Mercury vapor pressure, 20° to -80°C.—A modified Buckley ionization gage is described with a range from ${10}^{-2\mathrm{}}$ to 4×${10}^{-9\mathrm{}}$ mm. This was used to measure the vapor pressure of mercury. Correction for thermal effusion was made, ranging from 25 percent at -80°C to zero at 0°C. The gage was calibrated at 0°C from Knudsen's value. The results correspond to the equations ${log}_{10}P\left(\mathrm{liquid}\right)=8.86-3.44\left(\frac{{10}^3}{T}\right)$; ${log}_{10}p\left(\mathrm{solid}\right)=9.35-3.55\left(\frac{{10}^3}{T}\right)$.

Heat of vaporization of mercury, 20° to -80°.—From a large-scale plot of $logp$ AGAINST $\frac{1}{T}$ the heats of vaporization were calculated from the relation $\lambda =-\frac{Rd\left(logp\right)}{d\left(\frac{1}{T}\right)}$. Above the melting point the results correspond to the equation $\lambda =15,900-10t$ where $t$ is temperature centigrade. A slight discontinuity in slope of the ($logp,\frac{1}{T}$) curve gives at the melting point a sudden increase in the heat of vaporization of some 600 calories, which is in good agreement with the heat of fusion by direct experiment.

• Received 13 February 1925

DOI:https://doi.org/10.1103/PhysRev.26.859