See also the French version: La découverte de la radioactivité par Henri Becquel
On Wednesday, February 26, 1896, in Paris, Henri Becquerel was conducting experiments on phosphorescent crystals. A material is phosphorescent when it emits light for some time after having absorbed light itself.
The experiment consisted of exposing the crystals to sunlight and then placing them next to a photographic plate to measure the amount of light they would produce via phosphorescence.
However, it wasn’t sunny in Paris that day. Becquerel developed the photograph anyway. Since the crystal had received little light, he expected to see only a faint image of it. He was shocked, then, to find instead a rather strong image of the crystal on the photograph.
Five days later, on March 2, 1896, Becquerel presented this surprising phenomenon to the French Academy of Sciences. He knew he had made a significant discovery, but no one, including him, could have predicted how much this event would change both science and the world.
The crystal that Becquerel was studying that day contained uranium, and the phenomenon he observed is now called radioactivity.
The discovery of radioactivity by Henri Becquerel is a fascinating topic for popular science because it can be understood without requiring any mathematics or complex reasoning. It is also one of the most significant discoveries in the history of science.
Becquerel regularly presented his work at the French Academy of Sciences, and transcripts of all these presentations can be found in the Comptes rendus hebdomadaires des séances de l’Académie des sciences [minutes of the weekly sessions of the Academy of Sciences], which are available on the website of the Bibliothèque nationale de France (BNF) [National Library of France], https://gallica.bnf.fr.
The documents on the BNF website are only scanned images, making them quite difficult to read. I digitized1 all of Henri Becquerel’s presentations at the French Academy of Sciences on the topic of radioactivity (see the complete list of his presentations at the Academy from 1896 onwards – in French), allowing readers to relive this great scientific adventure as if they were part of it, witnessing the hesitations, the mistakes and the successes of Becquerel in his research. The complete list of texts can be found at this address (in French), but here are some translated excerpts:
I would like to emphasize the following fact, which I believe is particularly important and unlike any phenomenon we could have expected to observe: the same crystal strips, placed in front of photographic plates under the exact same conditions and behind the exact same screens, but completely shielded from external radiation and kept in the dark, still produce the same photographs.
It is most remarkable to observe that, since March 3rd—that is, after more than one hundred and sixty hours—the intensity of the radiation emitted in darkness has not diminished in any perceptible degree
Why was Becquerel shocked by this observation? Because uranium appeared to emit radiation without any external source of energy. With other phosphorescent crystals, it is easy to determine where the energy comes from that allows them to emit light: it comes from the light they absorbed themselves. In fact, they emit light only for a short period before running out of energy. Other light-emitting phenomena also have an obvious source of energy: the flame of a fire, the filament of a light bulb, and an electric arc all stop producing light shortly after they stop receiving energy.
Becquerel reproduced the experiment, this time ensuring that the uranium salt did not receive any light.
At the bottom of an opaque cardboard box, I placed a photographic plate. Then, on its sensitive side, I placed a slice of uranium salt […]; the operation was carried out in a dark room. The box was closed, then placed inside another cardboard box, and finally put in a drawer.
The results are the same: a strong image of the crystal appears on the photograph, confirming that uranium emits radiation without requiring any external source of energy. Later, Becquerel observed that not only does uranium continue to emit radiation, but the intensity of this radiation does not decrease over time:
For more than two months, the same pieces of various salts, kept shielded from any known external excitatory radiation, have continued to emit this new radiation with almost no noticeable weakening. From March 3 to May 3, these substances were confined in an opaque cardboard box. Since May 3, they have been placed in a double lead box that never leaves the dark room.
This is not the only strange property of the “uranium” Becquerel has in his lab. The “radiation” it emits can be detected using a photographic plate from the time (“Une plaque Lumière, au gélatino-bromure d’argent” according to Becquerel), but it is invisible to the human eye. More importantly, this radiation can pass through materials that visible light cannot:
One can very easily confirm that the radiation emitted by this substance […] passes through not only sheets of black paper, but also various metals, such as an aluminum plate or a thin sheet of copper.
Arenʼt these properties even more incredible? Why does Becquerel say that the absence of an external source of energy is “particularly important and unlike any phenomenon we could have expected to observe,” rather than emphasizing the fact that this radiation is invisible and can pass through matter?
In fact, while these properties were indeed surprising in 1896, they were not entirely new. As early as 1801, the German physicist Johann Wilhelm Ritter had discovered ultraviolet light, which is invisible to the eye. Like Becquerel, Ritter used a photographic plate in his experiments.
More significantly, in 1895, Wilhelm Conrad Röntgen had discovered X-rays, which are not only invisible to the eye but also pass through matter.
By the time Becquerel discovered uranium radiation, the scientific community was already fascinated by X-rays and their ability to penetrate materials. In fact, if we examine the minutes of the other presentations from the March 2, 1896, session of the French Academy of Sciences (see them on gallica.bnf.fr), we find that 8 out of the 35 presentations that day—covering topics as diverse as geology and botany—were dedicated to X-rays.
This is why, of all the properties of uranium radiation, it is the absence of an external source of energy that shocks Becquerel and the scientific community. It is the only property that is completely unprecedented.
Unfortunately for Becquerel, the global fascination with X-rays made it very difficult to draw attention to his discovery—especially since the radiation emitted naturally by uranium was much weaker than the X-rays scientists were already producing in 1896, which were powerful enough to photograph people’s skeletons through their bodies.
