Absolute Zero and the Conquest of Cold (5 page)

The ancients could not agree on the character of the
primum frigidum,
so Boyle tested their guesses. Aristotle had opted for water as the source of cold; this Boyle refuted by showing that substances with no water content, such as gold, silver, and crystal, could become quite cold. He also reported the observations of correspondents that ice forms atop the sea, where it interacts with air, but not: at the sea's bottom; to Boyle this meant the
primum frigidum
was unlikely to be water. While on the subject, Boyle disposed of another Aristotelian-based contention, a theory called antiperistasis. Aristotle wrote that heated water cools more quickly than cold water; this was true, but only for previously boiled water, and only if the temperature at which one began to chill the water was not too high. More recent Aristotelians, without testing the limitations of the data, had pyramided the hot-begets-cold-quicker notion into an elaborate construct about the general action of opposites in spiritual as well as physical matters. Boyle exploded this rather silly theory by setting outdoors several vessels, each filled with water at a different temperature—hot, tepid, or cold—and collecting data showing that the rate at which the contents froze was not affected by the temperature of the water at the start.

With the same clarity of argument, Boyle dismissed two other candidates for
primum frigidum.
Plutarch had said it was the earth; but Boyle pointed out that the earth was known to be cooler and more solid near the surface and that other explanations had shown there was likely to be a central fire (not a central cold) at the core of the earth. Pierre Gassendi, a contemporary philosopher, suggested nitre as the
primum frigidum;
Boyle rejected this idea by pointing out that many cold substances exist without nitre or its "exhalations."

The "peripateticks," a group of scholastic natural philosophers, espoused air as the fourth candidate for
primum frigidum.
To refute that notion, Boyle pointed out that his correspondents had found ice in the middle depths of the sea, between top and bottom, which seemed to preclude air as the source of cold; moreover, he reminded readers, in his famous vacuum jar he had frozen water in the absence of all air. Boyle concluded that neither air nor earth nor nitre nor water could be the principal cause of cold, and that a
primum frigidum
was "but an unwarrantable conceit."

Now Boyle applied his ingenuity and pushed into virgin territory in a series of elegantly simple experiments. Many people had observed that when water barrels hooped with iron were left out in winter, they froze and the hoops broke. Various explanations had been advanced; Boyle thought them all nebulous, and that the only logical one was that the breakage was produced by the expansive power exerted by water as it froze. But how could he prove that his own guess was nearest to the truth? And kill two philosophic birds with a single stone? Another of Aristotle's theories was that substances were what nature had intended them to be and could not change; this led Aristotle to assert that when the form of a substance was altered—for example, when water became ice—that substance did not and could not gain or lose weight or size. A second assertion, from those who held that the
primum frigidum
was air, was that when a glass tube containing water was left outside overnight, and the next morning showed ice on its external surface, that ice must have been formed by cold air permeating through the glass from inside to out.

In a way few experimenters had done before, Boyle put down on paper the exact progression of his thoughts, so readers could follow his path to imagining the experiments he devised to refute outmoded contentions. His test to reveal whether anything had "migrated" from inside to outside was simplicity itself: he weighed the amount of water he put into a glass before letting it sit in the cold overnight, and the next morning again weighed the contents. Finding that the weight of the contents was the same in the morning as it had been before he put the vessel into the cold night air, he could then conclude that nothing had migrated from inside the vessel to cause the frost on its exterior. Boyle believed it most probable—he was very cautious in expressing his degree of certainty—that the ice that formed inside the glass overnight caused the glass to become cold and to foster condensation of water vapor on the exterior of the glass, which soon froze into the coating of frost.

