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Lord Kelvin and his allies used three kinds of argument.

The first of these referred to the rate of heat loss from the earth and the length of time it would have taken to form its solid crust.

This position came to be known as uniformitarianism, but within it we must distinguish between uniformity of natural law (which nearly all of us would accept) and the increasingly questionable assumptions of uniformity of process, uniformity of rate and uniformity of outcome.

Even less should we let that knowledge influence our judgment of the players, acting as they did in their own time, constrained by the concepts and data then available.

One outstanding feature of this drama is the role played by those who themselves were not, or not exclusively, geologists.

His achievements ran from helping formulate the laws of thermodynamics to advising on the first transatlantic telegraph cable. Shelton was a philosopher of science, critical (as shown in his contribution, the 1915 article “Sea-Salt and Geologic Time”) of loose thinking and a defender of evolution in debates.

Harlow Shapley, who wrote an article in 1919 on the subject, was an astronomer, responsible for the detection of the redshift in distant nebulae and hence, indirectly, for our present concept of an expanding universe. Russell, author of the 1921 article on radioactive dating, was familiar to me for his part in developing the Hetzsprung-Russell diagram for stars, but I was surprised to discover that he was also the Russell of Russell-Saunders coupling, important in atomic structure theory. The prologue to the drama is the mid-19th century recognition of the relation between heat and other kinds of energy (see the 1857 article “Source of the Sun’s Heat”).

It is a drama consisting of a prologue and three acts, complex characters, and no clear heroes or villains.

We, of course, know the final outcome, but we should not let that influence our appreciation of the story as it unfolds.The third act sees the entry of a newly discovered set of physical laws—those governing radioactivity.Radioactivity offered not only a resolution to the puzzle of the earth’s energy supply but also a chronology independent of questionable geologic assumptions and a depth of time more than adequate for the processes of evolution.And we should resist the temptation to blame them for their resistance. Different methods of measurement (such as the decay of uranium to helium versus its decay to lead) sometimes gave discordant values, and almost a decade passed between the first use of radiometric dating and the discovery of isotopes, let alone the working out of the three separate major decay chains in nature.The constancy of radioactive decay rates was regarded as an independent and questionable assumption because it was not known—and could not be known until the development of modern quantum mechanics—that these rates were fixed by the fundamental constants of physics.One referred to the depth of the sediments and the time they would have taken to accumulate; the other referred to the salinity of the oceans, compared with the rate at which rivers are supplying them with sodium salts.In hindsight, both theories were deeply misguided, for similar reasons.Florian Cajori, author of the 1908 article “The Age of the Sun and the Earth,” was a historian of science and, especially, of mathematics, and Ray Lankester, whom he quotes, was a zoologist. The first act consists in a direct attack, led by Lord Kelvin, on the extreme uniformitarianism of those such as Charles Lyell, who regarded the earth as indefinitely old and who, with great foresight (or great naivety, depending on your point of view: see the third installment of the 1900 “The Age of the Earth” article by W. Sollas), assumed that physical processes would eventually be discovered to power the great engine of erosion and uplift.The second act of the drama sees a prolonged attempt by a new generation of geologists to estimate the age of the earth from observational evidence, to come up with an answer that would satisfy the demands of newly dominant evolutionary thinking, and to reconcile this answer with the constraints imposed by thermodynamics.The third, which by the end was the most acute, presented a problem that outlasted the controversy itself.Thus, when in 1919 Shapley stated that for him the radiometric timescale was fully established, he acknowledged that there was as yet no explanation for the sun’s energy. In 1920 Sir Arthur Eddington came up with the answer: the fusion of hydrogen into helium.) In reply to Lord Kelvin’s attacks, the geologists used two principal lines of reasoning.

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