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CHAPTER 3

New Light

The Curies’ Radium and the Beauty of Patience

Paris, 1898–1902—In a cold and damp wooden shed at the School of Chemistry and Physics, the Polish scientist Marie Curie, occasionally assisted by her French husband Pierre, crushes and grinds and cooks literally tons of processed pitchblende, the dirty brown material left over from the mining of uranium. The Curies are convinced that this unpromising substance contains two new elements, which they name polonium and radium. After endless chemical extractions, Marie obtains solutions that glow with pale blue-green light: a sign that they contain the intensely radioactive element radium. The significance of the work is recognized straight away, as Marie and Pierre, still struggling to forge scientific careers in France, are awarded the Nobel prize in physics in 1903.

Marie Curie may well have felt ambivalent about becoming an icon for women’s place in science. Like several trail-blazing women scientists, she seemed eager that her sex be seen as irrelevant. Sadly, it was not. Those scientists, such as Albert Einstein and Ernest Rutherford, who accepted Marie without question as an equal, stand out for their lack of prejudice; most of the scientific community at the end of the nineteenth century was reluctant to believe that a woman could contribute new, bold and original ideas to science. When Marie was grudgingly awarded prizes for her groundbreaking studies of radioactivity, as likely as not the news would be communicated via her husband. It was only by a hair’s breadth that she was included in the decision of the 1903 Nobel committee. And when the Paris newspapers discovered that, several years after Pierre’s death, Marie had had an affair with one of his former colleagues, they bit on the scandal with relish, when similar behaviour from an eminent male scientist would probably have been deemed too trivial to mention.

It is hardly surprising, then, that Marie took great pride in her work, carefully emphasizing her own contributions and hastening to publish them in the face of Pierre’s habitual indifference to public acclaim. Marie knew that, to make her mark, she would have to achieve twice as much as her male colleagues. And she did – which is why she became the only scientist to win two Nobel prizes in science.

Her life has been so often romanticised – that process began as soon as the news came from Stockholm in 1903 – that Marie Curie herself has tended to disappear behind the stereotype of the tragic heroine. Yet it is true that her life was marred by several tragedies and by considerable adversity, and it is not surprising that in the end this left her hardened, appearing aloof and cold to those around her. Her determination and dedication to her work could translate as a certain prickliness and unfriendliness towards her colleagues. If she expected others to ignore the fact that she was a woman, likewise she herself had no concern about protecting brittle male egos.

In as much as she discovered new elements, Marie Curie did nothing that others had not done previously. But the elements that she unearthed in her long and arduous experiment were like nothing anyone had seen before.

New physics

The elements that debuted in the periodic table in the late nineteenth and early twentieth century hint at the unattractive nationalism of that age: they bear names like gallium, germanium, scandium, francium. We can hardly begrudge Marie Curie her polonium, however, since her own sense of national pride was born out of Russian oppression. Poland was then part of the Russian empire, and after a rebellion in 1863 (four years before Marie’s birth) the country suffered from an intensive programme of ‘Russification’, during which the tsar forbade the use of the Polish language in official circles. The struggle of the Polish intelligentsia against the Russian authorities was a dangerous business in which some lost their lives.

Figure 3 Marie Curie (1867–1934), the discoverer of radium and polonium

(© CORBIS)

When Maria Sklodowska (Figure 3) came to Paris to study science and mathematics at the Sorbonne, it must have seemed a land of opportunity – this was the Paris of Debussy, Mallarmé, Zola, Vuillard and Toulouse-Lautrec. Yet women risked their reputation simply by venturing out into the city alone, and Maria was more or less confined to her garret lodgings in the Quartier Latin. She graduated in 1894 and began working for her doctorate under the physicist Gabriel Lippmann, who later won the Nobel prize for his innovations in colour photography.

That year she met the 35-year-old Pierre Curie, who taught at the School of Chemistry and Physics and studied the symmetry properties of crystals. Pierre did not possess the right credentials to become part of the French scientific élite – he had studied at neither of the prestigious schools, the École Normale or the École Polytechnique – but nonetheless he had made a significant discovery in his early career. With his brother Jacques at the Sorbonne in 1880, he had found the phenomenon of piezoelectricity. When the mineral quartz is squeezed, the Curie brothers discovered, an electric field is generated within it. They used this effect to make the quartz balance, which was capable of measuring extremely small quantities of electrical charge. Pierre’s colleague Paul Langevin used piezoelectricity to develop sonar technology during the First World War.

Shy and rather awkward in public, Pierre had never married. His work was almost an obsession and he did not seem interested in acquiring a token wife. ‘Women of genius are rare’, he lamented in his diary in 1881. But he quickly recognized that Maria Sklodowska was just that kind of rarity, and in the summer of 1894, when she had returned temporarily to Poland, he wrote to her: ‘It would be a beautiful thing, a thing I dare not hope, if we could spend our life near each other hypnotized by our dreams: your patriotic dream, our humanitarian dream and our scientific dream’.

