Absolute Zero and the Conquest of Cold

by Tom Shachtman

  Like the Royal Society in London and the Académie des Sciences in Paris, the Accademia del Cimento was founded by distinguished experimenters and had highborn patronage. The Accademia lasted a mere ten years, though, and perhaps because of its short life it has not been invested with the same reverence by later generations—but it was crucial to the history of thermometers. It was an institution devoted to experimentation, as evidenced by its name, translated as the "academy of the concrete" or the "academy of experiments," and by its motto, Provando e Riprovando, "proving and proving again." The father of Ferdinand II and of Leopold Medici had been a student of Galileo's, and the sons were fascinated by the work of Torricelli; those ideals translated into their installing facilities for the use of ten scientists in the Pitti Palace in Florence in the late 1650s. The Medicis' Accademia flourished at a moment when the imprimatur of a scientific institution could have the greatest influence on the exploration of the cold, by influencing the acceptance of its critical tools over those produced by other groups.

  In contrast to the English and French patrons of science, the Medicis were actively involved in the institution's experimental work. Ferdinand was a corpulent man with a bulbous nose and a black mustache whose ends rose up toward his eyes. "The grand duke is affable with all men, easily moved to laughter and ready with a jest," a contemporary account stated. One of his heartaches was his inability to interest his son in scientific work; the "melancholy" teenager exhibited "the symptoms of a singular piety" that dismissed experimentation as incompatible with religious faith. Ferdinand's brother Leopold was quite devout, but that did not prevent him from being a serious scientist. He spent four hours a day reading books on literature, geography, science, architecture, and general curiosities; it was said that, like a little boy with a piece of bread, Leopold always kept "a book in his pocket to chew on whenever he [had] a moment to spare."

  The Accademia shared with the Royal Society the goal of discrediting the pseudoscience of Aristotle's followers. The Medicis corresponded with the Royal Society, and they employed a secretary to carefully note the Accademia's experiments on the incompressibility of water, the gravity of bodies, and the electrical properties of metals and liquids. But mostly the men in the Pitti Palace addressed the task of making more capable and subtler measuring implements of every sort then in use—barometers, hygrometers, telescopes (Ferdinand ground his own lenses), astrolabes, quadrants, calorimeters, microscopes, magnetic devices, and thermometers. One visitor reported seeing "weather glasses" (barometers or thermometers) even in the grand duke's bedchamber, and another marveled at thermometers displayed as works of art rather than as implements of science. There were thermometers to measure the air temperature, thermometers to measure heat and cold in liquids, thermometers for baths. During the winter, strumentini, little temperature-measuring thermometric instruments, hung in every room of the palace.

  Several members of Ferdinand II's court were, in the words of a food historian, "ice-mad," among them the secretary, who reverently called snow "the fifth element" and wrote novels in which characters begged one another for ice-based treats. Other courtiers were known to delight in the ice goblets and fruit bowls, the ice pyramids, and the table-sized ice-capped mountains of the Medici celebratory feasts.

  It was Ferdinand who invented the sealed-glass thermometer. When the Medici workshop devised one with a scale that marked off into fifty segments the bulb in which the measuring fluid was held, Sagredo accepted that innovation and used it as a base from which to further divide his new thermometers. On these, he marked off 360 divisions, like the gradation of the circle; after that, all scientists started calling the divisions "degrees."

  Most of the Florentine sealed-glass thermometers used "spirit of wine," distilled alcohol. Small glass bubbles filled with air at varying pressures hovered within the liquid, changing position as the temperature rose or fell. Later the Accademia flirted with the use of mercury, but its scientists soon—and for reasons that to this day remain obscure—abandoned this choice and went back to spirit of wine. Spirit thermometers did not work well at the low and high ends of the scale, because alcohol boils at a lower temperature than water, and it freezes lower, too; moreover, because the density of the distillate varied from batch to batch, spirit thermometers were often incompatible with one another. Counterbalancing such inadequacies was the high quality of the thermometers produced by the Accademia del Cimento, which was why those that bore its good name came to be requested and used throughout Europe.

