Chapter Four

PIERRE (Uranium)

FOR HER DOCTORAL research, Marie turned away from magnetism, choosing instead to pursue the startling new energy exuded by uranium. So-called “uranic rays” had been discovered early the previous year, 1896, by the eminent Parisian physicist Henri Becquerel of the nearby natural history institution Le Muséum d’Histoire Naturelle. Becquerel’s finding had thus far failed to draw much attention from other physicists, because his uranic rays paled in comparison with a prior discovery, by Wilhelm Roentgen in Bavaria late in 1895, of “X-rays” that could pierce human flesh to expose the bones and organs.

The uranic rays’ relative lack of popularity among scientists made them all the more attractive as a dissertation topic for a novice such as Marie. While more than a thousand scientific papers had already been published about X-rays, she faced little or no competition on the topic of Becquerel’s uranic rays. Moreover, she could make the measurements she intended using instruments invented by her husband and his brother.

Becquerel had come upon uranic rays by accident, finding one thing while looking for another. He suspected that sunlight might elicit the newfound X-rays from certain natural materials, and he conceived an experiment to separate the effects of visible light from those of the invisible X-radiation. First he double-wrapped a photographic plate in heavy black paper. Atop this base he set a sample of a uranium salt called potassium uranyl sulfate—a crystalline compound with the ability to fluoresce (that is, to absorb light and emit its own glow in response). Becquerel planned to expose the wrapped plate and topping of salt crystals to the sunshine of broad daylight. The crystals would glow, and if that glow contained X-rays, then the X-rays would penetrate the black paper—as visible light could not—to impress an image of the crystals on the photographic plate.

As it happened, the February weather proved dreary, and the lack of sunlight compelled Becquerel to stow the wrapped plate and fluorescent material in a drawer while waiting for the skies to clear. Days later, under a still-lingering cloud cover, he retrieved the materials and, perhaps in frustration, developed the plate without having carried out the planned experiment. To his surprise, the plate revealed a vague image formed by the crystals that had been its companions in the drawer. There in the darkness, with no assistance from the sun or any other light source, the uranium salt had somehow transmitted a signal through the black wrapping, into the photographic emulsion.

Follow-up experiments convinced Becquerel that every uranium-containing substance available in the museum’s collections could work this same magic. Uranic rays not only penetrated black paper but also passed almost as easily through coverings of cardboard, aluminum, copper, and platinum. Stranger still, the effect did not diminish with time. Month after month, the uranic materials retained their ability to release uranic rays.

Becquerel tried a few other experiments with uranic rays before giving them up. He knew that several investigators across Europe had shown X-rays to be capable of draining electric charge—of “discharging” any electrified bodies in their path. A glass rod, for example, that had become electrified by being rubbed with a cloth would lose its charge in the presence of X-rays. Becquerel found that uranic rays were likewise capable of discharging, though he could not explain the effect.

It fell to physicist J. J. Thomson of the Cavendish Laboratory in England to align these findings, in 1897, with a new theory of the atom. Thomson’s atom was electrically neutral overall, comprising subatomic bits of equal but opposite charge, which he named “ions.” X-rays or uranic rays traveling through the air split the individual atoms of atmospheric gases, separating the positive ions from the negative ones. Thus “ionized,” or fragmented into ion pairs, the air became a conductor of electricity, through which charge drained away.

Mme. Curie saw in ionization a way to quantify the uranic rays released from various substances—by measuring the electrical conductivity they excited in the air around them.

At Pierre’s request, the director of his school ceded Marie a small storage room on the ground floor to use as her laboratory. The space was dank and unheated, but it was hers. She spent the first six weeks in her new niche setting up, trying out, and mastering her experimental apparatus. It consisted of a series of connected devices, both homemade and commercial.

