The record is ambiguous but suggestive. The English scientific tradition was historically genteel. It generally disdained research patents and any other legal and commercial restraints that threatened the open dissemination of scientific results. In practice that guard of scientific liberty could molder into clubbish distaste for “vulgar commercialism.” Ernest Marsden, a Rutherford-trained physicist and an insightful biographer, heard that “in his early days at Cambridge there were some few who said that Rutherford was not a cultured man.” One component of that canard may have been contempt for his eagerness to make a profit from radio.124
It seems that J. J. Thomson intervened. A grand new work had abruptly offered itself. On November 8, 1895, one month after Rutherford arrived at Cambridge, the German physicist Wilhelm Röntgen discovered X rays radiating from the fluorescing glass wall of a cathode-ray tube. Röntgen reported his discovery in December and stunned the world. The strange radiation was a new growing point for science and Thomson began studying it almost immediately. At the same time he also continued his experiments with cathode rays, experiments that would culminate in 1897 in his identification of what he called the “negative corpuscle”—the electron, the first atomic particle to be identified. He must have needed help. He would also have understood the extraordinary opportunity for original research that radiation offered a young man of Rutherford’s skill at experiment.
To settle the issue Thomson wrote the grand old man of British science, Lord Kelvin, then seventy-two, asking his opinion of the commercial possibilities of radio—“before tempting Rutherford to turn to the new subject,” Marsden says. Kelvin after all, vulgar commercialism or not, had developed the transoceanic telegraph cable. “The reply of the great man was that [radio] might justify a capital expenditure of a £100,000 Company on its promotion, but no more.”125
By April 24 Rutherford has seen the light. He writes Mary Newton: “I hope to make both ends meet somehow, but I must expect to dub out my first year. . . . My scientific work at present is progressing slowly. I am working with the Professor this term on Röntgen Rays. I am a little full up of my old subject and am glad of a change. I expect it will be a good thing for me to work with the Professor for a time. I have done one research to show I can work by myself.”126 The tone is chastened and not nearly convinced, as if a ghostly, parental J. J. Thomson were speaking through Rutherford to his fiancée. He has not yet appeared before the Royal Society, where he was hardly “a little full up” of his subject. But the turnabout is accomplished. Hereafter Rutherford’s healthy ambition will go to scientific honors, not commercial success.
It seems probable that J. J. Thomson sat eager young Ernest Rutherford down in the darkly paneled rooms of the Gothic Revival Cavendish Laboratory that Clerk Maxwell had founded, at the university where Newton wrote his great Principia, and kindly told him he could not serve God and Mammon at the same time. It seems probable that the news that the distinguished director of the Cavendish had written the Olympian Lord Kelvin about the commercial ambitions of a brash New Zealander chagrined Rutherford to the bone and that he went away from the encounter feeling grotesquely like a parvenu. He would never make the same mistake again, even if it meant strapping his laboratories for funds, even if it meant driving away the best of his protégés, as eventually it did. Even if it meant that energy from his cherished atom could be nothing more than moonshine. But if Rutherford gave up commercial wealth for holy science, he won the atom in exchange. He found its constituent parts and named them. With string and sealing wax he made the atom real.
* * *
The sealing wax was blood red and it was the Bank of England’s most visible contribution to science. British experimenters used Bank of England sealing wax to make glass tubes airtight.127 Rutherford’s earliest work on the atom, like J. J. Thomson’s work with cathode rays, grew out of nineteenth-century examination of the fascinating effects produced by evacuating the air from a glass tube that had metal plates sealed into its ends and then connecting the metal plates to a battery or an induction coil. Thus charged with electricity, the emptiness inside the sealed tube glowed. The glow emerged from the negative plate—the cathode—and disappeared into the positive plate—the anode. If you made the anode into a cylinder and sealed the cylinder into the middle of the tube you could project a beam of glow—of cathode rays—through the cylinder and on into the end of the tube opposite the cathode. If the beam was energetic enough to hit the glass it would make the glass fluoresce. The cathode-ray tube, suitably modified, its all-glass end flattened and covered with phosphors to increase the fluorescence, is the television tube of today.