His courtship was a little unorthodox – he dedicated to Maria his paper ‘Symmetry in physical phenomena’. But it seemed to work: they were married in 1895, the same year in which Pierre (never one to rush his research) completed his doctoral thesis on magnetism.* The marriage delayed Marie (as she now called herself) from starting on her own doctorate, for she soon had a daughter, Irène, who was later to become a Nobel laureate too. This hiatus turned out to be doubly fertile, for Marie’s eventual research topic was a phenomenon discovered only in March 1896.

The fin-du-siècle produced something of a public fad for the latest science and technology. Gustave Eiffel’s steel tower rose above the Paris skyline in 1889 as a monument to technological modernism, and was quickly embraced as such by Parisian artists like Raoul Dufy and Robert Delaunay. Emile Zola claimed to be writing novels with a scientific spirit, and his book Lourdes (1894), which Pierre gave to Marie, was a staunch defence of science against religious mysticism. When Wilhelm Conrad Roentgen discovered X-rays in 1895 and found that they could ‘look inside’ matter by imprinting a person’s skeleton on a photographic plate, the public’s imagination was quickly captured – the Paris carnival parade of 1897 even had an ‘X-ray float’.

Roentgen made his discovery while investigating so-called cathode rays, which were emitted from negatively charged metal electrodes. These mysterious rays were typically produced in a sealed glass tube containing gases at very low pressure: the cathode ray tube. In 1895 the French physicist Jean Perrin, later a firm friend of the Curies, showed that cathode rays deposited electrical charge when they struck a surface. J. J. Thomson at Cambridge showed two years later that cathode rays were deflected by electric fields, and he concluded that they were in fact streams of electrically charged particles, which he called electrons.

Roentgen was studying cathode rays in 1895 when he noticed that some rays seemed to escape from the glass tube, causing a nearby phosphorescent screen to glow. This effect had already been noted previously by the German physicist Philipp Lenard, but Roentgen investigated it more closely. He found that the rays were capable of penetrating black cardboard placed around the tube. And when he placed his hand in front of the glowing screen, he saw in shadow a crude outline of his bones. In December of 1895 he showed that the rays would trigger the darkening of photographic emulsion, and in that way he took a photograph of the skeleton of his wife’s left hand.

These were evidently not cathode rays. It was already known that cathode rays were deflected by magnets, but a magnetic field had no effect on these new, penetrating rays. Roentgen called them X-rays, and scientists soon deduced that they were a form of electromagnetic radiation: like light, but with a shorter wavelength. The French scientist Henri Poincaré described Roentgen’s discovery to the Académie des Sciences in January 1896, and among those who heard his report was Henri Becquerel. Becquerel’s father had made extensive investigations of phosphorescence – the dim glow emitted by some materials after they have been illuminated and then plunged into darkness – and he wondered ‘whether . . . all phosphorescent bodies would not emit similar rays’. This was actually a rather strange hypothesis, for the phosphors on Roentgen’s screens were clearly receiving X-rays, not emitting them. All the same, Becquerel went looking for X-rays from phosphorescent materials.

That February he wrapped photographic plates in black paper and then placed phosphorescent substances on top and exposed them to the sun to stimulate their emission. But most of these materials generated no sign of X-rays – the plates stayed blank. Uranium salts, however, would imprint the developed plates with their own ‘shadow’. At first, Becquerel assumed that sunlight was needed to cause this effect, since after all that was what induced phosphorescence. He set up one experiment in which a copper foil cross was placed between the uranium salt and the plate, expecting that the foil would shield the photographic emulsion from the rays apparently emanating from uranium. A shadow of the cross should then be imprinted on the developed plate. But February is seldom a sunny month in northern Europe, and on the day that Becquerel set out to perform this experiment the sky was overcast. So he put the apparatus in a cupboard for later use. But the weather remained gloomy, and after several days Becquerel gave up. Again we have cause to be thankful for the fluid logic of Becquerel’s mind, for rather than just writing the experiment off and casting the photographic plate aside, he went ahead and developed it anyway. The uranium had received a little of the winter sun’s diffuse rays, after all, so there might at least be some kind of feeble image in the emulsion.

To his amazement, he found that ‘on the contrary, the silhouettes [of the copper mask] appeared with great intensity’. Thus, sunlight wasn’t needed to stimulate the ‘uranic rays’. Still in thrall to the idea of phosphorescence, Becquerel dubbed this ‘invisible phosphorescence’ or hyperphosphorescence.

At first his discovery made little impact. These ‘uranic rays’ were too weak to take good skeletal photographs, and most scientists remained more interested in X-rays. The Curies, however, recognized that Becquerel’s result was pointing to something quite unprecedented, and in early 1898 Marie decided to make this the topic of her doctorate. ‘The subject seemed to us very attractive’, she later wrote, ‘and all the more so because the question was entirely new and nothing yet had been written upon it’.

