Invisible Light:

The Discovery of Radioactivity

It is perhaps, along with Fleming’s discovery of penicillin, the most famous accidental discovery in science, the best demonstration that chance favors the prepared mind. The dogged researcher has learned that after exposure to sunlight, a certain mineral containing uranium--a "uranium salt"--blackens a photographic plate wrapped in dark paper. Was the uranium salt somehow storing the sun’s rays? As he is about to repeat the experiment, the weather in Paris turns cloudy, and the researcher puts the unused plate with the uranium crystal atop it into a drawer. Several days later, impatient with the weather, he decides to develop the plate anyway and--to his eternal astonishment--finds it blackened without any prior exposure to sunlight of the mineral. The uranium salt has been emitting invisible and hitherto undiscovered rays, which have the ability to penetrate paper and expose photographic emulsion.

Henri Becquerel, of course, discovering radioactivity in 1896. No, Abel Niepce de Saint Victor discovering radioactivity in 1857.

One wonders what thoughts were running through Michele Chevreul’s mind when in 1842, as head of a commission appointed by the Ministry of the Army, he traveled to Montauban, charged with evaluating a proposal on the dyeing of uniforms made by one lieutenant of the dragoons, Claude-Félix-Abel Niepce de Saint Victor.

One imagines the lot of them meeting in an abandoned factory on the riverbank, or perhaps in an old warehouse, peering into vats of dye, or examining with serious visages the uniforms colored by the new process. We don’t know any of this. We don’t even know whether Chevreul actually traveled to Montauban. We know only that Michele-Eugène Chevreul, already 56 and Director of Dyeing at the Manufacture Nationale des Gobelins, had been appointed by the Ministry of the Army to evaluate Niepce’s proposal, whatever it may have been, and that Niepce was then stationed at Montauben. The lieutenant, a cousin of the famous Nicéphore Niepce who had taken the original photograph, was then about 37, having been born on July 26, 1805 in Saint-Cyr, near Chalon-sur-Saone. Like many of the time he seemed destined for a military career, and in 1827 entered calvary school; by 1842 he had become a lieutenant in the first regiment of dragoons at Montauban. We don’t know what he looked like--if a portrait exists, it has long been buried. It is pleasant to picture him as a dashing officer outfitted in brilliant blue and red, carrying a sabre and polishing his dress hat. But it is more difficult to imagine that either he or Chevreul had any inkling of the great and perplexing discovery they were to make, the controversy it would cause, or that the world would utterly forget it.

Regardless of what actually took place in 1842, we know that in 1845 Niepce received a transfer to Paris where he could work under the direction of Chevreul, who was a professor of chemistry at the Museum of Natural History and a member of the Academy of Sciences. Given that Nicéphore Niepce had taken his first photograph only three decades earlier and Louis Daguerre had announced his Daguerrotype in 1839, it is hardly suprising that Chevreul had an interest in photography, even less, Niepce de Saint Victor.* Chevreul encouraged his protégé to work on the development of new photographic processes and emulsions and even today Niepce is credited as having been first to use albumen in photographs and the first to produce negatives on glass (sometimes one hears steel, maybe both).

Photography is, if nothing else, the interaction of chemical substances with light, and we can be sure Niepce investigated dozens, if not hundreds, of variations over the next decade. By 1856 he was sufficiently astonished by the behavior of certain chemicals to write a memoir, "On a new action of light," which was communicated by Chevreul to the French Academy of Sciences on 16 November 1857 and published in the Comptes Rendus of the Academy. Niepce begins:

Does a body, after having been struck by light, or having been subjected to exposure, retain in darkness some impression of this light? This is the problem that I have tried to resolve through photography. Phosphorescence and fluorescence of bodies are known: but, as far as I am aware, no one before me has ever done the experiments I am about to describe. What Niepce had been doing was to expose plates covered with a variety of chemicals to light, then cover the plates with photographic paper and put everything in a dark drawer for several days. To his amazement, he found that certain chemicals exposed the photographic paper in absolute darkness. As both the title of his memoir and the opening paragraph make clear, Niepce was convinced he had discovered something new. They also make clear that, initially, he believed that this new phenomenon had something to do with exposure of the substances he was testing to light. But what exactly? Fluorescence, the ability of certain chemicals to glow under exposure to light, had been known for hundreds of years. Phosphorescence, the ability of some substances to emit light after exposure has ceased, had been observed at least since the seventeenth century. Niepce understood he was not witnessing either--no visible light was being emitted. In which case, what?

