Elegant Solutions. Philip Ball

‘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’.

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      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