Throughout the 19th century, unaware of this approaching crisis, geologists worked away at establishing our familiar geological column. They worked out the order of the strata from which lay on top of which others, and later from the complexity of the fossils they contained, and made estimates of the duration of each geological period from the thicknesses of its best preserved sediments. Not realising that they had only a very incomplete part of the total depositional record, they came up with an estimated age of around 100 million years upwards, in tolerable agreement with Kelvin.
The age of the Sun presented a much more serious problem. We know how large the Sun is, how far away, and how much solar energy reaches us. From this, it is relatively straightforward to calculate its total energy output. Where is this energy coming from? Not from any chemical process, for no chemical process is energetic enough. So Kelvin, building on suggestions by Helmholtz and others, suggested that a more useful source might be the gravitational energy released during the Sun’s formation. Knowing the total mass of the Sun, and using Newton’s Laws of gravitational attraction, Kelvin could work out how much energy must have been given out by this process. This would first be converted into the kinetic energy of the infalling matter, and that kinetic energy would then by well-known physical processes be converted to heat and ultimately to light, all in strict obedience to the laws of thermodynamics. Divide the amount of energy available by the rate of output, and you get an upper probable limit of 100 million years for the Sun’s total productive life. This is also, by implication, an upper limit to the age of the Earth as we know it. "As for the future, we may say, with equal certainty, that inhabitants of the Earth can not continue to enjoy the light and heat essential to their life for many million years longer unless sources now unknown to us are prepared in the great storehouse of creation." Kelvin wrote these words in 1862, and published them in a popular journal (Macmillan’s Magazine).
In subsequent refinements of this calculation, he would add further arguments, based for example on tidal friction and the dynamics of the Earth–Moon system, and in the light of fresh information about the thermal properties of rocks lower the range to some 20–40 million years, "and probably much nearer 20 than 40."
The impact was sensational. For by this time, as Kelvin well knew, a great deal was at stake. Darwin’s Origin of Species had appeared just three years before the Macmillan’s Magazine article. This had revolutionised our perspective on the world. It stated for the first time with complete clarity the modern view that species were not separately created but had evolved from simpler common ancestors by the operation of natural selection on the variations between individuals. The origin of these variations (what we now call mutations) was completely unknown, but it was clear that descent from a common ancestor must have been an extremely slow process, requiring what Darwin himself had described as "incomprehensibly vast… periods of time", with 20 to 40 million years much too little for all this to have occurred by natural selection. Nor did it help when Kelvin revised his 100 million year estimate of the age of the Earth sharply downwards, in the light of new evidence about the melting points of rocks. Indeed, Charles Darwin referred to Kelvin as an "odious spectre" and among his sorest troubles, and his son George was among the geologists most concerned with trying to find flaws in Kelvin’s reasoning.
Reprinted from From Stars to Stalagmites: How Everything Connects, by Paul S. Braterman, with permission from World Scientific Publishing (U.K.), Ltd. Copyright © Paul S. Braterman, 2012.