One reads Niepce’s memoirs with a sense of frustration. Looking over his shoulder you can’t help but advise, "Stop exposing these plates to light; it has nothing to do with that!" But hindsight is twenty-twenty. In the thick of things one never has any idea of what is going on, then or now, and every insight is territory claimed by blood. Niepce would never get over the idea that this new phenomenon must be some sort of stored light, but he did make progress. By the second memoir, presented to the Academy in 1858, he realizes that if one wants to get a quick and "vigorous" image, then one should impregnate a piece of paper with uranium nitrate--exactly one of the uranium salts Becquerel would use forty years later. He is still first exposing the uranium nitrate to the sun, and even thinks that tartaric acid works as well, which is nonsense. (One can only guess that he contaminated his tartaric acid with uranium nitrate.) Nevertheless, Niepce is able to say, "In conclusion, I have observed that the bodies which conserve the most activity after exposure are those, with the exception of the uranium salts, that are the least fluorescent."

Despite the confusion, director and researcher gradually began to perceive the importance of the discovery. After Niepce’s third memoir to the Academy, Chevreul presents his own. "The facts set forth in M. Niepce’s last memoir are important," he says, "not only for their connection to questions concerning chemical phenomena produced or assisted by the action of light, but also because above all they are new and concern light itself, its dynamical force. This is a capital discovery...M. Niepce has established that the exposed cardboard, kept in a tin cylinder in darkness is still active six months after exposure."

And finally, in 1861, Niepce states conclusively, "This persistent activity... cannot even be phosphorescence, because it would not continue for such a long time, as has been shown by Edmond Becquerel. It is then more probable that this is some sort of rays invisible to our eyes, as M. Leon Foucault believes, rays that do not cross glass."

That Foucault, one of the most eminent scientists of the time, commented on Niepce’s findings, indicates that they stimulated some interest. To be sure, Niepce presented no fewer than six memoirs to the Academy between 1857 and 1867. In one he notes that "the action of light is perhaps very favorable on certain wines; it can give them the qualitiy of an old wine..." In a few he responds to his critics. "Many hypotheses have been given. Certain people have denied the facts, which is the easiest thing to do. But no one has given an explanation of this phenomenon." Edmond Becquerel (1820-1891), Henri’s father, who worked at the Museum of Natural History with Chevreul, also commented on the discoveries and gave his own, incorrect, explanation.

One would have thought with such prolonged replication of the findings, that other researchers would have followed up Niepce’s claims. Perhaps the reports were too fantastic to be taken seriously, so far ahead of their time that no one had a conceptual framework in which to fit them. For whatever reasons, Niepce’s facts were denied, then forgotten. Niepce died in 1870, "leaving for posterity a widow, two children and nothing!" Chevreul followed him nineteen years later, at the extraordinary age of 103. Perhaps his longevity had something to do with the action of light on wine. But he was gone, and that left the stage empty for the appearance of Henri Becquerel.

It is easier to imagine what was going on in Henri Becquerel’s mind, when in 1895, three years after he had taken his father’s chair at the Museum of Natural History, he read William Conrad Röntgen’s paper on the discovery of X rays. Röntgen had been experimenting with cathode-ray tubes, also known as Crookes’ tubes or Geissler tubes depending whether you are English or German, which are no more than glass tubes partially evacuated of air, and through which high-voltage electricity can be discharged. Today we call them television tubes; cathode-rays are electrons. Since about the time Niepce submitted his second memoir to the Academy, scientists had known that cathode-rays striking the end of such a tube could cause it to phosphoresce. Röntgen, in another of history’s famous accidents, discovered that the cathode rays striking one end of the tube were causing it to emit invisible radiation which passed through cardboard and air and humans. The forty-three-year-old Becquerel, who like his father had an interest in studying solar spectra with photography, reasoned that certain substances struck by light might also emit invisible radiation. He exposed a uranium salt to light, the weather in Paris turned cloudy. The rest is history.

