Becquerel had discovered a new and much more mysterious power than Roentgen’s rays – the power of a uranic salt to emit a penetrating radiation that could fog a photographic plate, and in a way that had nothing to do with exposure to light or X-rays or, seemingly, any other external source of energy. Becquerel, his son later wrote, was ‘stupefied’ at this finding (’Henri Becquerel fut stupefait’) – as Roentgen had been by his X-rays – but then, like Roentgen, he investigated the ‘impossible.’ He found that the rays retained all their potency even if the uranic salt was kept for two months in a drawer; and that they had the power not only to darken photographic plates but also to ionize air, render it conducting, so that electrically charged bodies in their vicinity would lose their charge. This indeed provided a very sensitive way of measuring the intensity of Becquerel’s rays, using an electroscope.
Investigating other substances, he found that this power was possessed not only by uranic salts but uranous ones too, even though these were not phosphorescent or fluorescent. On the other hand, barium sulphide, zinc sulphide, and certain other fluorescent or phosphorescent substances had no such power. Thus the ‘uranium rays’, as Becquerel now called them, had nothing to do with fluorescence or phosphorescence as such – and everything to do with the element uranium. They had, like X-rays, a very considerable power of penetrating materials opaque to light, but unlike X-rays, they were apparently emitted spontaneously. What were they? And how could uranium continue to radiate them, with no apparent diminution, for months at a time?
Uncle Abe encouraged me to repeat Becquerel’s discovery in my own lab, giving me a chunk of pitchblende rich in uranium oxide. I took the heavy chunk home, wrapped in lead foil, in my school satchel. The pitchblende had been sectioned cleanly through the middle, to show its structure, and I placed the cut face flat on some film – I had begged a sheet of special X-ray film from Uncle Yitzchak, and I kept this wrapped in its dark paper. I left the pitchblende lying on the covered film for three days, then took it along to him to develop. I was wild with excitement when Uncle Yitzchak developed it in front of me, for now one could see the glares of radioactivity in the mineral – radiation and energy whose existence, without the film, one would never have guessed at.
I was doubly thrilled by this, because photography was becoming a hobby, and I now had my first picture taken by invisible rays! I had read that thorium, too, was radioactive, and, knowing that gas mantles contained this, I detached one of the delicate, thoria-impregnated mantles at home from its base and carefully spread it over another piece of X-ray film. This time I had to wait longer, but after two weeks I got a beautiful ‘autoradiography’ the fine texture of the mantle picked out by the thorium rays.
Though uranium had been known since the 1780s, it had taken more than a century before its radioactivity was discovered. Radioactivity might have been discovered, perhaps, in the eighteenth century, had anyone chanced to place a piece of pitchblende close to a charged Leyden jar or an electroscope. Or it might have been discovered in the middle of the nineteenth century, had a piece of pitchblende, or some other uranium ore or salt, been left in accidental proximity to a photographic plate. (This had in fact happened to one chemist, who, not realizing what had happened, sent the plates back to the manufacturer with an indignant note saying that they were ‘spoilt.’) Yet had radioactivity been discovered earlier, it would have been seen simply as a curiosity, a freak, a lusus naturae, its enormous significance wholly unsuspected. Its discovery would have been premature, in the sense that there would have been no nexus of knowledge, no context, to give it meaning. Indeed, when radioactivity was finally discovered in 1896, there was very little reaction at first, for even then its significance could barely be grasped. So in contrast to Roentgen’s discovery of X-rays, which instantly captured the public’s attention, Becquerel’s discovery of uranium rays was virtually ignored.
21. Madame Curie’s Element
My 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.
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In 1998 I spoke at a meeting for the centennial of the discovery of polonium and radium. I said that I had been given this book when I was ten, and that it was my favorite biography. As I was talking I became conscious of a very old lady in the audience, with high Slavic cheekbones and a smile going from one ear to the other. I thought, ‘It can’t be!’ But it was – it was Eve Curie, and she signed her book for me sixty years after it was published, fifty-five years after I got it.