Becquerel's Radioactivity
On February 24th, 1896, Henri Becquerel read before the French Academy of Science his conclusion that some “phosphorescent” metal ores emit rays that are able to pass through opaque materials and react with the silver salts of photographic films. This revelation was happening around the same time Wilhelm Röntgen discovered x-rays, and therefore these two phenomena were often compared—with the difference being that x-rays required electricity while phosphorescence was often spontaneous. Three days after his initial exposition, Becquerel reported further on his observations: he focused now on crystals of double sulfate of uranyl and potassium, and showed that the imprints left on photographic films could be masked by denser materials. He spoke of the different shadows that aluminum and copper cast, but Bequerel’s most important finding was that this was not a result of exposure to sunlight; they were spontaneous and their effect “infinitely longer” than their visible luminosity. Becquerel dares to call them “invisible rays”, but finding himself in Röntgen’s shadow, he hesitates, and says that “the present experiments” “do not warrant this conclusion” yet. Despite his hesitation, Becquerel had everything a Particle Physics experiment needs: a source (uranium salts) and a detector (photographic film)—he was the first to dip his toes into the ocean of Particle Physics, and the first to see the effects of radioactivity.
Source: Becquerel, H. Sur les radiations invisibles émises par les corps phosphorescents. Comptes Rendus Hebdomadaires de l’Académie des Science 102, 501–503 (1896).
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Full disclosure, I may occasionally borrow a sentence from Will Durant's Story of Civilization. I absolutely love that collection!