Absolute Zero and the Conquest of Cold (25 page)

Onnes, chastened himself, wrote Dewar a long letter detailing the reasons for his mistaken claim of liquefaction, and in an article he recorded that during the preliminary trials, he had found that "the utmost was demanded of the necessary vacuum glasses," and he worried that "the bursting of the vacuum glasses during the experiment would not only be a most unpleasant incident, but might at the same time annihilate the work of many months." To Dewar, Onnes explained that when he had produced what he thought was a liquid condensate in the midst of a cloud of helium gas, "the tube broke and so I could not have more certainty about the nature of the cloud."

On July 9, 1908, preliminary operations toward the liquefaction of helium began at Leiden, with the cranking up of the first three steps of the cascade, which used chloromethane to liquefy ethylene, the ethylene to liquefy oxygen, and the oxygen to liquefy air. Then the liquid air was used in the fourth step, to make liquid hydrogen.

July 10,1908, began early for the Low-Temperature laboratory at Leiden; at 5:45 in the morning, preparations started for the fifth step, the liquefaction of helium. As Onnes would shortly write in
Communication No. 108,
everything was in place for this descent: the prepared bottles of liquid hydrogen, the supply of helium purified from the monazite sands and held "ready for use in silvered vacuum glasses," a gas thermometer based on helium gas kept at low pressure, the heat exchangers, the tubing and stopcock apparatus for using Joule-Thomson expansion, the adapted Cailletet compressor with a mercury piston for pressuring the gas—seven years had been required to put it into working condition—and the vacuum cryostats, reinforced glass vessels into which the results of the experiment would flow. The apparatus consisted of both massive iron components and delicate pipettes, boilers and cryostats, sealed bolts and fan belts, slabs of metal and thin wires. Onnes, his chief assistant Gerrit Flim, some colleagues, and several "blue boys" (instrument-maker students) had rehearsed the procedure many times.

Some among the onlookers knew that this laboratory was just steps away from the spot where in the eighteenth century the great chemist Boerhaave had taught his students about the near boundaries of the country of the cold, using as a text Boyle's
Experiments Touching the Cold.
Now the explorers were nearing the epicenter of that country.

It took Onnes's team until 1:30 in the afternoon to make certain that the helium in the apparatus had been entirely cleared of the last traces of air, "by conduction over charcoal in liquid air" through a
side conduit. Every stray nook and cranny of the apparatus was filled with liquid hydrogen to protect from the unwanted influx of heat, and to purge any atmospheric air left in the apparatus, as that would have solidified at the low temperatures needed for the liquefaction of helium, producing a snow that could have clouded the glass and made observation of the liquid helium impossible.

At 3:00 in the afternoon, work was so intense that when Onnes's wife, Elisabeth, showed up with sandwiches for lunch, he could not stop to eat them, and Elisabeth fed them to him, bit by bit, as he gave instructions, turned dials, and watched gauges.

They were using a thermometer based on helium—employing helium gas at low pressure to measure the temperature of helium gas approaching liquefaction, a technique dependent on the pressure-volume equation in a way that might have delighted Robert Boyle. Since pressure multiplied by volume is proportional to temperature, and since the volume of helium gas in the thermometer was constant, measuring the pressure revealed the temperature.

At 4:20 in the afternoon, the apparatus and the helium gas in the canister were both at the proper temperature,—180°C. A gauge was turned to let helium gas into the apparatus, and the protecting glass was filled with liquid hydrogen. Since the experimenters could not see into the interior of the apparatus, the thermometer alone would tell them what was happening inside. At first, Onnes wrote, "the fall of the helium thermometer which indicated the temperature under the expansion cock, was so insignificant, that we feared it had got defect [sic].... After a long time, however, the at first insignificant fall began to be appreciable, and then to accelerate."

They added more liquid hydrogen, and increased the pressure on the helium; by 6:35 in the evening, the temperature for the first time fell below that of liquid hydrogen. Combinations of more and less pressure, and varying expansion volumes, were tried; the thermometer once dipped as low as 6 K, then wavered upward.

