  We compare the experimental data of the first measurement of a temperature
dependence of the Casimir-Polder force by Obrecht et al. [Phys. Rev. Lett. {\bf
98}, 063201 (2007)] with the theory taking into account small, but physically
real, static conductivity of the dielectric substrate. The theory is found to
be inconsistent with the data. The conclusion is drawn that the conductivity of
dielectric materials should not be included in the model of the dielectric
response in the Lifshitz theory. This conclusion obtained from the long
separation measurement is consistent with related but different results
obtained for semiconductors and metals at short separations.