It was Marie Curie who later succeeded in drawing significant attention to this phenomenon. However, Henri Becquerel still made important contributions to the study of radioactivity, paving the way for Marie Curie’s success.
Becquerel’s main contribution was probably a way to measure the amount of radiation emitted by uranium that is more precise than the use of a photographic plate. Becquerel reproduced an experiment that had been made with X-rays and consisted in exposing a device called an “electroscope”, which measures electric charges, to the radiation. The rays make the air conductive, leading to the electroscope losing its charge:
It is known that Mr. Hurmuzescu’s electroscope, when shielded from external electrical influences by a metallic enclosure and from ultraviolet radiation by yellow glass, remains charged for many months. If one of the yellow glass panes of the lantern be replaced with an aluminum sheet 0.12 millimeter in thickness, and if a slice of the phosphorescent substance be placed against the exterior of this aluminum sheet, the gold leaves of the electroscope may be observed to slowly approach one another, indicating a gradual discharge of the instrument.
This property of uranium was the one Becquerel studied most extensively and the one that would enable Marie Curie to make her first major discoveries. She used the piezoelectric electrometer invented by her husband, Pierre Curie, which was far more precise than Hurmuzescu’s electroscope.
One may wonder why Henri Becquerel had uranium in his laboratory. Today, when we hear about uranium, we think of nuclear power plants, the atomic bomb, and high-tech applications. However, in 1896, the scientific community had already been familiar with uranium for about a century, ever since its discovery by the Prussian chemist Martin Heinrich Klaproth, who named it after the recently discovered planet Uranus.
Uranium had also been used in glassmaking long before scientists identified it, to give glass a yellow or green tint. Uranium glass has been found dating back to antiquity (see this article by Peter Kurzmann).
Annales de Chimie et de Physique, tome XXXVIII contains a translation of a dissertation presented by George Gabriel Stokes at the Royal Society in 1852. Stokes mentions substances that, when illuminated by a given light, emit a different light. On page 500, one can read:
Some samples of yellow Bohemia glass were reported by Mr. Brewster as diffusing a nice yellow light. Mr. Stokes found the same property in yellow glass made in England and sold under the name canary glass.
Yellow Bohemia glass and canary glass both contain uranium.
In 1852, when Stokes presented his work, Henri Becquerel had just been born. But there was a French scientist of that time who was studying light and took an interest in Stokes’ research: Edmond Becquerel, Henri’s father. He is actually mentioned in the text from Annales de Chimie et de Physique, tome XXXVIII, referenced above:
In an experiment where a pure spectrum is cast upon a vase filled with a solution of quinine sulfate, the blue light that is diffused, extending beyond the violet, exhibits broad dark stripes […]. Mr. Stokes provided, in Fig. I, a drawing of these dark stripes as they appeared in his experiments, and this drawing bore a striking resemblance to that which Edmond Becquerel obtained through photographic experiments.
source: https://gallica.bnf.fr/ark:/12148/bpt6k34779r/f494.item
In 1857 and 1858, Edmond Becquerel published two essays compiled under the title “Recherches sur divers effets lumineux qui résultent de l’action de la lumière sur les corps”. He references Stokes’ work on light, as well as findings on colored glass, which he calls “uranium glass”:
This physicist hath observed these effects in bodies which are not phosphorescent, and among which may be mentioned, as is known, quinine bisulfate, uranium glass, and an alcoholic solution of chlorophyll.
source: https://gallica.bnf.fr/ark:/12148/bpt6k34796b/f60.item
Edmond Becquerel stated that these bodies “are not phosphorescent.” In reality, phosphorescence does occur in them, but it lasts for such a short time that it cannot be seen by the human eye. Edmond Becquerel understood this and built a device called the “phosphoroscope,” which could measure phosphorescence with extremely short durations.
It is because phosphorescence is so difficult to observe in uranium glass that Edmond Becquerel took such an interest in it. When Henri Becquerel continued his father’s work on phosphorescence, he therefore studied, among other things, substances that contained uranium. Henri Becquerel even mentioned his father in his presentations on uranium at the French Academy of Sciences:
The properties of the luminous radiation emitted by this substance were once studied by my father, and I had since the occasion to signal some of the interesting peculiarities of this luminous radiation.
In the end, the radioactivity of uranium that Henri Becquerel discovered has nothing to do with the phosphorescence of uranium that his father was studying. It is a discovery that is largely due to chance. But not entirely. Did scientists already have a sense that there was something odd about uranium? In the 1850s and 1860s, Abel Niépce de Saint-Victor regularly presented his work on photography to the French Academy of Sciences (see, for instance, his presentation on March 1, 1858), in which he frequently used uranium. He referenced the work of Edmond Becquerel and even suggested the existence of a “radiation that is invisible to our eyes”:
I then wrapped a needle within a sheet of paper soaked in uranium azotate […].
The results of my experiments lead me to conclude that this persistent activity, imparted by light to all these porous bodies—even the most inert among them—cannot be attributed to phosphorescence, for such an effect could not endure so long, according to the experiments of Mr. Edmond Becquerel. It is, therefore, more likely to be a radiation invisible to our eyes.
Thanks to Valeria Tettamanti for reviewing the French version of this text, and to Kathy “Kathy Loves Physics” Joseph for making the video that inspired me to write this article.
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using some partially digitized text from Wikisource ↩︎