By this time, word had gotten out to scientific colleagues at the university that the critical moment had come in the Low-Tempera ture laboratory, and people began to drift in to watch. Among them was Professor Franciscus Schreinmakers. Onnes was calm, but he could not refrain from remarking the moment when the last bottle of liquid hydrogen was let into the apparatus: if helium was not liquefied now, it would be some time before the stores were replaced and there could be another attempt.

Raising and then lowering the pressure to 75 atmospheres produced a "remarkably constant" reading of the thermometer at 5 K. It was 7:00 in the evening, and yet nothing could be seen in the glass receptacle. Schreinmakers suggested to Onnes that the refusal of the thermometer to budge from the 5 K reading was similar to what would occur "if the thermometer was placed in a liquid." Going beneath the vessel with an electric light, Onnes peered up at it, and saw clearly the outline of a liquid in the vessel, "pierced by the two wires of the thermoelement." "It was a wonderful moment," he later remembered: the surface of the liquid helium "stood out sharply defined like the edge of a knife against a glass wall." He had liquefied helium. His only regret, just then, was that he could not show liquefied helium to his friend van der Waals, "whose theory has been guide in the liquefaction up to the end." That gratification would have to wait a few days, since van der Waals was in Amsterdam, not in the small crowd of observers at the laboratory.

Though the liquefaction of helium had been predicted, it was nonetheless a spectacular achievement, a triumph of science harnessed to technology. In one last leap that built on all of the preceding ones, Onnes had lowered temperatures to a point scientists believed approximated the conditions of interstellar space, a point very near to a physical limitation of matter. Absolute zero lay ahead, but there was growing doubt that it could ever be reached, though not because of a lack of technological prowess.

Onnes sent a telegram announcing the liquefaction of helium to Dewar; it bore the wrong date, July 9, a date Dewar would repeat in a footnote to an article he was then in the process of composing.

Dewar's response encapsulated his complicated feelings at this event:

CONGRATULATIONS GLAD MY ANTICIPATIONS OF THE POSSIBILITY OF THE ACHIEVEMENT BY KNOWN METHODS CONFIRMED MY HELIUM WORK ARRESTED BY ILL HEALTH BUT HOPE TO CONTINUE LATER ON.

The Leiden team ran the machinery for two hours more, and then shut it down. "Not only had the apparatus been strained to the uttermost during this experiment and its preparation, but the utmost had also been demanded from my assistants." Onnes made certain in his
Communication No. 108
to express his "great indebtedness" to Flim for his "intelligent help" in having constructed the apparatus.

In that communication, written a few days after the event, Onnes recounted additional experiments he had conducted on the liquid helium, rapid attempts to discern its properties while he and his associates still had the liquid in the vacuum flask. He noted that several things about the liquid helium were what Dewar had predicted—the small surface tension, the difficulty of seeing the liquid, even the critical temperature, which Dewar had once postulated at 8 K and then revised to 5 K, and which was reached at 4.5 K. Dewar appeared to have been wrong only about the density of the liquid in relation to the saturated vapor; it was eleven times as dense, not seventeen times.

Beyond those expected parameters, there were some strange and inexplicable findings. Liquid helium had unprecedented low surface tension. Onnes believed that liquid helium's most astonishing property was its density, eight times lighter than that of water, which he thought might account for liquid helium's low surface tension—but then, hydrogen was also lighter than water, and it had a discernible surface tension. A second curious finding was the failure of the liquid helium to solidify when cooled further by the same tech
niques Dewar had used to solidify hydrogen. No current theory could account for either of these results. Nor had Onnes explored such areas as magnetism, electrical conductivity, and other properties of matter already known to be grandly affected in the nether regions of temperature.

For all these reasons, it became clear to Kamerlingh Onnes that, having reached the penultimate landmark of Frigor, the task of learning more about the behavior of matter in the ultracold environment was just beginning.