Becquerel, it needs to be said, does not come off well in the remainder of this drama. Shortly after he announced "invisible radiation emitted by uranium salts" to the Academy in 1896, several respected scientists pointed out that the same discovery with the same mineral and virtually the same methods had been made forty years earlier, and published in the same journal. One eminent critic, Professor de Heen of the University of Liège and member of the Belgian Royal Academy went so far as to publish a pamphlet in 1901 asking, scandalized tone audible between the quotation marks, "Who is the discoverer of the phenomenon called radioactivity?" Despite the clamor Becquerel refused to mention his predecessor for seven years. When he finally did, in 1903, it was only to dismiss Niepce’s work. "Uranium is in such small quantities on those papers," he wrote, "that in order to produce an appreciable effect on the [photographic] plates used by the author, it would have been necessary to keep them in place several months. Therefore Niepce could not have observed rays of uranium."

To make matters worse, Becquerel rewrote the history of his own contributions. In reading his first memoirs to the Academy, it is quite clear that Becquerel, as Niepce before him, initially thought that he had discovered some sort of lumière noire, which could be reflected, polarized and refracted (shades of Young) just as ordinary light could be. (At the same time though, it seemed to have electrical properties.) Becquerel’s most vocal critic, one of those who pointed out Niepce’s priority in the matter, was physician and sociologist Gustave Le Bon. It was actually Le Bon’s experiments in the years 1896-1899 that showed that the new rays did not have the properties of light, but were identical to cathode rays, that is electrons. Which meant Becquerel’s first experiments must have been in error. Becquerel came around to this point of view himself by 1899 with no acknowledgement of Le Bon. Yet, in one of his own popular books, Becquerel charged that "it is sufficient to read M. Gustave Le Bon’s work in the Comptes Rendus to convince oneself that the author...had no idea of the phenomenon of radioactivity." Le Bon was not amused. Supporters of Becquerel continued to claim that he had shown that uranic rays were neither reflected, nor refracted. It was an inversion of history.

Apart from all this, the nagging question remains of whether Henri Becquerel had known of Niepce’s work and had conveniently forgotten it (Kekulé and even Einstein seem to have excelled at that sort of thing). There can be no question that Edmund, who worked with Chevreul at the Museum, knew of the discovery. One of Edmund’s memoirs on phosphorescence in fact appears directly after Niepce’s first report in the Comptes Rendus. Le Bon, no friend of Becquerel’s, claims that his opponent’s initial misunderstandings about the discovery were due to the fact that he was "always under the influence of the ideas of Niepce de Saint Victor." On the other hand, Henri’s son, Jean, no neutral observer either, always claimed that the path of discovery went from Edmond to Henri and credit should, above all, go to the Museum. All in all, there seems to be no hard evidence that Edmund told his son about Niepce’s work and such accusations should be consigned to the arm chair.

Niepce’s discovery, of course, was before its time. It is convenient, for that reason, to nod with sympathetic understanding that it would inevitably find the Oblivion File. By 1896, we understand: Crookes’ tubes were commonplace, X rays had been discovered and the academicians were more receptive to the idea that Becquerel had uncovered something new. Of course it was new; this was not stored light, but radiation emitted from the disintegration of the atomic nucleus itself. It portended not only an entire new branch of physics but a new danger to mankind.

Yet, there was more to this sad tale than a discovery made forty years before it should have been. Niepce’s work attracted enough interest at the time that it might have been pursued; the action of cathode-rays on glass had been noticed. History could have been otherwise. In any case, the clamor raised by Le Bon at the turn of the century should have ensured that Niepce’s name would be resurrected. It was not. Another factor, an inescapable one, clearly contributed to Niepce de Saint Victor’s perpetual interment. One of the largest and certainly the most colorful exhibit at the Panopticon is devoted to this phenomenon: the Nobel Prize. Once Becquerel had been awarded the Prize in 1903 with his student Marie Curie and her husband Pierre, that was the end of the story. Nobel Prizes are given for discoveries, not always for understanding, and they have had the effect of clamping history shut. In our own time no story more closely parallels the discovery of radioacticity than the discovery of the cosmic microwave background radiation. The background radiation, detected first by accident by Arno Penzia and Robert Wilson at Bell Labs in 1964, is considered the most important cosmological observation of the twentieth century, after the discovery of the expansion of the universe itself, for it was the microwave background that convinced cosmologists that the big bang theory was correct. Today there is no doubt in anyone’s mind that the cosmic background radiation is exactly the heat left over from the big bang.

