It seems, therefore, on the whole most probable that the sun has not illuminated the earth for one hundred million years, and almost certain that he has not done so for five hundred million years. 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.

As I shall describe in the next chapter (and explain in detail in chapter 8), the last sentence proved to be truly farseeing.

The fact that the calculated ages of the Sun and the Earth turned out to be comparable—even though the estimates were determined independently—made Kelvin’s calculation more compelling, since there was every reason to suspect that the entire solar system had formed around the same time. Still, quite a few British geologists remained unconvinced. It almost seemed as though, for some of them, it was more convenient to explain everything not by the laws of physics but rather by what the American geologist Thomas Chamberlin cynically termed in 1899 “reckless drafts on the bank of time.” The best illustration of the skeptical attitude toward Kelvin’s findings is a fascinating exchange Kelvin had in 1867 with the Scottish geologist Andrew Ramsay. The occasion was a lecture by the geologist Archibald Geikie on the geological history of Scotland. Kelvin later described the conversation he had with Ramsay immediately following the talk, noting that almost every word of it remained “stamped on my mind”:

I asked Ramsay how long a time he allowed for that history. He answered that he could suggest no limit to it. I said, “You don’t suppose geological history has run through 1,000,000,000 [one billion] years?” “Certainly I do!” “10,000,000,000 [ten billion] years?” “Yes!” “The sun is a finite body. You can tell how many tons it is. Do you think it has been shining for a million million years?” “I am as incapable of estimating and understanding the reasons which you physicists have for limiting geological time as you are incapable of understanding the geological reasons for our unlimited estimates.” I answered, “You can understand the physicists’ reasoning perfectly if you give your mind to it.”

Kelvin was absolutely right. Ignoring for a moment the question of how solid his physical assumptions were and the mathematical details of his calculations, Kelvin’s main point was accessible. Since the Sun and the Earth are both losing energy, and they don’t possess any known sources that could replenish the losses, he argued, the Earth’s geological past must have been more active than the present. A hotter Sun would have caused more evaporation, with the associated higher rate of erosion by precipitation. At the same time, a hotter Earth would have experienced heightened volcanic activity. Consequently, Kelvin concluded, the uniformitarian assumption of an Earth in an almost indefinite quasi–steady state was untenable.