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John Joly (1857–1933) was one of Ireland’s most eminent scientists of the late nineteenth and early twentieth centuries who made important discoveries in physics, geology and photography. He was also a respected and influential diplomat for Trinity College, Dublin, and various Irish organizations, including the Royal Dublin Society. Measuring the age of the Earth occupied his mind for some considerable time – a problem he was to address using a diverse range of methods. His sodium method of 1899, for which he is best known, was hailed by many as revolutionary, but it was later superseded by other techniques, including the utilization of radiometric dating methodologies. Although Joly himself carried out much research in this area, he never fully accepted the large age estimates that radioactivity yielded. Nevertheless, Joly’s work in geochronology was innovative and important, for it challenged earlier methods of arriving at the Earth’s age, particularly those of Lord Kelvin. Although his findings and conclusions were later discredited, he should be remembered for his valuable contribution to this important and fundamental debate in the geological sciences.

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