  We report measurements of the susceptibility in the temperature range from
$3.5$ K to $6.0$ K of a series of Mn$_{12}$-ac and Mn$_{12}$-ac-MeOH samples in
the shape of rectangular prisms of length $l_c$ and square cross-section of
side $l_a$. The susceptibility obeys a Curie-Weiss Law, $\chi=C/(T-\theta)$,
where $\theta$ varies systematically with sample aspect ratio. Using published
demagnetization factors, we obtain $\theta$ for an infinitely long sample
corresponding to intrinsic ordering temperatures $T_c \approx 0.85$ K and
$\approx 0.74$ K for Mn$_{12}$-ac and Mn$_{12}$-ac-MeOH, respectively. The
difference in $T_c$ for two materials that have nearly identical unit cell
volumes and lattice constant ratios suggests that, in addition to dipolar
interactions, there is a non-dipolar (exchange) contribution to the Weiss
temperature that differs in the two materials because of the difference in
ligand molecules.
