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Measurements have been made of the temperature coefficients of viscosity of the vapours of tetrafluoro-ethylene, hexafluoro-ethane, perfluoro-n-pentane and perfluoro-nhexane. For all of these vapours the temperature coefficients of viscosity are smaller than those for the corresponding hydrocarbons. This has been interpreted in terms of the intermolecular forces between fluorocarbon molecules which change steeply with the distance between molecular centres.

The collision integrals necessary for the calculation of transport coefficients of gases have been computed for an intermolecular potential of the form 4(r) = A/+ -B/r7, where r is the distance between molecular centres. These integrals have been used in combination with existing second virial coefficient computations to test the suggestion that the 28 : 7potential describes the form of the intermolecular interactions which may be considered to occur between the centres of quasi-spherical molecules such as CF4, SF6, SiF4, or C(CH3)4.It is shown that such a potential approximation constitutes an improvement over the usual Lennard-Jones 12 : 6-potential for three of the quasi-spherical molecules, but it cannot be used to give a completely satisfactory correlation of the transport and equilibrium properties of these molecules.

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