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An empirical equation for the vapor pressure of liquid helium has been developed. This equation is valid throughout the entire range of temperatures below the critical point, and is in accord with the latest available data on the vapor pressure to within about ±0.002°K.

Heat capacities were measured for two samples of slightly impure magnesium identical with those whose transport properties have been measured. Although the samples exhibit markedly different transport properties, any systematic difference in their specific heats was found to be less than 5% in the range of measurement. Atomic heat values were in agreement with earlier measurements on pure Mg in this temperature range.

LETTERS TO THE EDITORprocesses occurring in rarefied gases, it is desirable to have a method of solution which deals with the entire range of pressures in a unified manner and which can satisfy general microscopic boundary conditions. The present communication sketches such a method for the special case of Maxwellian molecules, which repel each other with an inverse fifth-power law of force. The Boltzmann equation governing the distribution function f (y,x,t), in the absence of external forces, iswhere dA contains the geometrical variables specifying a collision and is independent of velocity; v' and v/ are velocities after collision and are expressed in terms of v,Vi and the geometrical variables.We introduce a cutoff in the form of a maximum impact parameter so that the integrals converge separately. We give the names "emission term" and "absorption term" to the first and second terms, respectively, on the right-hand side of (1). Our procedure is then the following: at each step the absorption term is left intact. For the first step we insert a locally Maxwellian distribution / 0 in the emission term. Here,where k is Boltzmann's constant and m is the mass of the molecule. Using the conservation laws v 2J rV\ 2 -v f2 -\-Vi 2 and v-f Vi = v'+v/, we obtain at a (3)where

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