At one end of a worktable, she placed the small ionization-testing chamber that she and Pierre had built of wood from grocery crates. Inside the chamber she mounted two metal discs, each eight centimeters (a little over three inches) in diameter, one above the other, just three centimeters (about an inch) apart. She attached the lower disc to a hundred-volt battery, and the upper one to an electrometer that gauged electric current in units named for French physicist André-Marie Ampère. The Curie piezoelectric quartz stood close by on its tripod, and was also hooked up to the electrometer. As Pierre and Jacques had shown in their pioneering work on piezoelectricity, a current could be elicited from the long, narrow quartz crystal in response to pressure, such as the stress of pulling on it.

To make a measurement, she sprinkled a thin layer of powdered uranium-containing material on the lower disc in the ionization chamber and flipped a switch closing the circuit between the disc and the battery. As the uranic rays ionized the air between the discs, current flowed from the charged bottom plate to the top one, at which point the electrometer responded, its needle rising from zero. Now Marie dropped a tiny weight into the balance pan suspended from the piezoelectric crystal, thus stretching it and causing it to generate an opposing current that nudged the needle back to zero. She needed to keep her eyes riveted on the readout and her right hand with its fistful of weights poised over the balance pan. When the current reached its peak value, or saturation point, and the crystal bore just enough weight to compensate, she clicked off the stopwatch in her left hand. The stronger the activity of the sample, the faster the current in the chamber reached saturation. Later she could calculate the current’s strength, in millionths of an ampere, from the weight that matched it, in grams.

She practiced the procedural steps repeatedly to achieve a smooth coordination between clocking and counterbalancing, but she could do little to control conditions in her laboratory. On February 6, 1898, she recorded a near-freezing room temperature of 6.25 degrees Celsius, which she emphasized in the lab notebook with ten exclamation points. The cold and damp affected the instruments nearly as much as they decreased her personal comfort, but she soon established the activity of pure uranium as a basis of comparison for all her other assessments. Patient testing proved that different compounds of uranium produced uranic rays in direct proportion to the amount of uranium they contained. The uranic-ray activity did not disappear or even dissipate when uranium combined chemically with other elements. Nor did the activity of her samples alter if she heated them to high temperatures, or exposed them to strong light, or bombarded them with X-rays. Nothing sapped uranium’s emissive power. On the basis of these observations, she concluded that the release of uranic rays must be an essential atomic property of uranium, as constant and defining as its atomic weight.

Yet uranium’s behavior defied the most fundamental physical principles. Basic “laws” of physics stipulated that energy could be neither created nor destroyed, but only changed from one form to another. What form of energy existed inside an atom of uranium to generate uranic rays? Was the emissive power the province of uranium alone? She began testing other materials to find out. Gold and copper, she noted, showed “no rays.”

At home, with similar diligence, she continued to record Irène’s progress. That February, for example, the five-month-old child, who could already “change her position on the bed by rolling” and “hold objects in her hand,” suddenly became “afraid of strangers and unfamiliar objects, loud voices, etc.”

When Marie had worked through all the substances she had on hand at the school, she borrowed others from fellow scientists. A uranium-rich ore known as pitchblende gave her a jolt when it registered more activity than pure uranium. The odd result made her repeat a number of prior measurements. As she had come to expect, the other uranium-containing compounds consistently evinced less activity than pure uranium. But again and again, and altogether contrary to her expectations, the pitchblende gave off more radiation than its content of uranium could explain.

As she tried to make sense of those results, she continued testing other materials. Soon she found that the element thorium, another heavy metal like uranium, emitted the same sort of spontaneous radiation. This meant that uranium was not the sole source of uranic rays. Indeed, she now suspected that pitchblende owed its strikingly high activity to some as yet unknown element hidden in the mix of its ingredients.

Pierre put aside his crystal-growth experiments to join Marie on the promising new tack her research had taken. Together they filled the remaining pages of the shared notebook with numbers and lists summarizing her findings, and with a graph comparing the density and composition of all the uranium- and thorium-containing compounds. In mid-March 1898 they started a new lab notebook in their joint pursuit of pitchblende’s active component. On March 31, at home, Marie documented the discovery of Irène’s first tooth.

