  Data from susceptibility measurements on Sr_2CuO_3 are compared with recent
theoretical predictions for the magnetic susceptibility of the
antiferromagnetic spin-1/2 Heisenberg chain. The experimental data fully
confirms the theoretical predictions and in turn we establish that Sr_2CuO_3
behaves almost perfectly like a one-dimensional antiferromagnet with an
exchange coupling of J = 1700^{+150}_{-100}K.