  Just when the Accademia had built up enough expertise to be well on the way to solving the remaining technical problems with thermometers, quarreling among the members reached an absurd height. In 1657 one member had written that "only disorder is to be found" at the Accademia. By 1666 some members could only be induced to speak at meetings if certain others were absent. The institution decayed. Louis the Sun King lured Christiaan Huygens to become the director of his Académie Royale and enlisted one of the key members of the Accademia del Cimento with a promise of a pension and the right to publish his experimental results under his own name; two others defected elsewhere, ostensibly to seek better climates for their health.

  Leopold was discouraged by the infighting, which was going on at the same time the Catholic Church was requesting that one of the Medicis become a cardinal. A former Accademia member's memoir suggests that shutting down the Accademia was made a condition for awarding Leopold the red hat of a cardinal, though other contemporary accounts dispute this notion. In any event, there was an abrupt end to the diary of the Accademia's experiments at the time Leopold was summoned to Rome. He sent a passing-the-baton letter to Constantijn Huygens, entreating him "to look over the great book of nature by means of experiments, and find new things never heard of before, and to purge books of those experimental errors that have been too easily believed, even by the most esteemed authors." In March 1668 Leopold journeyed from Florence to Rome to become a prince of the church; in the coach, as though in defiance of leaving science behind, he took along one of the Accademia's last strumentini and whiled away the hours of his journey recording its changing observations.

  Boyle obtained a Florentine thermometer, as did other English experimenters; Hooke tried to improve Boyle's in several ways, not the least of which was to substitute mercury as the liquid within the glass tubing; once again, however, for some unknown reason, mercury was soon rejected in favor of spirit of wine. Hooke adapted Florentines for himself and for his friend Christopher Wren. There was even an attempt to put the Royal Society's imprimatur on a thermometer adapted from a Florentine model with Hooke's modifications. Political considerations quashed that, but Hooke's own thermometric innovations advanced the field.

  By the 1660s this son of a parish curate, though still in his twenties, had matured from his stint as Boyle's assistant to become one of the most inventive and mechanically able of the Royal Society Fellows, having done significant work in microscopy, astronomy, geology, combustion, and meteorology. He also helped Wren rebuild London after the Great Fire of 1666. When the diarist Samuel Pepys became a member of the Royal Society, its president told him that Hooke did the most, and promised the least, of any of the Fellows. Bent almost completely askew, Hooke was described as "meanly ugly, very pale and lean"; John Aubrey agreed with that description but balanced the picture by adding that Hooke was "of great suavity and goodness." In later life, Hooke would run afoul first of Isaac Newton, then of Henry Oldenburg, secretary of the Royal Society; their combined opposition would succeed in downplaying his accomplishments and undercutting the acceptance of his work by the wider world.

  Those accomplishments were considerable, especially in regard to the scientific history of the cold. Hooke was the first person to minutely grade a thermometric scale with marks representing a precise volumetric quantity, each equal to one-thousandth of the expanding volume of spirit in the bulb. He was also among the first to assert that thermal expansion might be a general attribute of matter. "Th
is property of Expansion with Heat, and Contraction with Cold, is not peculiar to Liquors only, but to all kinds of solid Bodies also, especially Metals," he wrote, and italicized his conclusion: "Heat is a property of a body arising from the motion or agitation of its parts." Two hundred years would elapse before James Joule would prove this same conclusion experimentally and deduce from it the idea—central to the further exploitation of the cold—that heat is a form of energy related to the motion of atomic particles. Hooke was also among the first to propose establishing a permanent flagpost in the country of the cold, one that could be used for navigation by all later explorers: he sought to make the freezing point of water a fixed point of reference on a thermometer.

