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In the mid-18th century, the naturalist Mikhail Lomonosov suggested that Earth had been created separately from, and several hundred thousand years before, the rest of the universe. In 1779 the Comte du Buffon tried to obtain a value for the age of Earth using an experiment: He created a small globe that resembled Earth in composition and then measured its rate of cooling.

This led him to estimate that Earth was about 75,000 years old.

Kelvin stuck by his estimate of 100 million years, and later reduced it to about 20 million years.

The discovery of radioactivity introduced another factor in the calculation.

Their values were consistent with Thomson's calculations.In 1895 John Perry challenged Kelvin's figure on the basis of his assumptions on conductivity, and Oliver Heaviside entered the dialogue, considering it "a vehicle to display the ability of his operator method to solve problems of astonishing complexity." Other scientists backed up Thomson's figures. Darwin, proposed that Earth and Moon had broken apart in their early days when they were both molten.He calculated the amount of time it would have taken for tidal friction to give Earth its current 24-hour day.After Henri Becquerel's initial discovery in 1896, Marie and Pierre Curie discovered the radioactive elements polonium and radium in 1898; and in 1903, Pierre Curie and Albert Laborde announced that radium produces enough heat to melt its own weight in ice in less than an hour.Geologists quickly realized that this upset the assumptions underlying most calculations of the age of Earth.

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By measuring the concentration of the stable end product of the decay, coupled with knowledge of the half life and initial concentration of the decaying element, the age of the rock can be calculated.