In the spring of 1897 Thomson demonstrated that the beam of glowing matter in a cathode-ray tube was not made up of light waves, as (he wrote drily) “the almost unanimous opinion of German physicists” held. Rather, cathode rays were negatively charged particles boiling off the negative cathode and attracted to the positive anode. These particles could be deflected by an electric field and bent into curved paths by a magnetic field. They were much lighter than hydrogen atoms and were identical “whatever the gas through which the discharge passes” if gas was introduced into the tube.128 Since they were lighter than the lightest known kind of matter and identical regardless of the kind of matter they were born from, it followed that they must be some basic constituent part of matter, and if they were a part, then there must be a whole. The real, physical electron implied a real, physical atom: the particulate theory of matter was therefore justified for the first time convincingly by physical experiment. They sang J. J.’s success at the annual Cavendish dinner:
The corpuscle won the day129
And in freedom went away
And became a cathode ray.
Armed with the electron, and knowing from other experiments that what was left when electrons were stripped away from an atom was a much more massive remainder that was positively charged, Thomson went on in the next decade to develop a model of the atom that came to be called the “plum pudding” model. The Thomson atom, “a number of negatively-electrified corpuscles enclosed in a sphere of uniform positive electrification” like raisins in a pudding, was a hybrid: particulate electrons and diffuse remainder.130 It served the useful purpose of demonstrating mathematically that electrons could be arranged in stable configurations within an atom and that the mathematically stable arrangements could account for the similarities and regularities among chemical elements that the periodic table of the elements displays. It was becoming clear that electrons were responsible for chemical affinities between elements, that chemistry was ultimately electrical.
Thomson just missed discovering X rays in 1894. He was not so unlucky in legend as the Oxford physicist Frederick Smith, who found that photographic plates kept near a cathode-ray tube were liable to be fogged and merely told his assistant to move them to another place.131, 132 Thomson noticed that glass tubing held “at a distance of some feet from the discharge tube” fluoresced just as the wall of the tube itself did when bombarded with cathode rays, but he was too intent on studying the rays themselves to pursue the cause.133 Röntgen isolated the effect by covering his cathode-ray tube with black paper. When a nearby screen of fluorescent material still glowed he realized that whatever was causing the screen to glow was passing through the paper and the intervening air.134 If he held his hand between the covered tube and the screen, his hand slightly reduced the glow on the screen but in dark shadow he could see its bones.
Röntgen’s discovery intrigued other researchers besides J. J. Thomson and Ernest Rutherford. The Frenchman Henri Becquerel was a third-generation physicist who, like his father and grandfather before him, occupied the chair of physics at the Musée d’Histoire Naturelle in Paris; like them also he was an expert on phosphorescence and fluorescence—in his case, particularly of uranium. He heard a report of Röntgen’s work at the weekly meeting of the Académie des Sciences on January 20, 1896. He learned that the X rays emerged from the fluorescing glass, whic
h immediately suggested to him that he should test various fluorescing materials to see if they also emitted X rays. He worked for ten days without success, read an article on X rays on January 30 that encouraged him to keep working and decided to try a uranium salt, uranyl potassium sulfate.
His first experiment succeeded—he found that the uranium salt emitted radiation—but misled him. He had sealed a photographic plate in black paper, sprinkled a layer of the uranium salt onto the paper and “exposed the whole thing to the sun for several hours.” When he developed the photographic plate “I saw the silhouette of the phosphorescent substance in black on the negative.” He mistakenly thought sunlight activated the effect, much as cathode rays released Röntgen’s X rays from the glass.135
The story of Becquerel’s subsequent serendipity is famous. When he tried to repeat his experiment on February 26 and again on February 27 Paris was gray. He put the covered photographic plate away in a dark drawer, uranium salt in place. On March 1 he decided to go ahead and develop the plate, “expecting to find the images very feeble. On the contrary, the silhouettes appeared with great intensity. I thought at once that the action might be able to go on in the dark.” Energetic, penetrating radiation from inert matter unstimulated by rays or light: now Rutherford had his subject, as Marie and Pierre Curie, looking for the pure element that radiated, had their backbreaking work.136
* * *
Between 1898, when Rutherford first turned his attention to the phenomenon Henri Becquerel found and which Marie Curie named radioactivity, and 1911, when he made the most important discovery of his life, the young New Zealand physicist systematically dissected the atom.
