The conciliatory tone was apparently lost on Tait, since he continued to retort disparagingly: “I should like to have your answers to two questions: (1) What grounds have you for supposing the inner materials of the earth to be better conductors than the skin?” The second “question” was not really a question but a contemptuous remark about the insatiable expectations of the geologists: “(2) Do you fancy that any of the advanced geologists would thank you for the ten billion years instead of one hundred million? Their least demand is for a trillion:—for part of the mere secondary period!” (Figure 10 shows a copy of his note.) But Perry did not give up: “It is for Lord Kelvin to prove that there is not greater conductivity inside,” he insisted.

It goes without saying that Perry was correct in his assessment. In the absence of any definitive experimental evidence as to the Earth’s precise internal conditions, the fact that he was able to show that Kelvin could be wrong by a large factor was sufficient.

When he finally decided to respond, Kelvin was much less aggressive than Tait. While he stated, “I feel that we cannot assume as in any way probable the enormous differences of conductivity and thermal capacity at different depths which you [Perry] take for your calculations,” he also remarked in an uncharacteristically appeasing style, “I thought my range from 20 millions to 400 millions was probably wide enough, but it is quite possible that I should have put the superior limit a good deal higher, perhaps 4000 instead of 400.” Perhaps at no other time did Kelvin show such respect for opinions that contradicted his own. Most likely this magnanimity expressed his sense of obligation to empathize with a former student. He hastened to insist, however, that his estimate of the Sun’s age was still “refusing sunlight for more than a score or a very few scores of million years of past time.” As we shall see later in this chapter, Kelvin had no reason at the time to revise his calculation for the age of the Sun.

Perry’s challenge caused Kelvin to spend the following couple of months conducting experiments with heated basalt, marble, rock salt, and quartz. These experiments seemed to show, in agreement with new results by the Swiss geologist Robert Weber, that the conductivity either did not change much or even decreased slightly with increasing temperature. Unfortunately for Perry, Weber’s new results contradicted those of his own previous experiments—the very experiments Perry had used to support his case. The overjoyed Kelvin published the results in Nature on March 7, 1895, breaking the news that “Prof. Perry and I had not to wait long . . . to learn that there was no ground for the assumption of greater conductivity of rock at higher temperatures.” Kelvin further cited a conclusion of the American geologist Clarence King, who (without considering the possibility of convection by a fluid) stated: “We have no warrant for extending the earth’s age beyond 24 millions of years.” Kelvin pronounced gleefully that he was “not led to differ much from his [King’s] estimate of 24 million years.”

Perry, however, was not convinced. Concentrating on possible internal conditions, rather than trying, like Kelvin, to guess what the most probable conditions might be, he noted that King’s conclusion was still constrained by the assumption of a solid, homogeneous Earth. In a paper that appeared in Nature on April 18, 1895, Perry summarized his views on the impasse: “Now it is evident that if we take any probable law of temperature of convective equilibrium at the beginning and assume that there may be greater conductivity inside than on the surface rocks, Mr. King’s ingenious test for liquidity will not bar us from almost any great age.” Perry’s logic was clear: His goal was to demonstrate that the Earth could be older than Kelvin’s estimate, even if he was unable to identify the precise flaw in Kelvin’s argument, due to uncertainties concerning the Earth’s internal structure. The measurements of the conductivity of heated rocks might have disproved one of the ways in which heat could be transported more readily at great depths, but other possibilities were still open. In particular, convection by fluidlike mass was an attractive alternative.