When I was sixteen or seventeen, deeply in love now with marine biology, I wrote to marine biology laboratories all over the States – to Woods Hole in Massachusetts, to the Scripps Institution in La Jolla, to the Golden Gate Aquarium in San Francisco, and of course to Cannery Row in Monterey (by this time I knew that ‘Doc’ was a real person, Ed Ricketts). I got affable replies, I think, from them all, welcoming my interest and enthusiasm, but also indicating very clearly that I needed some real qualifications, too, and that I should think about recontacting them when I had a degree in biology (when I eventually made it to California, ten years later, it was not as a marine biologist, but as a neurologist).
CHAPTER TWENTY-THREE
«The World Set Free»
The Curies had noticed from the start that their radioactive substances showed a strange power to ‘induce’ radioactivity all around them. They found this both intriguing and irritating, for the contamination of their equipment made it nearly impossible to measure the radioactivity of the samples themselves:
The different objects used in the chemical laboratory [Marie wrote in her thesis]…soon acquire radioactivity. Dust particles, the air of the room, clothing, all become radioactive. The air of the room becomes a conductor. In our laboratory the evil has become acute, and we no longer have any apparatus properly insulated.«65»
I thought of our own house and of Uncle Abe’s house as I read this passage, wondering whether they, too, in their mild way, had become radioactive – whether the radium-painted dials of Uncle Abe’s clocks were inducing radioactivity in everything around them and filling the air, silently, with penetrating rays.
The Curies (like Becquerel) were at first inclined to attribute this ‘induced radioactivity’ to something immaterial, or to see it as a ‘resonance,’ perhaps analogous to phosphorescence or fluorescence. But there were also indications of a material emission. They had found, as early as 1897, that if thorium was kept in a tightly shut bottle its radioactivity increased, returning to its previous level as soon as the bottle was opened. But they did not follow up on this observation, and it was Ernest Rutherford who first realized the extraordinary implication of this: that a new substance was coming into being, being generated by the thorium; a far more radioactive substance than its parent.
Rutherford enlisted the help of the youngj chemist Frederick Soddy, and they were able to show that the ‘emanation’ of thorium was in fact a material substance, a gas, which could be isolated. It could be liquefied, almost as easily as chlorine, but it did not react with any chemical reagent; it was in fact just as inert as argon and the other newly discovered inert gases. At this point Soddy thought that the ‘emanation’ of thorium might be argon, and he was (as he wrote later)
…overwhelmed with something greater than joy – I cannot very well express it – a kind of exaltation…I remember quite well standing there transfixed as though stunned by the colossal impact of the thing and blurting out – or so it seemed at the time, ‘Rutherford, this is transmutation: the thorium is disintegrating and transmuting itself into argon gas.’
Rutherford’s reply was typically aware of more practical implications: ‘For Mike’s sake, Soddy, don’t call it transmutation. They’ll have our heads off as alchemists.’
But the new gas was not argon; it was a brand-new element with its own unique bright-line spectrum. It diffused very slowly and was exceedingly dense – III times as dense as hydrogen, whereas argon was only 20 times as dense. Assuming a molecule of the new gas was monatomic, containing only one atom like the other inert gases, its atomic weight would be 222. Thus it was the heaviest and last in the inert gas series, and as such could take its place in the periodic table, as the final member of Mendeleev’s Group o. Rutherford and Soddy provisionally named it thoron or Emanation.
Thoroir disappeared with great speed – half of it was gone in a minute, three-quarters in two minutes, and in ten minutes it was no longer detectable. It was the rapidity of this breakdown (and the appearance of a radioactive deposit in its place) which allowed Rutherford and Soddy to perceive what had not been clear with uranium or radium – that there was indeed a continuous disintegration of the atoms of radioactive elements, and with this their transformation to other atoms.
Each radioactive element, they found, had its own characteristic rate of breakdown, its own ‘half-life.’ The half-life of an element could be given with extraordinary precision, so that the half-life of one radon isotope, for instance, could be calculated as 3.8235 days. But the life of an individual atom could not be predicted in the least. I became more and more bewildered by this thought, and kept rereading Soddy’s account:
The chance at any instant whether an atom disintegrates or not in any particular second is fixed. It has nothing to do with any external or internal consideration we know of, and in particular is not increased by the fact that the atom has already survived any period of past time…All that can be said is that the immediate cause of atomic disintegration appears to be due to chance.
The life span of an individual atom, apparently, might vary from zero to infinity, and there was nothing to distinguish an atom ‘ready’ to disintegrate from one that still had a billion years before it.
I found this profoundly mystifying and disconcerting, that an atom might disintegrate at any time, without any ‘reason’ to do so. It seemed to remove radioactivity from the realm of continuity or process, from the intelligible, causal universe – and to hint at a realm where laws of the classical sort meant nothing whatsoever.