He studied the radiations emitted by uranium and thorium and named two of them: “There are present at least two distinct types of radiation—one that is very readily absorbed, which will be termed for convenience the α [alpha] radiation, and the other of a more penetrative character, which will be termed the β [beta] radiation.”137 (A Frenchman, P. V. Villard, later discovered the third distinct type, a form of high-energy X rays that was named gamma radiation in keeping with Rutherford’s scheme.138) The work was done at the Cavendish, but by the time he published it, in 1899, when he was twenty-seven, Rutherford had moved to Montreal to become professor of physics at McGill University. A Canadian tobacco merchant had given money there to build a physics laboratory and to endow a number of professorships, including Rutherford’s. “The McGill University has a good name,” Rutherford wrote his mother.139 “£500 is not so bad [a salary] and as the physical laboratory is the best of its kind in the world, I cannot complain.”
In 1900 Rutherford reported the discovery of a radioactive gas emanating from the radioactive element thorium.140 Marie and Pierre Curie soon discovered that radium (which they had purified from uranium ores in 1898) also gave off a radioactive gas. Rutherford needed a good chemist to help him establish whether the thorium “emanation” was thorium or something else; fortunately he was able to shanghai a young Oxford man at McGill, Frederick Soddy, of talent sufficient eventually to earn a Nobel Prize. “At the beginning of the winter [of 1900],” Soddy remembers, “Ernest Rutherford, the Junior Professor of Physics, called on me in the laboratory and told me about the discoveries he had made. He had just returned with his bride from New Zealand . . . but before leaving Canada for his trip he had discovered what he called the thorium emanation. . . . I was, of course, intensely interested and suggested that the chemical character of the [substance] ought to be examined.”141
The gas proved to have no chemical character whatsoever. That, says Soddy, “conveyed the tremendous and inevitable conclusion that the element thorium was slowly and spontaneously transmuting itself into [chemically inert] argon gas!” Soddy and Rutherford had observed the spontaneous disintegration of the radioactive elements, one of the major discoveries of twentieth-century physics.142 They set about tracing the way uranium, radium and thorium changed their elemental nature by radiating away part of their substance as alpha and beta particles. They discovered that each different radioactive product possessed a characteristic “half-life,” the time required for its radiation to reduce to half its previously measured intensity. The half-life measured the transmutation of half the atoms in an element into atoms of another element or of a physically variant form of the same element—an “isotope,” as Soddy later named it.143 Half-life became a way to detect the presence of amounts of transmuted substances—“decay products”—too small to detect chemically. The half-life of uranium proved to be 4.5 billion years, of radium 1,620 years, of one decay product of thorium 22 minutes, of another decay product of thorium 27 days. Some decay products appeared and transmuted themselves in minute fractions of a second—in the twinkle of an eye. It was work of immense importance to physics, opening up field after new field to excited view, and “for more than two years,” as Soddy remembered afterward, “life, scientific life, became hectic to a degree rare in the lifetime of an individual, rare perhaps in the lifetime of an institution.”144
Along the way Rutherford explored the radiation emanating from the radioactive elements in the course of their transmutation. He demonstrated that beta radiation consisted of high-energy electrons “similar in all respects to cathode rays.” He suspected, and later in England conclusively proved, that alpha particles were positively charged helium atoms ejected during radioactive decay.145 Helium is found captured in the crystalline spaces of uranium and thorium ores; now he knew why.
