}$ by $\frac{p}{\mathrm{RT}}$ in the complete equations for $C_V$ and $C_p$ which are obtained by use of the Beattie-Bridgeman equation of state and which express these quantities as functions of $V$ and $T$. The resulting relation for $C_p$ (Eq. (8)) reproduces the thermal data on air well, and those on ammonia fairly well.

The complete and approximate equations for $C_V$ and $C_p$ of gas mixtures expressing these quantities as functions of $V$ and $T$, and of $p$ and $T$, are given. They are derived by use of the generalized Beattie-Bridgeman equation of state for gas mixtures on the basis of one assumption; that of the additivity of energy of real gases at infinitely low pressures.

• Received 12 November 1929

DOI:https://doi.org/10.1103/PhysRev.34.1615