The coefficient S characterising the time dependence of the magnetisation of a magnetic material is calculated in terms of the irreversible susceptibility chi _irr are discussed in relation to experiment and computations. The Barbier plot of H_f against the coercive force H_c is shown to agree quantitatively with theoretical models of H_c. Correlation effects are discussed on the same basis as occurs in interpretations of the Vogel-Fulcher law of spin glasses.