  Today the need for a thermometer to have such a fixed point may seem obvious, but near the end of the seventeenth century, fixed points were a matter of controversy. Some people thought the freezing point of water varied with the time of day, the latitude, or the season. Following the tenets of Hobbes—the old notion that atmospheric conditions changed as one approached the planet's poles—even such usually astute men as the English astronomer Edmund Halley contended that the freezing point of water would not be the same in London as in Paris. Not until the 1730s was that notion finally disproved.*

  Among the candidates for fixed points proposed during the second half of the seventeenth century were, on the lower end, the temperature of mixtures of salt and ice, of ice and water, and of near-freezing water. In the middle range the suggested fixed points were the temperature of the deepest cellar of the Paris Observatory (believed not to vary between summer and winter); the congealing points of aniseed oil, linseed oil, and olive oil; the temperature at which butter melts; and that at which wax melts. On the higher end they were the heat of the healthy human body, measured under the arm or in the anus; the internal temperature of certain animals; the maximum summer temperature in Italy, Syria, and Senegal; the boiling points of pure alcohol, spirit of wine, and water; the heat of a kitchen fire hot enough to roast foods; and the presumed temperature of the sun's rays. Halley, another Fellow of the Royal Society, recommended the boiling point of spirit, yet he also revealed why it was so difficult to establish any fixed point: "The spirit of wine used to this purpose [must] be highly Rectified or Dephlegmed for otherwise the differing goodness of the spirit will occasion it to boil sooner or later, and thereby pervert the designed exactness."

  The questions of fixed reference points and measuring liquids even lured Isaac Newton. An eighteenth-century historian of thermometers commented that in the way Newton "carried everything he meddled with beyond what anybody had done before him, and generally with a greater than ordinary exactness and precision, so he laid down a method of adjusting thermometers in a more definite way than had been done hitherto."

  Newton forthrightly set his zero at the freezing point of water, but his indication of the exact location of that zero reveals the difficulty of being precise as to where a flag should be planted: he described the point as "[t]he heat of the air in winter, when the water begins to freeze; and it is discovered exactly by placing the thermometer in compressed snow, when it begins to thaw." Newton was wrong in many of his thermometric ideas. His scale erred in being self-referential—the meaning of its points could be understood only by reference to one another, and not by reference to any widely recognized standards—and in having a bias toward measuring the upper temperatures more finely than the lower. As Newton went up the scale, his descriptions became longer and more exact. Number 17 was "Greatest degree of heat of a bath, which a man can bear for some time without stirring his hand in it," but above that, his choices increasingly involved materials that ordinary people would seldom encounter, such as the boiling of mixtures of water and metals, and they also used ratios that smacked of the magical—for instance, he insisted that the heat of boiling water was three times that of the human body, six times that of melting tin, and eight times that of melting lead, and that the heat of a kitchen fire of heaped coals was sixteen or seventeen times that of the human body.

  Newton contributed in a more novel and useful way in the matter of choosing the range of information the instrument could display, by proposing linseed oil as the fluid inside the thermometer bulb. This extract of flax was a brilliant choice, because it addressed the basic requirement that thermometers be able to measure accurately on the high and low ends of the scale. Being extremely viscous, linseed oil could remain liquid at temperatures above that at which water boiled and below that at which water froze. But linseed oil's viscosity made it slow to register changes—when the temperature was raised just a few moderate degrees, linseed oil would drain in a sluggish way down the sides of the tube on which the scale was marked. This long adjustment time made the linseed-oil thermometer difficult to use and may have been why a 1701 article on it by Newton in the Royal Society's Transactions was unsigned and only later attributed to him.

  The next innovator in thermometry was French physicist Guillaume Amontons, regarded as a relatively obscure figure today, but one whose work provided a critical foundation for others in the exploration of the cold, even well into the twentieth century. The son of a provincial lawyer, Amontons was deaf since birth, and he had been largely self-taught in the sciences when in 1687 he first sought attention for his work from the Académie Royale. The twenty-four-year-old demonstrated a new hygrometer that used two liquids for measurement, one of them mercury, and followed this up a year later with ideas for multiliquid barometers and thermometers. More than the innovations of a craftsman, these were the constructions of a man who understood basic principles and how to apply them. The instrument maker for the Paris Observatory thought Amontons's ideas worthy enough to be discussed with members of the Royal Society on a visit to London. Put into a 1695 book, these ideas earned Amontons admission to the Académie.

