  In recent years, there has been increasing interest in the specific heat $C$
of insulators and semiconductors because of the availability of samples with
different isotopic masses and the possibility of performing \textit{ab initio}
calculations of its temperature dependence $C(T)$ using as a starting point the
electronic band structure. Most of the crystals investigated are elemental
(e.g., germanium) or binary (e.g., gallium nitride) semiconductors. The initial
electronic calculations were performed in the local density approximation and
did not include spin-orbit interaction. Agreement between experimental and
calculated results was usually found to be good, except for crystals containing
heavy atoms (e.g., PbS) for which discrepancies of the order of 20% existed at
the low temperature maximum found for $C/T^3$. It has been conjectured that
this discrepancies result from the neglect of spin-orbit interaction which is
large for heavy atoms ($\Delta_0\sim$1.3eV for the $p$ valence electrons of
atomic lead). Here we discuss measurements and \textit{ab initio} calculations
of $C(T)$ for crystalline bismuth ($\Delta_0\sim$1.7 eV), strictly speaking a
semimetal but in the temperature region accessible to us ($T >$ 2K) acting as a
semiconductor. We extend experimental data available in the literature and
notice that the \textit{ab initio} calculations without spin-orbit interaction
exhibit a maximum at $\sim$8K, about 20% lower than the measured one. Inclusion
of spin-orbit interaction decreases the discrepancy markedly: The maximum of
$C(T)$ is now only 7% larger than the measured one. Exact agreement is obtained
if the spin-orbit hamiltonian is reduced by a factor of $\sim$0.8.