The work Marie had accomplished to date merited presentation to the Académie des Sciences, the authoritative and influential body to which Henri Becquerel belonged. Marie could not deliver the report herself, because she did not belong to the Académie. No woman had ever been elected to membership. Nor had Pierre attained the lofty status of an Académicien. Marie turned for help to her former professor, Gabriel Lippmann, who read aloud her paper, “Rays Emitted by the Compounds of Uranium and Thorium,” at the April 12 meeting.

The most stunning remarks in Mme. Curie’s paper concerned the mineral pitchblende. She had examined three samples of the blackish ore, two from mines in eastern Europe and one from Cornwall in England. Each had yielded a different activity reading, with one of them tripling and another quadrupling the value for uranium. “This fact is very remarkable,” she affirmed, “and leads to the belief that these minerals may contain an element which is much more active than uranium.”

Now all she had to do was find it.

IN THE GROUND, a chunk of blackish-brown pitchblende rock had a dull, greasy look, akin to bubbling tar. The ore was mined for its chief component, uranium oxide, which found wide commercial use as a pigment for coloring glass and pottery a greenish shade of yellow. The means for extracting uranium oxide were already well known and long practiced by the time Marie Curie took an interest in pitchblende. But she was seeking a trace element, perhaps only 1 percent of the ore by weight. She had stumbled upon the unknown substance in her makeshift laboratory, using novel techniques that formed no part of the traditional chemist’s or prospector’s tool kit. Indeed, the sole known property of her supposed new element was its active emission of uranic rays.

As physicists, neither Pierre nor Marie possessed the knowledge and experience to chemically dissect pitchblende and identify the unknown radiation source it harbored. What they lacked in background, however, they more than made up for in motivation and advice from colleagues. They obtained a hundred-gram (quarter-pound) lump of pitchblende, pulverized it, and began attacking the powder with acids to break it down.

At each stage of their chemical assay, the Curies tested the breakdown products for uranic rays. After the initial acid attack, for example, they treated the solution with hydrogen sulfide, a colorless gas that smelled like rotten eggs; the uranium and thorium remained in the solution, but other substances reacted with the sulfur and fell to the bottom of the beaker as a solid precipitate that proved very active. It contained familiar elements—lead, bismuth, copper, arsenic, and antimony, all recognizable by their behavior in the reactions—and presumably also the mystery element, because Marie had already tested all the known elements, and none of them could account for the electrical effects that the precipitate excited in the ionization chamber.

Next they dissolved the precipitate in ammonium sulfide, and this time the arsenic and antimony stayed in solution, while everything else precipitated out. Continuing in careful stages with different reagents, they arrived at last at a smidgen of residue that behaved chemically like bismuth—a whitish metal similar to lead—except for the fact that it emitted the telltale rays. It was four hundred times more active than pure uranium.

Henri Becquerel, who had listened attentively to the reading of Marie’s April report, visited the Curies several times that spring at their lab in the industrial school. In July, when they were ready to release the next outcome of their research, he represented them at the Académie.

Marie had taken to calling uranic rays by the more general term “Becquerel rays” after she detected them in thorium. Now she introduced an altogether original term for the ability of select heavy elements such as uranium and thorium to radiate: “radio-active” appeared for the first time in the title of the report she coauthored with Pierre, “On a New Radio-active Substance Contained in Pitchblende,” which Becquerel read to the assembled Académiciens on July 18, 1898.

The Curies admitted they could not yet separate their new radio-active substance from bismuth by any means. Nevertheless, they felt so certain of the element’s existence that they had already christened it: “We propose to call it polonium from the name of the country of origin of one of us.”

