  We report on magnetisation measurements of Sr14-xCaxCu24O41, with 0 <= x <=
12, in magnetic fields up to 16 T. The low temperature magnetic response of the
CuO2 spin chains changes strongly upon doping. For x = 0, the ground state with
nearly independent dimers is confirmed. Reduction of the number of holes in the
chains through Ca-doping leads to an additional contribution to the
magnetisation, which depends linearly on the magnetic field. Remarkably, the
slope of this linear contribution increases with the Ca content. We argue that
antiferromagnetic spin chains do not account for this behaviour but that the
hole dynamics might be involved.
