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
