On the practical front, Onnes—like Dewar—was hampered by the lack of an adequate supply of helium gas. The two had traded letters about helium for some time, and an Onnes assistant, acting as an emissary from Leiden, had even visited Dewar's laboratory. Each scientist complained to the other of bouts of ill health that kept them from their research, and of the "fairly risky business," as Dewar put it, of working with helium. "I have already lost 1 cylinder of helium by the breaking of vacuum vessels during the course of its circulation at liquid air temperatures and I dread any repetition of the disaster," Dewar wrote. Worried he would not be physically well enough to complete the work, he penned a poignant wish to Onnes to possess again "the gift of youth so that I might begin my scientific career after a training in the great Dutch School of Science."
Perhaps the comradely and confessional tone of Dewar's letters encouraged Onnes in mid-1905 to take the extraordinary step of proposing to Dewar that they join forces. Dewar had built a plant to extract helium from the Bath Springs sands; Onnes begged to share that material with him, and he implied that they might jointly work through the "determination of the isotherms of helium at low temperatures as well as the magnetic dispersion of the plane of polarization."
"We both want the same material in quantity from the same place, at the same time, and the supply is not sufficient to great demands," Dewar wrote back, in anguished but firm handwriting. He added, "It is a mistake to suppose the Bath supply is so great. I have not been able so far to accumulate sufficient for my liquefaction experiments. If I could make some progress with my own work the time might come when I could give you a helping hand which would give me great pleasure." Then, perhaps remorseful that he had to be so tough with Onnes, Dewar confided that things in London were "in a sad way," that his illness prevented him from doing much work, and that he hoped to get away for a rest. The regretful tone continued in a note a month later, in which he apologized for having not cited Onnes's work in an article.
Dewar's refusal to share the Bath helium probably had more to do with his ego than with the difficulty of obtaining helium. He was not averse to collaboration, having toiled for many years with various scientists on jointly signed work. But he had been laboring by himself for nearly thirty years on reaching absolute zero, and he was not about to change his solo style when the competitors were on the last lap of the race.
Dewar's decision not to share the Bath Springs helium with Onnes may have prevented a competitor from using that source, but he had no joy of it, because when he transported the Bath Springs helium to the Royal Institution, he found it contaminated with other gases and needing further purification, which slowed his progress. Just when Dewar's disappointment was becoming obvious, Onnes found his own supply of helium.
Through the auspices of his brother Onno, who was in charge of the Dutch government's Office of Commercial Intelligence, in 1905 Onnes arranged to import large quantities of monazite sand from gravel pits in North Carolina, sand known to contain significant amounts of rare-earth metals and helium. After that, as Onnes later wrote, "the preparation of pure helium in large quantities became chiefly a matter of perseverance and care." That was an understatement, since the process of extracting helium gas from the sand was complex, involving exploding the sand with oxygen, using liquid hydrogen to freeze out certain gases, and compressing the helium gas until it was absorbed by a charcoal filter, from which it could be recovered. Four chemists, working continuously over the next three years, were able to produce barely enough helium for experimental purposes.
During that period, Onnes remained uncertain that his attempt to liquefy helium would succeed, because scientists believed that at very low temperatures the Joule-Thomson effect might not further lower the temperature of a gas but might actually raise it. More perplexing was the unknown temperature at which the gas would become a liquid. All three laboratories had used a test devised by Dewar to measure how much helium a charcoal filter would absorb at the temperature of liquid hydrogen, but the results differed. Based on this test and others, Olszewski guessed that the critical temperature of helium might be less than 2 degrees above absolute zero, Dewar now thought it would be closer to 8 degrees above, and Onnes's own best estimate was 5 to 6 degrees.
Onnes recognized that the differences between 8, 5, and 2 degrees were highly significant, and he wrote that the results of the various laboratories' charcoal tests not only "left room for doubt" in his mind; far worse, they begat "ample room for fear that helium should deviate from the law of corresponding states." For a quarter century, Onnes had been conducting low-temperature research to experimentally verify the theory of corresponding states articulated by van der Waals. If the critical temperature of helium was really significantly lower or higher than the 5 K predicted by van der Waals's law, then that law of corresponding states would not apply to all elements, which would mean it was not universal, and his friend's theory might have to be junked. Moreover, in practical terms, a 2 K critical temperature would also mean that even the most advanced state-of-the-art apparatus would not be able to force the gas to become a liquid. "So it remained a very exciting question what the critical temperature of helium would be. And in every direction ... we were confronted by great difficulties."
It was at this moment that Kamerlingh Onnes's determination of the isotherms rescued him from paralyzing doubt, because the mathematical calculations showed that the temperature at which helium ought to liquefy was between 4 and 5 degrees above absolute zero—not, as he had feared, significantly higher or lower than that. Dewar now began to agree with him that the critical temperature was more likely to be nearer 5 K than 8 K.
Onnes pored over the details of the work of Hampson and Olszewski, trying to figure out whether his adaptations of their machinery would do the job. Once, in March 1908, he thought that he had accomplished the task, but in an odd way—that he had compressed helium into becoming a solid, without its ever having stopped at the liquid state. He dashed off a telegram to Dewar to this effect, and received a congratulatory telegram in return, before having to retract his claim because the solid he had produced was composed not of helium but of impurities in the gas.
Before the retraction reached the Royal Institution, Dewar had written a letter to the London Times noting the feat. The letter was published, and then Dewar had to apologize for it to Onnes: "I felt a duty to inform the world ... that you had succeeded where I failed. Considering the enormous difficulty of such experiments we can all be mislead." In this letter to Onnes of April 15,1908, Dewar all but conceded that Onnes would shortly win the race. He expressed admiration for Onnes's ability to admit mistakes, to identify the problem that caused them, and to move ahead. As for himself, he wrote, his health was improving, though only slowly; more to the point, "the Royal Institution has no money to prosecute such many extensive experiments...[and] has no endowment to draw on." What Dewar did not inform Onnes directly about was a near catastrophe in his lab: during an attempt at lowering the temperature of helium, the impurities in the gas froze and clogged a capillary tube; in a too-quick reaction, one of Dewar's assistants turned a vent the wrong way, and many months' supply of hard-won helium gas escaped into the upper atmosphere. This, Dewar recognized, was fatal to his chances to win the race.
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.
11. A Sudden and Profound Disappearance
WHEN JAMES DEWAR LEARNED that Heike Kamerlin
gh 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.
Absolute Zero and the Conquest of Cold Page 19