The heat contents and entropies have been derived as a function of temperature for gallium arsenide and the gaseous species which exist in equilibrium with it, in the temperature range 800~176 when iodine or hydrogen chloride and hydrogen are also present. The important gaseous species are shown to be GaC13, GaIs, GaC1, GaI, HI, I2, I, HC1, As4, and H2. Under certain conditions the species As2, AsC13, and Ga (liq.) may also be present. The free energy changes for reactions among the above species are derived. They are shown to be consistent with those isothermal equilibria that have been directly measured. The free energy changes are used, together with the conservation equations for nondepositing components (I, C1, H, and As in excess of the stoichiometric Ga: As: 1:1 ratio) to give the partial pressures of all the important species at any temperature in a temperature gradient in a constant pressure system. The results are applicable to the calculation of the flux of gallium arsenide in open flow or closed tube (diffusive transport) systems, and they are in good agreement with experimentally measured rates of transport in the system GaAs-H2-C12 in an open flow apparatus.

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The theory of thermomolecular pressure differences in the form introduced by Weber is applied to measurements on argon and nitrogen. The integration of the fundamental differential equation for the pressure gradient is expanded to give two forms of solution. The experimental data for argon are found to fit one form and the experimental data for nitrogen the other. The equations obtained enable the thermomolecular pressure differences to be calculated for all values of pressure, temperature, and tube radius. Values so calculated fit the experimental data for three glass capillaries with a maximum uncertainty of 1 micron. In a metal capillary examined the deviations were less than 2 5 microns. Reasons for the larger discrepancy between theory and experiment for the metal capillary are sought. For argon an increase of Maxwell's reflection coefficient was found in the temperature region in which considerable adsorption takes place.For the accurate determination of small gas pressures in systems of which the parts connected to the pressure measuring device are subjected to large temperature gradients a knowledge of the thermomolecular pressure differences between the parts at different temperatures may be necessary. The thermomolecular

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