Marie mailed a copy of the discovery report to her cousin Józef Boguski at the Museum of Industry and Agriculture, who saw to its immediate publication in a monthly Warsaw magazine. At the same time, she received an incentive award of nearly 4,000 francs from the Académie des Sciences, which greatly enriched the Curies’ research fund. Although Marie had won the Prix Gegner in recognition of her work with magnetized steel and of her recent investigation into uranic rays, word of the honor reached her indirectly, via letters to Pierre. “I congratulate you most sincerely,” said one from an Académie official, “and beg you to present my respectful compliments to your wife.” Even the award certificate, though it had “Madame Curie” penned in at the top, addressed the honoree twice as “Monsieur.”

The Curies spent the summer holiday of 1898 in a small house they rented in the Auvergne region in south-central France. From their base at Auroux, they made bicycle tours to the surrounding towns, explored the hills and grottoes dotting the volcanic landscape, and introduced Irène to the pleasures of skinny-dipping. “For the past three days we have bathed her in the river,” Marie noted in her private journal on August 15.

When they returned to Paris, they ordered more of the expensive pitchblende ore from the St. Joachimsthal mine in Bohemia. Pitchblende had shown itself to be such a medley of materials that it merited further analysis. A chemist at Pierre’s school, Gustave Bémont, assisted the couple in systematically deconstructing the second quantity of pitchblende, leading to the discovery of a second new radio-active element. In the same way that polonium had adhered to bismuth—signaling its individuality by radio-activity alone—the Curies’ second find clung to barium. And Marie had already shown that pure barium lacked any hint of radio-activity.

Once again, the property of radio-activity indicated the presence of an otherwise undetectable element.

Before announcing their result publicly, the Curies sought corroborating evidence from the field of spectroscopy. The technique of spectrum analysis, developed in the 1860s, gave chemists the means to identify elements by the color of light they emitted when heated to incandescence. Each element proclaimed its presence through one or more wavelengths of light, in a pattern as distinctive as a fingerprint. A few elements that had been discovered by means of spectroscopy bore the names of the colors that revealed them, such as cesium (sky blue), rubidium (ruby red), and thallium (from the Greek word for a fresh green twig). The Curies had offered their quantum of polonium to the well-known spectroscopist Eugène Demarçay, who, unfortunately, had failed to find any spectral lines aside from those of bismuth in the tiny sample. They returned to Demarçay now, full of hope, and this time he detected a line in the near ultraviolet belonging to no known element. Better yet, the intensity of the line increased or decreased according to the level of radio-activity in each of the several specimens he was given to examine.

The new element practically named itself—not by any color but rather by its extraordinary degree of radio-activity, which multiplied that of uranium a thousandfold. The Curies and their collaborator Bémont thought the difference might be even greater, but they had run out of pitchblende and could go no further till they got more. On the day after Christmas in 1898, Henri Becquerel informed the Académie des Sciences of the discovery of “radium.”

Although Becquerel shared the Curies’ excitement, the news of the new element elicited little response outside a small circle of physicists.

WITHIN HER FAMILY circle in the five months between the announcements of polonium and radium, Marie lost the ever-ready companionship of her sister Bronya. The doctors Dluski moved in the autumn of 1898 to Zakopane, part of Austrian Poland in the Tatra Mountains, to create a modern tuberculosis sanitarium.

“You can’t imagine what a hole you have made in my life,” Marie wailed to Bronya in early December. “With you two, I have lost everything I clung to in Paris except my husband and child. It seems to me that Paris no longer exists, aside from our lodging and the school where we work.”

As she pressed forward at her laboratory over the following months, and chased after the toddler who could now walk unassisted, she wrote to Bronya in more even tones: “I miss my family enormously, above all you, my dears, and Father. I often think of my isolation with grief. I cannot complain of anything else, for our health is not bad, the child is growing well, and I have the best husband one could dream of; I could never have imagined finding one like him. He is a true gift of heaven, and the more we live together the more we love each other.”