Measurements have been made of the magnetic contributions to the specific heats of several paramagnetic salts, and of diluted specimens consisting of mixed crystals of the same paramagnetic salts with isomorphous diamagnetic salts. It is shown that the specific heats can be analyzed into a contribution characteristic of the isolated paramagnetic ions, due to the splitting of the ground state by nuclear and Stark effects, and a contribution due to interactions between the magnetic ions. While the specific heats of the isolated ions agree well with the values calculated from paramagnetic resonance data, the interaction contributions are in several cases too large to be accounted for by magnetic dipole interaction between the ions. It is concluded that exchange interaction is important, even in magnetically very dilute salts, and that it is predominantly of the anisotropic type postulated by Van Vleck and Opechowski. It is shown that in some salts the Stark splitting of the ground state of the magnetic ions is very sensitive to small distortions of the crystal lattice, so that in these cases it is not possible to apply the results of measurements on diluted specimens to calculate the properties of the pure substances.
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