by exposure to radiation from a uranium salt. The shadow of a metal
Maltese Cross placed between the plate and the uranium salt is clearly visible. Becquerel's discovery of
spontaneous radioactivity is a famous example of
serendipity, of how chance favours the prepared mind. Becquerel had long been interested in
phosphorescence, the emission of light of one colour following the object's exposure to light of another colour. In early 1896, there was a wave of excitement following
Wilhelm Röntgen's discovery of
X-rays in late 1895. During the experiment, Röntgen "found that the
Crookes tubes he had been using to study
cathode rays emitted a new kind of invisible ray that was capable of penetrating through black paper." Becquerel learned of Röntgen's discovery during a meeting of the
French Academy of Sciences on 20 January where his colleague
Henri Poincaré read out Röntgen's preprint paper. Becquerel "began looking for a connection between the
phosphorescence he had already been investigating and the newly discovered X-rays" There followed a period of intense research into radioactivity, including the determination that the element
thorium is also radioactive and the discovery of additional radioactive elements
polonium and
radium by
Marie Curie and her husband,
Pierre Curie. The intensive research of
radioactivity led to Becquerel publishing seven papers on the subject in 1896. As simultaneity often happens in science, radioactivity came close to being discovered nearly four decades earlier in 1857, when
Abel Niépce de Saint-Victor, who was investigating photography under
Michel Eugène Chevreul, observed that uranium salts emitted radiation that could darken photographic emulsions. By 1861, Niepce de Saint-Victor realized that uranium salts produce "a radiation that is invisible to our eyes". Niepce de Saint-Victor knew Edmond Becquerel, Henri Becquerel's father. In 1868, Edmond Becquerel published a book,
La lumière: ses causes et ses effets (Light: Its causes and its effects). On page 50 of volume 2, Edmond noted that Niepce de Saint-Victor had observed that some objects that had been exposed to sunlight could expose photographic plates even in the dark.
Experiments Describing them to the French Academy of Sciences on 27 February 1896, he said: One wraps a
Lumière photographic plate with a bromide emulsion in two sheets of very thick black paper, such that the plate does not become clouded upon being exposed to the sun for a day. One places on the sheet of paper, on the outside, a slab of the phosphorescent substance, and one exposes the whole to the sun for several hours. When one then develops the photographic plate, one recognizes that the silhouette of the phosphorescent substance appears in black on the negative. If one places between the phosphorescent substance and the paper a piece of money or a metal screen pierced with a cut-out design, one sees the image of these objects appear on the negative ... One must conclude from these experiments that the phosphorescent substance in question emits rays which pass through the opaque paper and reduce silver salts. But further experiments led him to doubt and then abandon this hypothesis. On 2 March 1896 he reported: I will insist particularly upon the following fact, which seems to me quite important and beyond the phenomena which one could expect to observe: The same crystalline crusts [of potassium uranyl sulfate], arranged the same way with respect to the photographic plates, in the same conditions and through the same screens, but sheltered from the excitation of incident rays and kept in darkness, still produce the same photographic images. Here is how I was led to make this observation: among the preceding experiments, some had been prepared on Wednesday the 26th and Thursday the 27th of February, and since the sun was out only intermittently on these days, I kept the apparatuses prepared and returned the cases to the darkness of a bureau drawer, leaving in place the crusts of the uranium salt. Since the sun did not come out in the following days, I developed the photographic plates on the 1st of March, expecting to find the images very weak. Instead the silhouettes appeared with great intensity ... One hypothesis which presents itself to the mind naturally enough would be to suppose that these rays, whose effects have a great similarity to the effects produced by the rays studied by M. Lenard and M. Röntgen, are invisible rays emitted by phosphorescence and persisting infinitely longer than the duration of the luminous rays emitted by these bodies. However, the present experiments, without being contrary to this hypothesis, do not warrant this conclusion. I hope that the experiments which I am pursuing at the moment will be able to bring some clarification to this new class of phenomena. == Later life and death ==