Return to the source

It was very much a joint project, which the Curies began in an empty store room of the School of Chemistry and Physics. They first found a method of quantifying the ‘activity’ of the uranic emissions by measuring their charging effect on a metal electrode. Becquerel had commented that the rays made air electrically conducting – as we’d now say, they ionize the air, knocking electrons out of the atoms and leaving them electrically charged. Pierre’s piezoelectric quartz balance now came into its own for measuring the amount of charge deposited on a metal plate due to a sample of uranium salt placed below it.

At first the Curies used relatively ‘pure’ materials: uranium salts given to them by the French chemist Henri Moissan. But in February 1898 Marie tested raw pitchblende – uranium ore, which was mined in the town of Joachimsthal in Saxony, where silver mining had been conducted since the Middle Ages. Remarkably, crude pitchblende turned out to be even more active than purified uranium. Likewise, whereas salts of the rare element thorium were also found to emit ‘uranic rays’, the raw mineral form of thorium (aeschynite) was more active than pure thorium compounds.

The Curies had a crucial insight: they hypothesized that the greater ionizing power of pitchblende was caused by an unknown element, more ‘active’ than uranium itself, which was present as an impurity in the mineral. To verify this, they compared another natural uranium mineral, chalcite, with ‘artificial’ chalcite synthesized chemically from uranium and copper phosphate. Superficially, the two materials should be identical; but the synthetic chalcite had only uranium-like activity, whereas natural chalcite was more active. So there was something else in this mineral too: some ingredient with a ‘uranic’ potency exceeding that of uranium. What they needed to do was to isolate it.

The Curies reported their findings and hypothesis to the Académie on 12 April. In effect, this report suggested that radioactivity could be used as a diagnostic signal to search for new elements: invisible to chemical analysis, the hypothetical new source of uranic rays betrayed its presence by its emission. ‘I had a passionate desire to verify this new hypothesis as rapidly as possible’, Marie wrote.

‘Passionate’ is not a word commonly associated with Marie Curie. She had been brought up to observe the genteel, reserved manners expected of a lady of that era. Even Einstein, who was fond of Marie, confessed that he found her ‘poor when it comes to the art of joy and pain’. There can be no doubt, judging from her own words, that she was devoted to her husband and her children, and her pain at the tragedies in her later life is clear and deeply felt. But she would, if she could, keep her passions for other people very private. The comment of Le Journal in 1911 on her affair with Paul Langevin was an example of pure tabloid lasciviousness – ‘The fire of radium had lit a flame in the heart of a scientist’ – and was met by her justifiably icy response in Le Temps: ‘I consider all intrusions of the press and of the public into my private life as abominable’. The only passion that Marie Curie permitted herself to reveal publicly was that for her work, and like Pierre’s, it bordered on obsession.

If there was a new element lurking in pitchblende, it would have to be separated by chemical ingenuity, and the Curies enlisted the help of a chemist named Gustave Bémont at the School of Chemistry and Physics. Two dissolved elements may be parted if one of them forms an insoluble compound while the other does not: the one can be precipitated and collected by filtering, while the other remains dissolved in solution. In such a procedure, an element present in only very small amounts can sometimes be separated by precipitating it along with some other element with which it shares chemical properties in common: the trace element gets entrained with the ‘carrier’. The Curies found that in fact pitchblende seemed to contain two new ‘active’ elements. One of them was chemically similar to barium, precipitating when chloride was added to a solution of the mixture to produce insoluble barium chloride. The other element seemed instead to ‘follow’ the element bismuth.

These separations involved laborious, repetitive procedures in which chemical products were crystallized from solution, washed and redissolved and then recrystallized. It was tedious, mind-numbing work. But the Curies tracked the progress of their labours by using the ionization apparatus to measure changes in the ‘activity’ of their samples. Since the new elements were more active than uranium, products in which they were enriched relative to pitchblende showed greater activity. By the summer of 1898 the Curies had increased the activity of their extract by a factor of around 300. They hoped that the new elements might be revealed in these enriched samples by the technique of spectroscopy. Elements irradiated by light re-emit some of the light in the form of distinct ‘spectral lines’ at specific wavelengths – this was how the element helium was discovered in 1868, when astronomers found previously unknown spectral lines in sunlight (the sun is rich in helium). But when the Curies gave their enriched samples to the French scientist Eugene Demarçay for analysis, he was unable to find any new spectral lines. There was still more purification to be done before the highly active elements would show themselves that way.

This did not prevent the Curies from presenting their findings to the Institut de France in July, in a paper read by Becquerel. That month, they had chosen a name for one of the new elements they were sure the samples contained. ‘We thus believe’, they said, ‘that the substance we have extracted from pitchblende contains a metal never before known, akin to bismuth in its analytical properties. If the existence of this metal is confirmed, we propose to call it polonium after the name of the country of origin of one of us’. The paper’s title introduced another new word: ‘On a new radio-active substance contained in pitchblende.’