The vapor pressure of solid calcium has been measured between 500 and 625°C, by the method of molecular effusion. The amount evaporated was determined by microtitration to better than ${10}^{-6\mathrm{}}$ g. The results are well represented by the empirical equation: $logp \left(\mathrm{in}\mathrm{mm}\right)=8.15-\frac{9670}{T}$. The present values for the pressure are about ten times smaller than those previously reported by Pilling for the same temperatures. Since the specific heat of solid calcium is known as a function of $T$, the "vapor pressure constant," or "chemical constant," can be computed. The new pressure data lead to the correct value for this constant, but the old ones do not. For the heat of evaporation at $T=0\mathrm{}$ the value 4.53×${10}^4$ cal./mole is obtained.

• Received 17 September 1934

DOI:https://doi.org/10.1103/PhysRev.46.763