Rates of evaporation IV. The rate of evaporation and vapour pressure of rhombic sulphur

Abstract

The rate of evaporation in vacuo and vapour pressure of bipyramidal crystals of rhombic sulphur ({111}) have been determined in the range 15 to 32$\cdot $5 degrees C; the rate of evaporation of a polycrystalline bead of rhombic sulphur has been determined at 30 degrees C over a range of air pressures. The diffusion coefficient of S$_{8}$ molecules in air is 0$\cdot $0884 cm.$^{2}$ sec.$^{-1}$ at 30 degrees C; the evaporation coefficient is 0$\cdot $73 $\pm $ 0$\cdot $05 and appears to be temperature independent over the range studied. The linear velocity of the interface on evaporation is given by 1$\cdot $91 $\times $ 10$^{11}$T$^{-\frac{1}{2}}$e$^{-23,890/\boldsymbol{R}T}$ cm. sec.$^{-1}$. The vapour pressure is given by log$_{10}$p cm. = -5240/T + 10$\cdot $763. The temperature independent factor for the rate of evaporation and the value of the evaporation coefficient are discussed in terms of statistical mechanics.