Coexistence Curve of Sulfur Hexafluoride

The coexistence curve of sulfur hexafluoride has been determined by interferometric measurements for the temperature range ${10}^{-5\mathrm{}}<\frac{(T_c-T)}{T_c}<{10}^{-1\mathrm{}}$. The refractive index difference $n_l-n_v$ is fitted by a power law. The difference $\Delta n$ is proportional to ${\left(\Delta T\right)}^{\beta }$, where $\beta =0.346\mathrm{}\pm 0.001$ for the temperature range ${10}^{-2\mathrm{}}<\frac{(T_c-T)}{T_c}<{10}^{-1\mathrm{}}$. The exponent $\beta$ decreases to 0.339 ± 0.003 as $\frac{(T_c-T)}{T_c}$ decreases to the lower limit of the data at ${10}^{-5\mathrm{}}$.