The Vapor Pressure, Critical Point, Heat of Vaporization, and Entropy of LiquidHe3

The vapor pressure of pure liquid ${\mathrm{He}}^3$ was measured from 1.025° to 3.35°K, and the data are represented accurately by the equation ${log}_{10}{p}_{\mathrm{mm}}=\frac{-0.97796\mathrm{}}{T}+2.5\mathrm{}{log}_{10}T+0.000302\mathrm{}{T}^3+1.91594\mathrm{}$. The normal boiling point is 3.195±0.001°K. The critical temperature and pressure were determined to be 3.35±0.02°K and 890±20 mm Hg, respectively. The calculated heat of vaporization has a maximum value of 11.4±0.4 cal. ${\mathrm{mole}}^{-1\mathrm{}}$ at 2.1°K, and the extrapolated value at 0°K is 4.47 cal. ${\mathrm{mole}}^{-1\mathrm{}}$. The calculated entropy of liquid ${\mathrm{He}}^3$, excluding the nuclear spin entropy, is 0.75±0.1 cal. ${\mathrm{deg}.\mathrm{}}^{-1\mathrm{}}$ ${\mathrm{mole}}^{-1\mathrm{}}$ at 1°K, and the extrapolated value at 0°K is 0.42±0.1 cal. ${\mathrm{deg}.\mathrm{}}^{-1\mathrm{}}$ ${\mathrm{mole}}^{-1\mathrm{}}$. This extrapolated value of the entropy suggests that below 1°K there is either a triple point or an anomaly in the heat capacity.