  His main contributions to thermometry and to the study of cold came by way of a detour. Amontons tried to create a "fire-wheel" that used the heat of a fire to expand air and make it move a wheel; he failed, but the principles he elucidated were sound and would be built on in later research by others on heat engines, horsepower, and friction. He also used some of the fire-wheel research to create better thermometers. Heating three unequal masses of air and water in glass bulbs submerged in boiling water, he demonstrated that the masses "increase [d] equally the force of their spring by equal degrees of heat," causing the air pressure to rise by one-third of an atmosphere in each bulb—1 atmosphere being equal to the pressure of air at sea level, 14.7 pounds per square inch.

  From this pressure work Amontons drew two important conclusions. First, even if the heated air had been afforded "the liberty of extending itself" instead of remaining confined within the glass bulbs, it still would not have increased its volume by more than one-third.* Second, since the water in all three bulbs boiled at the same temperature, despite variations in the volume of the water and air in the bulbs, the boiling point of water was proved to be a constant, one that could be used with confidence as a fixed point on a thermometer. Based on these results, Amontons designed a new air-based thermometer employing sealed glass; the sealing prevented distortions in the readings that would otherwise come from changes in atmospheric pressure.

  Amontons also advanced the cause of better thermometry by his tough criticism of that anonymous 1701 paper on the linseed-oil thermometer in the Transactions of the Royal Society—not knowing it had been written by Newton. He lambasted the author's contention that a body that on the author's self-referential scale had a temperature of 64 was twice as hot as one that registered at 32, charging that not enough was currently known about heat and cold to support such an assumption.

  As his critique of Newton's thermometry suggests, Amontons was a purist who did not often engage in the speculation rampant among researchers of the era. But he was a man who did pay attention to the implications of his mathematical computations; so, having proved that air contracts by a fixed proportional amount when cooled, he could not
avoid calculating what might happen to air if its temperature were radically reduced, to well below the freezing point of water. Would air become denser, and as the temperature dropped further, would air become a liquid? Would that liquid be water—current thinking favored that conclusion—or something else? In the total absence of heat, would there be any air pressure? In a 1703 paper, Amontons evolved a simple equation showing that a total absence of heat was theoretically possible.

  In the equation, the product of pressure times volume equals the product of temperature times an unknown constant. From this Amontons drew the clear implication that if by some means the product of pressure times volume became zero, on the other side of the equation the temperature could fall to an "absolute zero." Amontons did not come right out and say there was an absolute zero, because he considered such a thing incompatible with what was currently known about nature and with what he believed.

  For Amontons, absolute zero was a hypothetical construct to be imagined, not to be realistically pursued. Two years after his article was published, he died of an internal inflammation, at the age of forty-two. Central to his legacy was his understanding that in the grand scheme of things, human beings and most other life on earth lived not far, in temperature terms, from the freezing point of water, and that the country of the cold that began at the freezing point was far more vast than human beings had previously believed, promising temperatures below what any scale then in existence could measure, down to an almost mythical point, an absolute zero, the end of the end.

  Around 1702, while Amontons was doing his best work in Paris, in Copenhagen the astronomer Ole Rømer, who had calculated the finite speed of light, broke his leg. Confined to his home for some time, he took the opportunity of forced idleness to produce a thermometer having two fixed points, marking his scale at 7.5 for the melting point of ice and at 60 for the boiling point of water. His zero was thus well below the freezing mark, and supposedly represented the temperature of a mixture of salt and ice, while blood heat on this scale happened to fall at 22.5, or three times the melting-ice temperature. Rømer wasn't concerned very much with the upper and lower limits of his scale, because his primary work was in meteorology, which dealt with temperatures in the middle range. Six years after his broken-leg episode, Gabriel Daniel Fahrenheit visited him in Copenhagen. The young, Polish-born man had become fascinated by the making of scientific instruments and wanted tips on the techniques involved. And he may also have had another important reason for visiting Rømer.