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. 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
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. 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. 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. Rontgen 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. 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
Rontgen'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 Musee 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 Rontgen's work at the weekly meeting of the Acad6mie des Sciences on January 20, 1896. He learned that the X rays emerged from the fluorescing glass, which 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 Rontgen's X rays from the glass.
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.
Between 1898, when Rutherford first turned his attention to the phenomenon Henri Becquerel found and which Marie Curie named
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 a [alpha] radiation, and the other of a more penetrative character, which will be termed the ? [beta] radiation.” (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.) 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. “?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. 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.”
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. 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. 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.”
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. 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
