The hypothesis that the liquid core of the Earth represents a phase-change at high pressure (and suitable temperature) of the mantle material is further investigated. A more accurate series of two-zone models have been computed, and also a new series of three-zone models. The change of overall radius as between an original all-solid Earth and the present size is shown to be at least 370 km. In the outer regions, greater pressure may be needed with rising temperature to effect the transition to denser crystal form (associated with the 20$^\circ$-discontinuity), and from this cause acting alone slight expansion of the Earth would result but to an extent less than one-tenth the overall contraction. Epochs of rapid contraction (mountain-building eras) could thus be separated by longer intervals of very slow expansion. The initial liquefaction of the central regions brings about pressure increase at the boundary of the core that renders the Earth unstable in that about 6 per cent of the entire mass liquefies extremely rapidly to cause a sudden collapse of the planet as a whole. The accompanying decrease of outer radius is about 70 km. Thereafter the planet remains thoroughly stable and contracts only slowly. The total contraction to date would have reduced the moment of inertia by a factor about 4/5, and the corresponding reduction in rotation period (through conservation of angular momentum) would be an effect comparable with tidal friction. The contraction also leads to release of gravitational energy at an average rate comparable with that from radioactive sources. An important consequence of the phase-change hypothesis is that the melting-point gradient changes sign after sufficient depth, thereby permitting melting of the central regions to occur at moderate temperatures explicable by a reasonable content of radioactive elements.