The half-life of radium was much longer than that of its emanation, radon – about 1,600 years. But this was still very small compared to the age of the earth – why, then, if it steadily decayed, had all the earth’s radium not disappeared long ago? The answer, Rutherford inferred, and was soon able to demonstrate, was that radium itself was produced by elements with a much longer half-life, a whole train of substances that he could trace back to the parent element, uranium. Uranium in turn had a half-life of four and a half billion years, roughly the age of the earth itself. Other cascades of radioactive elements were derived from thorium, which had an even longer half-life than uranium. Thus the earth was still living, in terms of atomic energy, on the uranium and thorium that had been present when the earth formed.
These discoveries had a crucial impact on a long-standing debate about the age of the earth. The great physicist Kelvin, writing in the early 1860s, soon after the publication of The Origin of Species, had argued that, based on its rate of cooling, and assuming no source of heat other than the sun, the earth could be no more than twenty million years old, and that in another five million years it would become too cold to support life. This calculation was not only dismaying in itself, but was impossible to reconcile with the fossil record, which indicated that life had been present for hundreds of millions of years – and yet there seemed no way of rebutting it. Darwin was greatly disturbed by this.
It was only with the discovery of radioactivity that the conundrum was solved. The young Rutherford, it was said, nervously facing the famous Lord Kelvin, now eighty years old, suggested that Kelvin’s calculation had been based on a false assumption. There was another source of warmth besides the sun, Rutherford said, and a very important one for the earth. Radioactive elements (chiefly uranium and thorium, and their breakdown products, but also a radioactive isotope of potassium) had served to keep the earth warm for billions of years and to protect it from the premature heat-death that Kelvin had predicted. Rutherford held up a piece of pitchblende, the age of which he had estimated from the amount of helium it contained. This piece of the earth, he said, was at least 500 million years old.
Rutherford and Soddy were ultimately able to delineate three separate radioactive cascades, each containing a dozen or so breakdown products emanating from the disintegration of the original parent elements. Could all of these breakdown products be different elements? There was no room in the periodic table for three dozen elements between bismuth and
thorium – room for half a dozen, perhaps, but not much more. Only gradually did it become clear that many of the elements were just versions of one another; the emanations of radium and thorium and actinium, for example, though they had widely differing half-lives, were chemically identical, all the same element, though with slightly different atomic weights. (Soddy later named these isotopes.) And the end points of each series were similar – radium G, actinium E, and thorium E, so-called, were all isotopes of lead.
Every substance in these cascades of radioactivity had its own unique radio signature, a half-life of fixed and invariable duration, as well as a characteristic radiation emission, and it was this which allowed Rutherford and Soddy to sort them all out, and in so doing to found the new science of radiochemistry.
The idea of atomic disintegration, first raised and then retreated from by Marie Curie, could no longer be denied. It was evident that every radioactive substance disintegrated in the act of giving off energy and turned into another element, that transmutation lay at the heart of radioactivity.
I loved chemistry in part because it was a science of transformations, of innumerable compounds based on a few dozen elements, themselves fixed and invariant and eternal. The feeling of the elements’ stability and invariance was crucial to me psychologically, for I felt them as fixed points, as anchors, in an unstable world. But now, with radioactivity, came transformations of the most incredible sort. What chemist would have conceived that out of uranium, a hard, tungsteny metal, there could come an alkaline earth metal like radium; an inert gas like radon; a tellurium-like element, polonium; radioactive forms of bismuth and thallium; and, finally, lead – exemplars of almost every group in the periodic table?
No chemist would have conceived this (though an alchemist might), because the transformations lay beyond the sphere of chemistry. No chemical process, no chemical attack, could ever alter the identity of an element, and this applied to the radioactive elements too. Radium, chemically, behaved similarly to barium; its radioactivity was a different property altogether, wholly unrelated to its chemical or physical properties. Radioactivity was a marvelous (or terrible) addition to these, a wholly other property (and one that annoyed me at times, for I loved the tungstenlike density of metallic uranium, and the fluorescence and beauty of its minerals and salts, but I felt I could not handle them safely for long; similarly I was infuriated by the intense radioactivity of radon, which otherwise would have made an ideal heavy gas).
Radioactivity did not alter the realities of chemistry, or the notion of elements; it did not shake the idea of their stability and identity. What it did do was to hint at two realms in the atom – a relatively superficial and accessible realm governing chemical reactivity and combination, and a deeper realm, inaccessible to all the usual chemical and physical agents and their relatively small energies, where any change produced a fundamental alteration of the element’s identity.
Uncle Abe had in his house a ‘spinthariscope,’ just like the ones advertised on the cover of Marie Curie’s thesis. It was a beautifully simple instrument, consisting of a fluorescent screen and a magnifying eyepiece, and inside, an infinitesimal speck of radium. Looking through the eyepiece, one could see dozens of scintillations a second – when Uncle Abe handed me this, and I held it up to my eye, I found the spectacle enchanting, magical, like looking at an endless display of meteors or shooting stars.
