Antoine Henri Becquerel: The Accident That Opened the Atom
The Paris sky clouded over in late February 1896, and a physicist waiting for sunshine put his photographic plates and his uranium crystals away in a drawer. On 1 March he developed the plates anyway, expecting a faint smudge at most. The image was sharp. Nothing had shone on the uranium; the uranium had been shining by itself. Matter, it turned out, could emit energy out of nowhere, and the tidy nineteenth-century atom was finished.
Four Generations of Becquerels
Antoine Henri Becquerel was born in Paris on 15 December 1852 into what amounted to a hereditary physics chair. His grandfather, Antoine César Becquerel, was a Fellow of the Royal Society who devised an electrolytic method for extracting metals from their ores. His father, Alexander Edmond Becquerel, was professor of applied physics and one of the period's leading authorities on fluorescence and phosphorescence — the study of substances that glow after being illuminated. Henri's son Jean, born in 1878, would become a physicist too. Four generations occupied, in succession, the same professional territory and often the same actual chair.
That inheritance decided what he looked at and, crucially, what he had lying around the laboratory. From 1883 he worked on phosphorescence, his father's subject, alongside the plane polarisation of light, the absorption of light by crystals, and terrestrial magnetism.
The Engineer's Route
His training was practical before it was academic. He entered the École Polytechnique in 1872 and in 1874 joined the government's Ponts-et-Chaussées, the corps of bridge and road engineers, becoming an ingénieur in 1877 and rising to ingénieur-en-chef in 1894. The doctorate came comparatively late, in 1888.
The posts accumulated in parallel. He was an assistant at the Museum of Natural History from 1878, took over his father's chair of applied physics at the Conservatoire des Arts et Métiers, became professor of applied physics at the Paris Museum in 1892 and professor at the Polytechnique in 1895. He was elected to the Académie des Sciences de France in 1889 and would later serve as its Life Secretary, succeeding Marcellin Berthelot.
A Drawer in Paris
In January 1896 the Académie heard the news from Würzburg: Wilhelm Conrad Röntgen had found a penetrating radiation he called X-rays. The room was electrified, and Becquerel drew from it a hypothesis that fitted his own expertise. Perhaps phosphorescent substances, having soaked up sunlight, re-emitted part of it as X-rays. Uranium salts were the obvious test case; he had them to hand.
The method was straightforward. Wrap a photographic plate in black paper so no light can reach it. Lay crystals of potassium uranyl sulfate on top. Put the assembly in the sun. Develop. If something penetrating comes off the crystals, the plate will carry their silhouette.
It did. He got outlines, including the outline of a copper cross laid between crystal and plate, and concluded he had confirmed his theory. Then the weather intervened. With Paris overcast, the prepared plates went into a drawer with the uranium sitting on them, unexposed and unstimulated. On 1 March he developed them expecting almost nothing, and instead found an image of startling clarity.
He announced it to the Académie on 2 March 1896: the uranium salts emitted radiation with no stimulation from sunlight whatever. His original hypothesis was wrong, and the wrongness was worth more than the hypothesis.
What the Uranium Meant
He pressed on. Uranium compounds that were not phosphorescent at all produced the same effect, which killed the phosphorescence explanation outright. Pure uranium metal produced it too. By May 1896 he had identified the element itself as the source: the radiation was a property of uranium atoms, independent of their chemical state, their history, or any external supply of energy.
That last clause is the whole revolution. Energy was coming out of matter continuously and apparently for free, which no chemistry and no classical physics could account for. Something was going on inside the atom, and the atom had been assumed to have no inside.
The Curies, and the Prize
Becquerel did not chase the discovery to its conclusion; others did. From 1898 Marie and Pierre Curie took up what were then called uranium rays, isolating polonium in June 1898 and radium that December, and finding the same behaviour in thorium. It was Marie Curie who coined the word radioactivity for the phenomenon Becquerel had stumbled into.
In 1903 the Nobel Prize in Physics was divided: half to Becquerel for the discovery of spontaneous radioactivity, half shared by the Curies for their work on the radiation he had found. He was already an Officer of the Legion of Honour, from 1900, and belonged to the Accademia dei Lincei in Rome and the Royal Academy in Berlin.
Why Henri Is Called a Genius
The honest description of the central quality is not conceptual daring. Becquerel's hypothesis was conventional and it was wrong; the crucial plate was developed on a whim during a spell of bad weather. What separates him from anyone else who has ever had a lucky accident is what he did in the following ten weeks.
The discipline is in the follow-through. Presented with a result that contradicted his own theory, he did not explain it away — he dismantled his explanation systematically. He tested uranium compounds that could not phosphoresce. He tested uranium metal itself. Within two months he had located the effect in the element rather than in any chemical arrangement of it, and had reported the whole sequence, including the failure of his starting idea, to the Académie. The cliché about chance favouring the prepared mind is, for once, precise: Becquerel had spent thirteen years on phosphorescence and knew exactly what an anomalous plate meant.
The counter-case is straightforward and should be stated. He did not name radioactivity, did not isolate a radioactive element, did not explain the mechanism, and did not pursue the field with the ferocity the Curies brought to it. He inherited his laboratory, his subject and very nearly his chair. The Nobel committee split the prize for a reason. What he supplied was the observation everything else grew from, and the intellectual honesty to report it against his own interest — which is rarer among scientists than brilliance is.
Legacy
He died at Le Croisic in Brittany on 25 August 1908, aged fifty-five, twelve years after the discovery and long before anyone understood what radiation did to the people handling it. The SI unit of radioactivity is the becquerel: one nuclear disintegration per second, a quantity so small that ordinary objects register in the hundreds. Every medical scan that uses a tracer, every reactor safety figure, every measurement of fallout is quoted in his name. The drawer he left the plates in is the reason nuclear physics exists.
Achievements
- Knight of the Legion of Honour — 1882
- Foreign Member of the Royal Society — 1908
- Nobel Prize in Physics — 1903
- Rumford Medal — 1900
- Officer of the Legion of Honour — 1900
- Notable work: natural radioactivity
- Held posts at Muséum national d'histoire naturelle, École Nationale des Ponts et Chaussées and École polytechnique
- Educated at Lycée Louis-le-Grand, École Nationale des Ponts et Chaussées and École polytechnique
- Fields of research: chemistry and physics
