Lord Kelvin's estimates of the Earth's age were not necessary consequences of his physics. Religion influenced his physics and his arguments for a limited age of the Earth. Kelvin's primary aim was to destroy Charles Darwin's theory of evolution by natural selection by attacking the uniformitarian geology on which Darwin's theory was founded. His calculations of the age of the Earth contained a fundamental contradiction. He assumed that the Earth began as a hot liquid sphere, but Fourier's mathematics, which he used to calculate the rate of cooling, applied only to heat conducted through a solid. Kelvin's assumption of an initially hot liquid Earth was a necessary consequence of his thermodynamics. Energy could neither be created nor destroyed. The heat within the Earth must, therefore, be derived from its first creation by God. Kelvin never admitted the contradiction between the original hot liquid Earth and his calculation of its cooling on the assumption that the Earth was solid throughout, but in 1897 his imagined account of the initial Earth was a search for a solid Earth amenable to his calculations. The heat flow through the solid crust was very small in proportion to the total internal heat of the Earth. If Kelvin had included the total internal heat in his calculations, he would have arrived at much higher figures for the age of the Earth.

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