Only after the preamble of that experiment could he proceed to the main task of demonstrating that freezing water did, indeed, change in size when it became ice. The least complicated way to show this would have been to measure the volume of water in a container before freezing, and compare it with the volume of ice after it froze. But Boyle knew that if he did not carefully control the conditions, the proponents of various candidates for
primum frigidum
would say any increase in the volume of ice was caused by migrating air, or by migrating particles from the iron or pottery or wood of the vessel's walls. So he decided to moot those possibilities by using a vessel made of glass, putting water in it, and then sealing the vessel. In other experiments, he had placed vessels outside to freeze, but to do so in this instance, he reasoned, might cause the glass to break. To prevent that, he worked inside a house, where he immersed the bottom of the vessel in a mixture of snow and salt. This ensured that the freezing of the water inside the vessel would proceed from bottom to top—allowing him to stop the process before the expansion had gone too far. Contending that his rigorous conditions had eliminated all other explanations, Boyle was able to convincingly conclude that ice was nothing more and nothing other than an expanded state of water.

But what were the parameters of that expansion? How greatly did the water expand in becoming ice? What amount of force did that entail? Aristotle and his followers had not even asked these questions, since they did not believe water expanded when it became ice. "No body has yet, that we know of, made any particular trials on purpose to make discoveries in this matter," Boyle noted. He ingeniously froze water in pottery and metal vessels with weights placed on top to retard expansion; he was astounded to discover that a weight of 74 pounds was required to prevent expanding ice from pushing out a cork. These results allowed Boyle to counter other faulty explanations of the action of cold. René Descartes, the "master of first principles" whose theories were very popular just after his death in 1650, contended that cold was only the absence of heat, which he defined as a free-floating "ethereal" substance that had neither weight nor mass; Cartesians believed cold was caused—in Boyle's words—by "the recess [receding] of that ethereal substance, which agitated the little eel-like particles of the water." Boyle wondered dryly how the ethereal particles could be so strong in their "recess" as to expand the water "with so stupendous a force." Epicureans held a related explanation: "cold corpuscles" that worked—again in Boyle's words—by "stealing insensibly into the liquors they insinuate themselves into, without any shew of boisterousness or violence." Were the vessels to be permeated in so calm a manner, Boyle observed, ice would never break them. There were, he concluded triumphantly, no "swarms of frigorifick atoms," nor was there any other explanation, other than his own, that could adequately describe or predict the phenomena he observed in his myriad experiments on the power of cold.

Boyle's most effective antagonist was Thomas Hobbes, who by mid-century had become one of England's leading philosophers. The disagreement between Hobbes and Boyle was, in part, that between a former "amanuensis" of Francis Bacon's and Bacon's leading scientific disciple. Hobbes championed Bacon's emphasis on the need to discover axioms and construct a comprehensive natural philosophy, while Boyle and his fellow members of the Royal Society more faithfully adhered to Bacon's insistence on properly conducted experiments, including deliberate attempts to manipulate nature. But the antagonism between Boyle and Hobbes was even more fundamental, involving two antithetical ways of seeing the world and of discovering facts about it. Following Aristotle more than Bacon, Hobbes's natural philosophy used deduction from "first principles," a process that replicated the path he had taken to reach his unusual understandings of civil law and ethics. In Hobbes's view, when a philosopher wanted to know something about the workings of the natural world, to begin with he made a "first principles" hypothesis, then inferred phenomena and rules from it, and from these drew his conclusions; in accordance with this way of determining knowledge, experimentation had no value, because it could not reveal anything about the world that the first principles had not already predicted.

This was directly counter to Boyle's experimentalist way of viewing, questioning, and assessing the world; both he and Hobbes knew how sharply opposed their viewpoints were, and in a series of books published over several decades, they directly attacked one another's contentions, methodology, and conclusions. In
Dialogus Physicus
and in
De Corpore,
Hobbes mounted a strong critique of experimentation and of Boyle's use of it in regard to the vacuum pump. He contended that experiments were unable to "establish" facts. As an example, he noted that Boyle's air pump had leaked, so whatever results it produced were faulty and could not be the basis for making explanations. Moreover, Hobbes argued, for the results obtained, there were alternative causal explanations to those advanced by Boyle.

