Chapman–Enskog theory also predicts a simple relation between thermal conductivity, λ λ , and viscosity, μ μ , in the form λ λ = f μ μ c v , where c v is the specific heat at constant volume and f is a purely numerical factor. For spherically symmetric molecules, its value is predicted to be very close to 2.5 in a slightly model-dependent way. For instance, rigid elastic spheres have f ≈ ≈ 2.522 , and molecules with repulsive force ∝ ∝ r − − 13 have f ≈ ≈ 2.511 (the latter deviation is ignored in table 1). The special case of Maxwell molecules (repulsive force ∝ ∝ r − − 5 ) has f = 2.5 exactly. Since λ λ , μ μ , and c v can be measured directly in experiments, a simple experimental test of Chapman–Enskog theory is to measure f for the spherically symmetric noble gases. Table 2 shows that there is reasonable agreement between theory and experiment.