Elegant Solutions. Philip Ball

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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 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.’