But it had been predicted as early as 1950 by Ralph Alpher and Robert Hermann, who didn’t take their own prediction seriously enough to follow up; at least three research groups around the world independently repredicted it in the early 1960s, two Princeton cosmologists were completing the apparatus to search for it in 1964, and when Penzias and Wilson stumbled across the signal, the Princeton group explained to them what they had found. This does not count all the spurious, near and ignored detections over the previous two decades. Physicists know the story, but in textbooks it is reduced to a single line. Who deserved the Prize?

As a brash graduate student I once remarked to Tullio Regge, a famous Italian physicist, that the Nobel Prize should be abolished. Regge countered that that would only make the Prizes in circulation more valuable. Rather, he said, the Nobel Prize should be inflated. The Committee should hand out more and more every year until the Prize was worthless. During that memorable drive with Dyson, Schwarz and Feynman to the lake the subject, inevitably, turned to Nobel Prizes and I mentioned Regge’s suggestion. Feynman, in his usual growl rejoined, "Regge just said that because he’s an Italian. He’s always thinking about inflation. Tell me, kid, what do you think about my Nobel Prize?"

That’s another story, but if Abel Niepce de Saint Victor’s tale teaches us anything, it is that the history of science should be divorced from the history of the Nobel Prize. It may be that justice will eventually have its say: over the last few years Abel Niepce de Saint Victor seems to have become recognized in France as the discoverer of radioactivity, only the news has yet to cross the Atlantic. We can only hope it someday makes the journey.

References and notes for Niepce

Insofar as possible this chapter has been based on the original memoirs of Niepce de Saint Victor, Eugène Chevreul, Henri Becquerel and Gustave Le Bon that appeared in the Comptes Rendus of the French Academy of Science. They are:

N. de Saint Victor, Première Memoire "Sun une nouvelle action de la lumière," C. R. Acad. Sci. 45, 811 (1857). N. de Saint Victor, Deuxième Mémoire, C. R. Acad. Sci. 46, 448 (1858).

N. de Saint Victor, Troisième Mémoire, C. R. Acad. Sci. 47, 866 (1858).

N. de Saint Victor, Quatrième Mémoire, C. R. Acad. Sci. 47, 1002 (1858).

N. de Saint Victor, Note sure l’activité, C. R. Acad. Sci. 48, 741 (1859).

N. de Saint Victor, De l’action..., C. R. Acad. Sci. 49, 815 (1859).

N. de Saint Victor, Cinquième Mémoire, C. R. Acad. Sci. 53, 33 (1861).

N. de Saint Victor, Sixième Mémoire, C. R. Acad. Sci. 65, 505 (1867).

M. E. Chevreul, C. R. Acad. Sci. 47, 1006 (1858).

H. Becquerel, C. R. Acad. Sci. 120, 559 (1896).

H. Becquerel, C. R. Acad. Sci. 120, 691 (1896).

H. Becquerel, C. R. Acad. Sci. 120, 762 (1896).

H. Becquerel, C. R. Acad. Sci. 122, 1086 (1896).

H. Becquerel, C. R. Acad. Sci. 128, 771 (1899).

G. Le Bon, C. R. Acad. Sci. 124, 755 (1897).

G. Le Bon, C. R. Acad. Sci. 124, 892 (1897).

Also helpful have been

P. Fournier and J. Fournier, New J. Chem. 14, 785 (1990).

M. Meyer and E. Gonthier, "Y a-t-il encore polémique autour de la découvert des phénomènes dit radioactifs?" Science Tribune (July 1997); www.tribunes.com/tribune/art97/meyer.htm

G. Le Bon, L’Evolution de la Matière (Flammarion: Paris, 1908), chaps. 2 and 14.

Notes

pp. 3-4: "Does a body...?" Niepce, Première Mémoire, p. 811.

"I have definitely verified..." Niepce, Deuxième Mémoire, p. 452.

"The facts set forth..." Chevreul, p. 1009.

"The activity persists..." Niepce, Cinquième Memoire, p. 34.

"The action of light is perhaps very favorable on certain wines..." Niepce, "De l’action..." p. 817.

p 5-7: For de Heen’s pamphlet and Le Bon’s side of the story, see Le Bon, L’Evolution, chap 14. For his remark implying that Becquerel knew of Niepce’s discovery, see chap. 2, p. 24.

For the Becquerel’s side, see J. Becquerel, Les inventeurs celèbres, sciences physicqes et applications (Mazenod: Paris, 1962), p. 298.