Stung by the criticism of his famous device and of his experimental method, Boyle took steps to shore up both, repairing some of the pump's inadequacies before his next series of experiments on atmospheric pressure, and inserting into his subsequent definitions of the experimental method the need to generate a hypothesis consistent with all known facts about nature—a hypothesis that could explain not only the results of the experiments at hand but also of all similar experiments, and of future tests not yet conceived. These were salutary results of Hobbes's critique, in that they forced the leading English practitioner of the experimental method toward greater rigor of methodology.

Hobbes left himself vulnerable to significant counterattack from Boyle by venturing opinions in an area about which Boyle knew far more than Hobbes: the cold. Boyle began his riposte by claiming an obligation to respond because Hobbes's "fame and confident way of writing might prejudice experimental philosophy in the minds of those who are yet strangers to it," and who might "mistake confidence for evidence." According to Boyle, Hobbes's theory about the origin and actions of cold was "so inconsiderately pitched upon, and so slightly made out" that it should not merit more than a passing mention, but since it was popular, it had to be refuted. Boyle cited Hobbes's discussion of how ice forms, in which Hobbes wrote that the source of all cold was wind, which

rakes the superficies of the earth, and that with a motion so much the stronger, by how much the parallel circles towards the poles grow less and less. From whence must arise a wind, which will force together the uppermost parts of the water, and withal raise them a little, weakening their endeavour towards the center of the earth.

Citing such convoluted reasoning, Boyle noted that Hobbes gave no proofs or demonstrations of his theories, only explanations that were "partly precarious, partly insufficient, and partly scarce intelligible."

Hobbes's contention that cold winds made the exterior parts of a body coagulate and go inward, thus transmitting the cold, was wrong in almost every detail, Boyle wrote. Boyle had put live animals in his vacuum apparatus, extracted the air, and then frozen the animals in the absence of all air—which disproved Hobbes's thesis that winds were what cause bodies to feel cold. He also showed that when a cake of ice served as a stopper in a vessel between the wind and the remaining water inside, freezing continued despite the absence of wind. There was no way, Boyle concluded, that Hobbes's theory of wind-as-source-of-cold could explain Boyle's experimental results.

It was with such counterattacks, in this and other areas of physics and chemistry, that Boyle eclipsed Hobbes in natural philosophy and relegated Hobbes's contributions in science to the dustbin of history. Hobbes never became a Fellow of the Royal Society—blackballed, it appears, by Boyle and a few others—though he did remain on good terms with many of the Royal Society's early members. Within a few years of his death in 1679, at the age of ninety, his reputation had come to rest mainly on his contributions to political philosophy, ethics, and morals, for which he is still principally known today.

In 1665 the effacement of Hobbes by Boyle in natural philosophy lay in the future, and Boyle used Hobbes's arguments as a sort of governor on his own exuberance. This was most evident in a late chapter of the book on the cold, a "sceptical dialogue" involving the fictional character Carneades, who was a stand-in for Boyle. In it, Carneades agreed with Bacon that cold "must be a privation [deprivation]" of motion of some sort but admitted he could not demonstrate precisely how it worked, and he could not completely refute all other explanations, such as that cold might be transmitted through the walls of vessels, in the manner of rays of light. Boyle was in effect conceding Hobbes's point that there could be alternative explanations for certain of his experimental results. But if Boyle could not definitively assert that his theory about the deprivation of motion as the cause of cold was the only correct explanation, he could and did contend that he had succeeded in disproving every other explanation, and that his had reached Huygens's threshold of the fairly probable, which was the best that could be achieved at that moment.

"Future industry," Boyle predicted, would be able to build on his work, venturing beyond the frontiers to chart and explain to the country of the cold. In that future exploration, its Columbus wrote, if any one thing was needed more than another, it was a good and reliable thermometer, the lack of which had forced Boyle to leave some important experiments on cold "untried."