  We demonstrate that the magnetic susceptibility of strongly alternating
antiferromagnetic spin-1/2 chains is an entanglement witness. Specifically,
magnetic susceptibility of copper nitrate (CN) measured in 1963 (Berger et al.,
Phys. Rev. 132, 1057 (1963)) cannot be described without presence of
entanglement. A detailed analysis of the spin correlations in CN as obtained
from neutron scattering experiments (Xu et al., Phys. Rev. Lett. 84, 4465
(2000)) provides microscopic support for this interpretation. We present a
quantitative analysis resulting in the critical temperature of 5K in both,
completely independent, experiments below which entanglement exists.
