The history of ideas about the earth's core is reviewed, from the l9th century to the present. Following the determination of the outer core boundary by B. Gutenberg and the establishment of the fluidity of the core by H. Jeffreys, the current model for the overall physical structure was completed by Inge Lehmann's discovery of the inner core and the proposal by F. Birch and K.E. Bullen that the inner core is solid >
The traditional assumption that the core is primarily iron was challenged by several scientists in the 1940's, especially W.H. Ramsey, who proposed that the core boundary marks a change in physical but not chemical state. His hypothesis, that the core is a liquid ‘metallized silicate,’ was refuted by research on the properties of silicates at high pressures, but it raised the question whether a theory of the present state of the earth's interior should be consistent with some plausible theory of its origin and development. While Western geophysicists tended to ignore this criterion, a group of Russian scientists developed a theory which satisfied it, although it was difficult to maintain the metallized silicate hypothesis. A compromise model, proposed in the 1970s, involves iron and oxygen in proportions chosen to satisfy density conditions but also derivable by physicochemical evolution from an initially homogeneous earth.