They continued rendering their pitchblende residue down, and in July of 1898 they were able to make a bismuth extract four hundred times more radioactive than uranium itself. Knowing that spectroscopy could be thousands of times more sensitive than traditional chemical analysis, they now approached the eminent rare-earth spectroscopist Eugene Demarcay to see if they could get a spectroscopic confirmation of their new element. Disappointingly, no new spectral signature could be obtained at this point; but nonetheless, the Curies wrote,
… we believe the substance we have extracted from pitchblende contains a metal not yet observed, related to bismuth by its analytical properties. If the existence of this new metal is confirmed we propose to call it polonium, from the name of the original country of one of us.
They were convinced, moreover, that there must be still another radioactive element waiting to be discovered, for the bismuth extraction of polonium accounted for only a portion of the pitchblende’s radioactivity.
They were unhurried – no one else, after all, it seemed, was even interested in the phenomenon of radioactivity, apart from their good friend Becquerel – and at this point took off on a leisurely summer holiday. (They were unaware at the time that there was another eager and intense observer of Becquerel’s rays, the brilliant young New Zealander Ernest Rutherford, who had come to work in J.J. Thomson’s lab in Cambridge.) In September the Curies returned to the chase, concentrating on precipitation with barium – this seemed particularly effective in mopping up the remaining radioactivity, presumably because it had close chemical affinities with the second as-yet-unknown element they were now seeking. Things moved swiftly, and within six weeks they had a bismuth-free (and presumably polonium-free) barium chloride solution which was nearly a thousand times as radioactive as uranium. Demarcay’s help was sought once again, and this time, to their joy, he found a spectral line (and later several lines: ‘two beautiful red bands, one line in the blue-green, and two faint lines in the violet’) belonging to no known element. Emboldened by this, the Curies claimed a second new element a few days before the close of 1898. They decided to call it radium, and since there was only a trace of it mixed in with the barium, they felt its radioactivity ‘must therefore be enormous.’
It was easy to claim a new element: there had been more than two hundred such claims in the course of the nineteenth century, most of which turned out to be cases of mistaken identity, either ‘discoveries’ of already known elements or mixtures of elements. Now, in a single year, the Curies had claimed the existence of not one but two new elements, solely on the basis of a heightened radioactivity and its material association with bismuth and barium (and, in the case of radium, a single new spectral line). Yet neither of their new elements had been isolated, even in microscopic amounts.
Pierre Curie was fundamentally a physicist and theorist (though dexterous and ingenious in the lab, often devising new and original apparatus – one such was an electrometer, another a delicate balance based on a new piezo-electric principle – both subsequently used in their radioactivity studies). For him, the incredible phenomenon of radioactivity was enough – it invited a vast new realm of research, a new continent where countless new ideas could be tested.
But for Marie, the emphasis was different: she was clearly enchanted by the physicality of radium as well as its strange new powers; she wanted to see it, to feel it, to put it in chemical combination, to find its atomic weight and its position in the periodic table.
Up to this point the Curies’ work had been essentially chemical, removing calcium, lead, silicon, aluminium, iron, and a dozen rare-earth elements – all the elements other than barium – from the pitchblende. Finally, after a year of this, there came a time when chemical methods alone no longer sufficed. There seemed no chemical way of separating radium from barium, so Marie Curie now began to look for a physical difference between their compounds. It seemed probable that radium would be an alkaline earth element like barium and might therefore follow the trends of the group. Calcium chloride is highly soluble; strontium chloride less so; barium chloride still less so – radium chloride, Marie Curie predicted, would be virtually insoluble. Perhaps one could make use of this to separate the chlorides of barium and radium, using the technique of fractional crystallization. As a warm solution is cooled, the less soluble solute will crystallize out first, and this was a technique which had been pioneered by the rare-earth chemists, striving to separate elements that were chemically almost indistinguishable. It was one that required great patience, for hundreds, even thousands, of fractional crystallizations might be needed, and it was this repetitive and tantalizingly slow process that now caused the months to extend into years.
The Curies had hoped they might isolate radium by 1900, but it was to take nearly four years from the time they announced its probable existence to obtain a pure radium salt, a decigram of radium chloride – less than a ten-millionth part of the original. Fighting against all manner of physical difficulties, fighting the doubts and skepticisms of most of their peers, and sometimes their own hopelessness and exhaustion; fighting (although they did not know it) against the insidious effects of radioactivity on their own bodies, the Curies finally triumphed and obtained a few grains of pure white crystalline radium chloride – enough to calculate radium’s atomic weight (226), and to give it its rightful place, below barium, in the periodic table.
To obtain a decigram of an element from several tons of ore was an achievement with no precedent; never had an element been so hard to obtain. Chemistry alone could not have succeeded in this, nor could spectroscopy alone, for the ore had to be concentrated a thousandfold before the first faint spectral lines of radium could even be seen. It had required a wholly new approach – the use of radioactivity itself – to identify the infinitesimal concentration of radium in its vast mass of surrounding material, and to monitor it as it was slowly, reluctantly, forced into a state of purity.
With this achievement, public interest in the Curies exploded, spreading equally to their magical new element and the romantic, heroic husband-and-wife team who had dedicated themselves so totally to its exploration. In 1903, Marie Curie summarized the work of the previous six years in her doctoral thesis, and in the same year she received (with Pierre Curie and Becquerel) the Nobel Prize in physics.
Her thesis was immediately translated into English and published (by William Crookes in his Chemical News), and my mother had a copy of this in the form of a little booklet. I loved the minute descriptions of the elaborate chemical processes the Curies performed, the careful, systematic examination of radium’s properties, and especially the sense of intellectual excitement and wonder that seemed to simmer beneath the even-toned scientific prose. It was all down-to-earth, even prosaic – but it was a sort of poetry, too. And I was attracted by the notices on its covers for radium, thorium, polonium, uranium – all of these were freely available, to anyone, for fun or experiment.
There was an advertisement from A.C. Cossor, in Farringdon Road, a few doors from Uncle Tungsten’s place, selling ‘pure radium bromide (when available), pitchblende… Crooke’s high-vacuum tubes, showing the fluorescence of various minerals…[and] other scientific materials.’ Harrington Brothers (in Oliver’s Yard, not far away) sold a variety of radium salts and uranium minerals. J.J. Griffin and Sons (later to become Griffin & Tatlock, where I went for my own chemical supplies) were selling ‘Kunzite – the new mineral, responding in a high degree to the emanations from radium’, while Armbrecht, Nelson & Co. (a cut above the rest, in Grosvenor Square) had polonium sulphide (in tubes of one gram, twenty-one shillings) and screens of fluorescent willemite (sixpence for a square inch). ‘Our newly invented Thorium inhalers’, they added, ‘may be had on hire.’ What, I wondered, was a thorium inhaler? Would one feel braced, strengthened, inhaling the radioactive element?