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Langmuir free-evaporation rates for solid BN and AIN were measured with a microbalance built inside a vacuum system. This system has been used previously for several Knudsen effusion and Langmuir studies at high and low temperatures.2

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One assumes the validity of the modified Clausius-Clapeyroii equation in which A x is the mean heat of vaporization between the teniperatures TI and Tz. At the high temperatures concerned, this equation should be fairly accurate.At the normal boiling point of PbO, the heat of vaporization is 51 kcal. per sole.il(no If the value 70 mm. at llOO°C., as re-ported by Preston and Turner, is substituted into the equation, the impossible value of 29 kcal. is obtained for A H . However, if a value of 10 mm. is assumed as the vapor pressure of PbO a t 1100"C., one arrives a t a reasonable value of 55 kcal. per mole for A H between 1472'' and llOO°C. Utilizing this value, it is possible to calculate the approximate vapor pressure of liquid PbO at other temperatures assuming a linear dependence of with the temperature. The calculated pressure of 0.2 mm. a t 900°C. has been shown to cr~rrcspond quite well with observations. The sublimation of chromic oxide, Crz03, has been observed in vacuum by the Langmuir technique using induction and solar heating. Extensive sublimation did not yield any new phases on the basis of X-ray powder studies, and condensates of CrZ03 were always obtained. Flash vaporization and flow experiments in CO or O2 atmospheres and in vacuum indicated no appreciable differences in rates of sublimation. Weight-loss experiments showed that the rate of sublimation was slightly higher than predicted for decomposition to the elements and suggested that small amounts of complex molecules, e.g. CrO and CrO,, were also present in the equilibrium vapor.

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