  We demonstrate that the presence of entanglement in macroscopic bodies (e.g.
solids) in thermodynamical equilibrium could be revealed by measuring
heat-capacity. The idea is that if the system were in a separable state, then
for certain Hamiltonians heat capacity would not tend asymptotically to zero as
the temperature approaches absolute zero. Since this would contradict the third
law of thermodynamics, one concludes that the system must contain entanglement.
The separable bounds are obtained by minimization of the heat capacity over
separable states and using its universal low-temperature behavior. Our results
open up a possibility to use standard experimental techniques of solid state
physics -- namely, heat capacity measurements -- to detect entanglement in
macroscopic samples.
