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Joly, John

rate age radioactive geological

(1857–1933) Irish geologist and physicist.

Joly studied a range of sciences and engineering at Trinity College, Dublin, became professor of geology there in 1897 and held this post until his death. Early in his career he devised the steam calorimeter always linked with his name; he used it to find the specific heat of minerals and, for the first time, the specific heat of gases at constant volume.

Had proposed from 1693 that the age of the Earth (since water condensed on it) could be deduced from the rate at which the salt content of the oceans is increased by leaching of the salt of land masses by water, which rivers carry to the seas; he doubted both the age of a few thousand years deduced from the Bible and the idea that the Earth was eternal. In 1899 Joly used Halley’s methods, by measuring the rate of increase of oceanic sodium content, and arrived at an age of 80–90 × 10 6 years. Although now seen as far too short a time, Joly’s estimate was valuable then in supporting the geologist’s need for a much longer period than that given by work from 1862 based on the supposed rate of radiative cooling. In 1903 Joly pointed out that radioactivity (then recently discovered) would provide terrestrial heating and so affect Kelvin’s calculation.

Soon afterwards new estimates of the Earth’s age were made based on the rate of radioactive decay of uranium to lead and the U:Pb ratio in old rocks. Joly aided this work in 1907 by showing that the dark rings (pleochroic halos) found in some minerals had been formed by radioactive inclusions within them. He went on to use these microscopic halos present in rocks of differing geological age to show that radioactive decay had occurred at a constant rate during geological time. Only after Joly’s work could geological dating by radioactivity have a logically secure basis.

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