Sur les radiations invisibles émises par les corps phosphorescents; Sur les radiations émises par phosphorescence.

Paris: Gauthier-Villars, 1896.

First edition, in original printed wrappers, of the discovery of radioactivity. The Nobel Prize in Physics 1903 was shared between Becquerel, “in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity,” and Marie and Pierre Curie, “in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel.”

“Becquerel was born in Paris and studied civil engineering at the École Polytechnique. Having obtained his diploma in 1877, he opted for a career as a scientist. He did research in optics, took his PhD in 1888, and was elected a member of the Academy of Sciences a year later. In 1892 Becquerel became professor at the Natural History Museum in Paris, like his father and grandfather before him (and later his son after him). In 1895 he was also appointed professor at the École Polytechnique. His speciality (as that of his father and grandfather) was the study of uorescence and phosphorescence. These are properties of materials to emit light if properly stimulated, usually by irradiation with light of shorter wavelength than that of the emitted light… In the Académie des Sciences Röntgen’s discovery [of X-rays] was discussed in January 1896. It was quite natural for Becquerel to wonder, as Poincaré had suggested, whether X-rays were produced by some kind of phosphorescence.

“Becquerel began investigations with materials which were phosphorescent if exposed to sunlight. Because of its scienti c tradition the Museum possessed many samples of such material. This was a decisive advantage which he had over his competitors. His technique was clear and simple. He wrapped a photographic plate in thick black paper, protecting it from light, placed on it the phosphorescent substance, exposed the arrangement for some time to the direct sunlight, and nally developed the plate. Since X-rays were known to traverse the paper, the plate would be blackened if X-rays were produced. At rst the results were negative. But when he used a sample of potassium uranyl disulphate K2UO2 (SO4)22H2O, he observed an effect which he reported to the Academy on 24 February [in ‘Sur les radiations émises par phosphorescence’].

“A week later, on 2 March 1896, Becquerel reported again to the Academy [in ‘Sur les radiations invisibles émises par les corps phosphorescents’]. It is this report that contained the discovery which would make him famous [our emphasis]. We rst quote from a paragraph dealing with repetitions of his previous experiment:

A photographic plate, gelatine with silver bromide, was enclosed in an opaque frame in black cloth, closed on one side by a sheet of aluminium; if one exposed the frame in full sunshine, even for a whole day, the plate was not fogged; but, if one had xed on the aluminium sheet, on the outside, a lamella of uranium salt [...] and if one exposed it for several hours to the sun, one recognised, after one had developed the plate in the usual way, that the silhouette of the crystalline lamella appeared in black on the sensitive plate.

“The unexpected discovery is described two paragraphs further down [p. 502]:

I shall insist in particular on the following fact which seems to me very important and outside the phenomena one could expect to observe: The same crystalline lamellas, placed with respect to photographic plates in the same way [...] but kept in the dark, produce again the same photographic prints. Here is how I have been led to make that observation: Of the previous experiments some had been prepared on Wednesday 26th and on Thursday 27th of February and, since on these days the sun had shown itself only intermittently, I had kept the experiments prepared and placed the frames back in the dark into a drawer, leaving the lamellas of uranium salt in place. Since the sun had also not shown itself the following days, I developed the plates, expecting to nd very feeble images. The silhouettes appeared, on the contrary, with great intensity.

“Becquerel then went on to describe how he repeated experiments in complete darkness with the same result. Thus besides X-rays (or Röntgen rays) there was yet another kind of new rays, which were emitted seemingly without cause by a certain phosphorescent substance although that substance had not been stimulated to phosphoresce.

“During the course of 1896 Becquerel continued to investigate these rays. Still in March he found that they discharge an electroscope, i.e., they give some electric conductivity to air [reported in ‘Sur quelques propriétés nouvelles des radiations invisibles émises par divers corps phosphorescents,’ pp. 559-564]. He also performed observations, which later turned out to be erroneous, namely, that the rays could be re ected, refracted, and could possess polarization. These erroneous ndings seemed to attribute to Becquerel’s rays most of the properties of ordinary light; they seemed to be better understood than Röntgen rays. It can be assumed that for this reason there was little interest in them for the next two years. In May 1896, Becquerel found out that all uranium compounds, phosphorescent or not, which he had studied, emitted rays. He concluded that pure metallic uranium should show the strongest radiation and con rmed that hypothesis by experiment. Becquerel now called his rays uranic rays. At the end of 1896, he reported on the absorbing power of different materials with respect to his rays. Moreover, he stressed the fact that the source of energy, which makes uranium emit rays, was completely unknown.

“We can summarize the events of 1896 in Paris by stating that Becquerel discovered radioactivity. He found it to be a property of the element uranium. It could be detected photographically and electrically. An unknown source of energy had to exist to keep the radiation going” (Brandt, The Harvest of a Century, pp. 10-12).

There is some confusion in the literature as to which of Becquerel’s six papers in Vol. 122 of Comptes Rendus represents the discovery of radioactivity. Garrison & Morton ascribe the discovery of radioactivity to ‘Sur les radiations émises par phosphorescence,’ in which Becquerel observes the effect of uranium sulphate on photographic paper but wrongly believes it to be due to phosphorescence caused by the effect of sunlight on the salt. PMM incorrectly states that the discovery was first reported in Becquerel’s third paper ‘Sur quelques propriétés nouvelles des radiations invisibles émises par divers corps phosphorescents,’ which only describes the effect of the rays on an electroscope. As Brandt correctly states, and as is clearly established by the quotation above, it was in Becquerel’s second paper in this volume, ‘Sur les radiations invisibles émises par les corps phosphorescents,’ that the discovery was made: the effect of the uranium salt on a photographic plate was observed even though the effect of sunlight on the salt had been excluded.

Garrison & Morton 2001. Dibner, Heralds of Science 163; PMM 393 (both referring to the 1903 book edition of Becquerel’s radioactivity papers). Norman 158 (complete journal volume).

pp. 501-503 in Comptes Rendus Hebdomadaires des Séances de L'Académie des Sciences, Tome 122, No. 9, 3 March 1896. [With:] ‘Sur les radiations émises par phosphorescence,’ pp. 420-421 in ibid., No. 8, 24 February 1896. Paris: Gauthier-Villars, 1896. Original printed wrappers.

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