The magnitude of the thermomolecular pressure difference across capillary tubes has been measured for the gases argon, nitrogen, oxygen, carbon monoxide, and methane at mean temperatures of 162.3, 233.5, 292.0, and 340.5°K. From these data, the translational part of the thermal conductivity (Eucken factor) has been determined for each of the polyatomic gases. Estimates have been made for the rotational relaxation number and the total thermal conductivity (Eucken factor) for these gases using the thermal conductivity formulas of Mason and Monchick (in the full and linearized forms) and of Saxena and co-workers. It is observed that: (a) These three sets of formulas are equivalent, except for a scaling of the rotational relaxation number; (b) the rotational relaxation number for these gases is an increasing function of temperature; and (c) the values of the Eucken factor and rotational relaxation number obtained here are in very good agreement with the data for these parameters gotten by other methods.

The analysis of thermomolecular pressure difference experiments to obtain information on thermal transport properties is critically examined. It is concluded that the finite difference form of the dusty-gas equation, which has been used to analyze the thermal transpiration phenomenon, is internally inconsistent. A form of the integrated dusty-gas equation developed in this paper, while more difficult to use than the finite difference equation, yields results for the Eucken factor which are in better agreement with direct measurements of this quantity than are obtained with the approximate equations. It is observed, however, that when using the integrated dusty-gas equation there is a temperature variation of the apparent capillary diameter which cannot be explained.