Thermal Diffusion in Polyatomic Gases: Isotopic 15N2–14N2

The thermal diffusion factor, ${\alpha }_T$ , for the isotopic gas mixture ¹⁵N₂–¹⁴N₂ has been measured as a function of temperature in the range 100°–740°K, using a ten‐tube, stainless‐steel trennschaukel. ${\alpha }_T$ values are negative below 130°K and at higher temperatures have the practically constant value 0.36 within the experimental uncertainties. Monatomic gas theory as developed by Chapman and Enskog is unable to explain the results for any realistic potential model. The dimensional treatment of Waldman is also ineffective. Internal consistency and the agreement with the data of other workers suggest a rather dominant effect of the internal degrees of freedom and inelastic collisions on thermal diffusion in polyatomic gases.