An important 1903 paper written with Soddy, “Radioactive change,” offered the first informed calculations of the amount of energy released by radioactive decay:
It may therefore be stated that the total energy of radiation during the disintegration of one gram of radium cannot be less than 108 [i.e., 100,000,000] gram-calories, and may be between 109 and 1010 gram-calories. . . . The union of hydrogen and oxygen liberates approximately 4 × 103 [i.e., 4,000] gram-calories per gram of water produced, and this reaction sets free more energy for a given weight than any other chemical change known. The energy of radioactive change must therefore be at least twenty-thousand times, and may be a million times, as great as the energy of any molecular change.146
That was the formal scientific statement; informally Rutherford inclined to whimsical eschatology. A Cambridge associate writing an article on radioactivity that year, 1903, considered quoting Rutherford’s “playful suggestion that, could a proper detonator be found, it was just conceivable that a wave of atomic disintegration might be started through matter, which would indeed make this old world vanish in smoke.” Rutherford liked to quip that “some fool in a laboratory might blow up the universe unawares.” If atomic energy would never be useful, it might still be dangerous.147, 148
Soddy, who returned to England that year, examined the theme more seriously. Lecturing on radium to the Corps of Royal Engineers in 1904, he speculated presciently on the uses to which atomic energy might be put:
It is probable that all heavy matter possesses—latent and bound up with the structure of the atom—a similar quantity of energy to that possessed by radium. If it could be tapped and controlled what an agent it would be in shaping the world’s destiny! The man who put his hand on the lever by which a parsimonious nature regulates so jealously the output of this store of energy would possess a weapon by which he could destroy the earth if he chose.149
Soddy did not think the possibility likely: “The fact that we exist is a proof that [massive energetic release] did not occur; that it has not occurred is the best possible assurance that it never will. We may trust Nature to guard her secret.”
H. G. Wells thought Nature less trustworthy when he read similar statements in Soddy’s 1909 book Interpretation of Radium. “My idea is taken from Soddy,” he wrote of The World Set Free. “One of the good old scientific romances,” he called his novel; it was important enough to him that he interrupted a series of social novels to write it.150 Rutherford’s and Soddy’s discussions of radioactive change theref
ore inspired the science fiction novel that eventually started Leo Szilard thinking about chain reactions and atomic bombs.
In the summer of 1903 the Rutherfords visited the Curies in Paris. Mme. Curie happened to be receiving her doctorate in science on the day of their arrival; mutual friends arranged a celebration. “After a very lively evening,” Rutherford recalled, “we retired about 11 o’clock in the garden, where Professor Curie brought out a tube coated in part with zinc sulphide and containing a large quantity of radium in solution.151 The luminosity was brilliant in the darkness and it was a splendid finale to an unforgettable day.” The zinc-sulfide coating fluoresced white, making the radium’s ejection of energetic particles on its progress down the periodic table from uranium to lead visible in the darkness of the Paris evening. The light was bright enough to show Rutherford Pierre Curie’s hands, “in a very inflamed and painful state due to exposure to radium rays.” Hands swollen with radiation burns was another object lesson in what the energy of matter could do.
A twenty-six-year-old German chemist from Frankfurt, Otto Hahn, came to Montreal in 1905 to work with Rutherford. Hahn had already discovered a new “element,” radiothorium, later understood to be one of thorium’s twelve isotopes. He studied thorium radiation with Rutherford; together they determined that the alpha particles ejected from thorium had the same mass as the alpha particles ejected from radium and those from another radioactive element, actinium. The various particles were probably therefore identical—one conclusion along the way to Rutherford’s proof in 1908 that the alpha particle was inevitably a charged helium atom. Hahn went back to Germany in 1906 to begin a distinguished career as a discoverer of isotopes and elements; Leo Szilard encountered him working with physicist Lise Meitner at the Kaiser Wilhelm Institute for Chemistry in the 1920s in Berlin.
Making of the Atomic Bomb Page 6