Spinthariscopes, at a few shillings each, were fashionable scientific toys in Edwardian drawing rooms – a new and uniquely twentieth-century accession, next to the stereoscopes and Geissler tubes inherited from Victorian times. But if they made their appearance as a sort of toy, it was rapidly appreciated that they also showed one something fundamentally important, for the tiny sparks or scintillations one saw came from the disintegration of individual atoms of radium, from the individual alpha particles each shot off as it exploded. No one would have imagined, Uncle Abe said, that we would ever be able to see the effects of individual atoms, much less to count them individually.
‘Here there is less than a millionth of a milligram of radium, and yet, on the small area of the screen, there are dozens of scintillations a second. Imagine how many there would be if we had a gram of radium – a thousand million times this amount.’
‘A hundred thousand million,’ I calculated.
‘Close,’ Uncle said. ‘One hundred and thirty-six thousand million, to be exact – the number never varies. Every second, one hundred and thirty-six thousand million atoms in a gram of radium disintegrate, shoot off their alpha particles – and if you think of this going on for thousands of years, you’ll get some idea of how many atoms there are in a single gram of radium.’
Experiments around the turn of the century had shown that not only alpha rays but several other sorts of ray were being emitted by radium. Most of the phenomena of radioactivity could be attributed to these different sorts of rays: the ability to ionize air was especially the prerogative of the alpha rays, while the ability to elicit fluorescence or affect photographic plates was more marked with the beta rays. Every radioactive element had its own characteristic emissions: thus radium preparations emitted both alpha and beta rays, where polonium preparations emitted only alpha rays. Uranium affected a photographic plate more quickly than thorium, but thorium was more potent in discharging an electroscope.
The alpha particles emitted by radioactive decay (they were later shown to be helium nuclei) were positively charged and relatively massive – thousands of times more massive than beta particles or electrons – and they traveled in undeviating straight lines, passing straight through matter, ignoring it, without any scattering or deflection (although they might lose some of their velocity in so doing). This, at least, appeared to be the case, though in 1906 Rutherford observed that there might be, very occasionally, small deflections. Others ignored this, but to Rutherford these observations were fraught with possible significance. Would not alpha particles be ideal projectiles, projectiles of atomic proportions, with which to bombard other atoms and sound out their structure? He asked his young assistant Hans Geiger and a student, Ernest Marsden, to set up a scintillation experiment using screens of thin metal foils, so that one could keep count of every alpha particle that bombarded these. Firing alpha particles at a piece of gold foil, they found that roughly one in eight thousand particles showed a massive deflection – of more than 90 degrees, and sometimes even 180 degrees. Rutherford was later to say, ‘It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you.’
Rutherford pondered these curious results for almost a year, and then, one day, as Geiger recorded, he ‘came into my room, obviously in the best of moods, and told me that now he knew what the atom looked like and what the strange scatterings signified.’
Atoms, Rutherford had realized, could not be a homogenous jelly of positivity stuck with electrons like raisins (as J.J. Thomson had suggested, in his ‘plum pudding’ model of the atom), for then the alpha particles would always go through them. Given the great energy and charge of these alpha particles, one had to assume that they had been deflected, on occasion, by something even more positively charged than themselves. Yet this happened only once in eight thousand times. The other 7,999 particles might whiz through, undeflected, as if most of the gold atoms consisted of empty space; but the eight-thousandth was stopped, flung back in its tracks, like a tennis ball hitting a globe of solid tungsten. The mass of the gold atom, Rutherford inferred, had to be concentrated at the center, in a minute space, not easy to hit – as a nucleus of almost inconceivable density. The atom, he proposed, must consist overwhelmingly of empty space, with a dense, positively charged nucleus only a hundred-thousandth its diameter, and a relatively few, negatively charged electrons in orbit about this nucleus – a miniature solar system, in effect.
Rutherford’s experiments, his nuclear model of the atom, provid
ed a structural basis for the enormous differences between radioactive and chemical processes, the millionfold differences of energy involved (Soddy would dramatize this, in his popular lectures, by holding a one-pound jar of uranium oxide aloft in one hand – this, he would say, had the energy of a hundred and sixty tons of coal).
Chemical change or ionization involved the addition or removal of an electron or two, and this required only a modest energy of two or three electron-volts, such as could be produced easily – by a chemical reaction, by heat, by light, or by a simple 3-volt battery. But radioactive processes involved the nuclei of atoms, and since these were held together by far greater forces, their disintegration could release energies of far greater magnitude – some millions of electron-volts.
Soddy coined the term atomic energy soon after the turn of the nineteenth century, ten years or more before the nucleus was discovered. No one had known, or been able to make a remotely plausible guess, as to how the sun and stars could radiate so much energy, and continue to do so for millions of years. Chemical energy would be ludicrously inadequate – a sun made of coal would burn itself out in ten thousand years. Could radioactivity, atomic energy, provide the answer?
Supposing [wrote Soddy]…our sun…were made of pure radium…there would be no difficulty in accounting for its out-pourings of energy.
Could transmutation, which occurs naturally in radioactive substances, be produced artificially, Soddy wondered.«66» He was moved by this thought to rapturous, millennial, and almost mystical heights: