Uncle Tungsten: Memories of a Chemical Boyhood (2001) (29 page)

Madame Curie’s Element

M
y mother worked at many hospitals, including the Marie Curie Hospital in Hampstead, a hospital that specialized in radium treatments and radiotherapy. I was not too sure, as a child, what radium was, but I understood it had healing powers and could be used to treat different conditions. My mother said the hospital possessed a radium ‘bomb.’ I had seen pictures of bombs and read about them in my children’s encyclopedia, and I imagined this radium bomb as a great winged thing that might explode at any moment. Less alarming were the radon ‘seeds’ which were implanted in patients – little gold needles full of a mysterious gas – and once or twice she brought an exhausted one home. I knew my mother admired Marie Curie hugely – she had met her once, and would tell me, even when I was quite small, how the Curies had discovered radium, and how difficult this had been, because they had had to work through tons and tons of heavy mineral ore to get the merest speck of it.

Eve Curie’s biography of her mother – which my own mother gave me when I was ten – was the first portrait of a scientist I ever read, and one that deeply impressed me.«60» It was no dry recital of a life’s achievements, but full of evocative, poignant images – Marie Curie plunging her hands into the sacks of pitchblende residue, still mixed with pine needles from the Joachimsthal mine; inhaling acid fumes as she stood amid vast steaming vats and crucibles, stirring them with an iron rod almost as big as herself; transforming the huge, tarry masses to tall vessels of colorless solutions, more and more radioactive, and steadily concentrating these, in turn, in her drafty shed, with dust and grit continually getting into the solutions and undoing the endless work. (These images were reinforced by the film
Madame Curie
, which I saw soon after reading the book.)

Even though the rest of the scientific community had ignored the news of Becquerel’s rays, the Curies were galvanized by it: this was a phenomenon without precedent or parallel, the revelation of a new, mysterious source of energy; and nobody, apparently, was paying any attention to it. They wondered at once whether there were any substances besides uranium that emitted similar rays, and started on a systematic search (not confined, as Becquerel’s had been, to fluorescent substances) of everything they could lay their hands on, including samples of almost all the seventy known elements in some form or other. They found only one other substance besides uranium that emitted Becquerel’s rays, another element of very high atomic weight – thorium. Testing a variety of pure uranium and thorium salts, they found the intensity of the radioactivity seemed to be related only to the amount of uranium or thorium present; thus one gram of metallic uranium or thorium was more radioactive than one gram of any of their compounds.

But when they extended their survey to some of the common minerals containing uranium and thorium, they found a curious anomaly, for some of these were actually more active than the element itself. Samples of pitchblende, for instance, might be up to four times as radioactive as pure uranium. Could this mean, they wondered, in an inspired leap, that another, as-yet-unknown element was also present in small amounts, one that was far more radioactive than uranium itself?

In 1897 the Curies launched upon an elaborate chemical analysis of pitchblende, separating the many elements it contained into analytic groups: salts of alkali metals, of alkaline earth elements, of rare-earth elements – groups basically similar to those of the periodic table – to see if the unknown radioactive element had chemical affinities with any of them. Soon it became clear that a good part of the radioactivity could be concentrated by precipitation with bismuth.

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?

No one seemed to have any idea of the danger of these stuffs at this time.«61» Marie Curie herself mentioned in her thesis how ‘if a radio-active substance is placed in the dark in the vicinity of the closed eye or of the temple, a sensation of light fills the eye,’ and I often tried this myself, using one of the luminous clocks in our house, their figures and hands painted with Uncle Abe’s luminous paint.

I was particularly moved by the description in Eve Curie’s book of how her parents, restless one evening and curious as to how the fractional crystallizations were going, returned to their shed late one night and saw in the darkness a magical glowing everywhere, from all the tubes and vessels and basins containing the radium concentrates, and realized for the first time that their element was spontaneously luminous. The luminosity of phosphorus required the presence of oxygen, but the luminosity of radium arose entirely from within, from its own radioactivity. Marie Curie wrote in lyrical terms of this luminosity:

One of our joys was to go into our workroom at night when we perceived the feebly luminous silhouettes of the bottles and capsules containing our products…It was really a lovely sight and always new to us. The glowing tubes looked like faint fairy lights.

Uncle Abe still had some radium in his possession, left over from his work on luminous paint, and he would show me this, pulling out a vial with a few milligrams of radium bromide – it appeared to be a grain of ordinary salt – at the bottom. He had three little screens painted with platinocyanides – lithium, sodium, and barium platinocyanide – and as he waved the tube of radium (gripped in a pair of tongs) near the darkened screens, these lit up suddenly, becoming sheets of red, then yellow, then green fire, each fading suddenly as he moved the tube away again.

‘Radium has lots of interesting effects on substances around it,’ he said. ‘The photographic effects you know, but radium also browns paper, burns it, pits it, like a colander. Radium decomposes the atoms of the air, and then they recombine in different forms – so you smell ozone and nitrogen peroxide when you are around it. It affects glass – it turns soft glasses blue, and hard glasses brown; it can also color diamonds and turn rock salt a deep, intense violet.’ Uncle Abe showed me a piece of fluorspar which he had exposed to radium for a few days. Its original color had been purple, he said, but now it was pale, charged with strange energy. He heated the fluorspar a little, far below red heat, and it suddenly gave off a brilliant flash, as if it were white-hot, and returned to its original purple.