SaturatedHe4near Its Critical Temperature

Measurements of the refractive index $n$ of saturated ${\mathrm{He}}^4$ liquid and vapor have been made at $\lambda =5462.27\mathrm{}$ Å from 4.2°K nearly to the critical temperature ($T_c=5.1994\mathrm{}$°K). Careful determinations of $n$ and $\frac{\mathrm{dn}}{\mathrm{dT}}$ as a function of temperature were made using a metal optical cryostat and a Jamin interferometer modified to produce a chart record of the interference fringes. The measurements extended to within 50 mdeg of $T_c$ for saturated vapor, and to within 36 mdeg of $T_c$ for saturated liquid. These results, combined with the Lorenz-Lorentz equation, then give the density $\rho$ of both saturated liquid and saturated vapor, and the expansion coefficient ${\beta }_s$ of the saturated liquid. These $n$, $\rho$, and ${\beta }_s$ measurements are the first ever made above 4.4°K except for two early measurements of density at 4.6 and 4.7°K. The critical density is found to be ${\rho }_c=(0.06948\mathrm{}\pm 0.00030$ g ${\mathrm{cm}}^{-3\mathrm{}}$, or the critical molar volume, $V_c=(57.628\mathrm{}\pm 0.25)$ ${\mathrm{cm}}^3$ ${\mathrm{mole}}^{-1\mathrm{}}$.