W
HEN JAMES DEWAR LEARNED
that Heike Kamerlingh Onnes had liquefied helium, he upbraided his assistant Robert Lennox for failure to provide him with a good enough separator of helium gas from the Bath Springs sands so he could have reached that goal in advance of his rival. The two Scots quarreled, and Lennox walked out of the Royal Institution, vowing never to return until Dewar was dead.
^*
The defection of Lennox was a heavy blow to Dewar's low-temperature research, and after Lennox's departure, Dewar turned back from his march toward the cold pole and he never made another attempt on absolute zero, dropping his work on liquefaction to pursue other scientific endeavors. He did, however, dutifully report Onnes's achievement in liquefying helium to a meeting of the BAAS; and when Elisabeth Onnes sent him a telegram saying that her husband had taken ill, he wrote back that he was "grieved to learn" that the "great" man was ill, "but not surprised after the strain of his epoch-making work." In time, Dewar consoled himself with the belief, as he put it in a letter to Onnes, that "we must not forget what we have done someone else might have done," provided they had the aptitude, the funding, and the resources to do the work.

Dewar was even sicker now than in past years; before long, he would be operated on for cancer of the vocal cords and would require some time to recover. Unwilling to leave low-temperature research behind completely, he let Onnes enlist him, along with Olszewski and Linde, in a new institute to draw up standards for the refrigeration industry and for laboratories working in the ultracold environment. Shortly, Onnes became known in the press as "the gentleman of absolute zero," but it must be recognized that Dewar, at least in his relations with the man who had beaten him to the liquefaction of helium, also behaved as a gentleman.

After the liquefaction of helium, Kamerlingh Onnes was virtually alone in the field. Only he possessed the ability to produce liquid helium, and although his process did not make much in each run, if he managed carefully there would be enough liquid helium for conducting experiments. The knowledge that he had something of a monopoly made him redirect the thrust of all four fields of physics at Leiden; from 1908 on, 75 percent of the research done in thermodynamics, electricity, magnetism, and optics used low temperatures as a tool. For his own research, the first experiments were to continue the drive toward the cold pole. Very soon, by pumping off helium vapor, he depressed the temperature of the liquid helium to within 1.04 K of absolute zero. However, when it seemed apparent that manipulating the pressure did not push the liquid helium into the solid state or further reduce the temperature, Onnes called a halt in the attempts to reach absolute zero and decided instead to use liquefied helium to test the properties of matter in the neighborhood of a few degrees above absolute zero.

Part of the reason for Onnes's abandoning his prior single-minded quest for the cold pole was that absolute zero was now agreed to be impossible to achieve. In 1905 Walther Nernst had shown definitively that it was effectively at an infinite distance.

The vague idea that absolute zero existed but was unreachable had been around for a while. Nernst solidified it by logically relating it to Rudolf Clausius's concept of entropy. Examining the results of liquid-hydrogen experiments, Nernst contended that what lowered the temperature was the extraction of heat by evaporation, which reduced the entropy of the liquid. Nernst argued that the total energy of a system was constant so long as it was isolated. But when the system interacted, the change in energy equaled the sum of the work done on it and the heat absorbed by it. Nernst concluded that as the system's temperature neared absolute zero, its entropy would steadily vanish. This notion he formulated into what came to be called the third law of thermodynamics: as temperature approaches absolute zero, entropy approaches a constant value, taken to equal zero. This was very different from Amontons's belief that at absolute zero, all
energy
would vanish; but Amontons wrote 150 years before Clausius invented the concept of entropy. Nernst's third law implied that absolute zero could never be reached because the closer it was approached, the more difficult became the conversion of heat energy into entropy. So absolute zero was effectively at an infinite distance, and therefore unattainable.

The feeling that the cold pole was beyond reach refocused post-helium-liquefaction research, aiming it again at where Dewar and Fleming had left off in the late 1890s, the altered properties of matter at ultra-low temperatures. Among the most interesting one they had investigated was the deep drop in electrical resistance. A substance's electrical resistance is the degree to which it retards the passage of an electric current through it. Low resistance or "resistivity" means a substance is a good conductor; high resistance, a good insulator. As Onnes wrote, he and Jacob Clay undertook "to corroborate and extend earlier measurements by Dewar" on the decline of resistance at low temperatures.